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Trilobite

Trilobites (/ˈtrləˌbts, ˈtrɪlə-/;[4][5][6] meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (521 million years ago) and they flourished throughout the lower Paleozoic before slipping into a long decline, when, during the Devonian, all trilobite orders except the Proetida died out. The last extant trilobites finally disappeared in the mass extinction at the end of the Permian about 252 million years ago. Trilobites were among the most successful of all early animals, existing in oceans for almost 270 million years, with over 22,000 species having been described.

By the time trilobites first appeared in the fossil record, they were already highly diversified and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton, they left an extensive fossil record. The study of their fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology, and plate tectonics. Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata),[7] although several alternative taxonomies are found in the literature. More recently they have been placed within the Artiopoda, which includes many organisms that are morphologically similar to trilobites, but are largely unmineralised.

Trilobites evolved into many ecological niches; some moved over the seabed as predators, scavengers, or filter feeders, and some swam, feeding on plankton. Some even crawled onto land.[8] Most lifestyles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where scientific debate continues).[9] Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.[10] The largest trilobites were more than 45 centimetres (18 in) long and may have weighed as much as 4.5 kilograms (9.9 lb).[11]

Evolution

Trilobite relatives

Trilobites belong to the Artiopoda, a group of extinct arthropods generally morphologically similar to trilobites, but aside from trilobites lacking mineralised exoskeletons. Because of their lack of mineralised exoskeletons, non-trilobite artipodans are typically only found in exceptionally preserved deposits, mostly during the Cambrian period. The exact relationships of artiopods to other arthropods is uncertain. They have been considered closely related to chelicerates (which include horseshoe crabs and arachnids) as part of a clade called Arachnomorpha, while others consider them to be more closely related to Mandibulata (which contains insects, crustaceans and myriapods) as part of a clade called Antennulata.[12]

Fossil record of early trilobites

 
Redlichiida, such as this Paradoxides, may represent the ancestral trilobites
 
Meroperix, from the Silurian of Wisconsin
 
Walliserops trifurcatus, from Jebel Oufatene mountain near Fezzou, Morocco

The earliest trilobites known from the fossil record are redlichiids and ptychopariid bigotinids dated to around 520 million years ago.[1][2] Contenders for the earliest trilobites include Profallotaspis jakutensis (Siberia), Fritzaspis spp. (western USA), Hupetina antiqua (Morocco)[13] and Serrania gordaensis (Spain).[14] All trilobites are thought to have originated in present-day Siberia, with subsequent distribution and radiation from this location.[1]

All Olenellina lack facial sutures (see below), and this is thought to represent the original state. The earliest sutured trilobite found so far (Lemdadella), occurs almost at the same time as the earliest Olenellina, suggesting the trilobites origin lies before the start of the Atdabanian, but without leaving fossils.[1] Other groups show secondary lost facial sutures, such as all Agnostina and some Phacopina. Another common feature of the Olenellina also suggests this suborder to be the ancestral trilobite stock: early protaspid stages have not been found, supposedly because these were not calcified, and this also is supposed to represent the original state.[15] Earlier trilobites may be found and could shed more light on the origin of trilobites.[1][16][17]

Three specimens of a trilobite from Morocco, Megistaspis hammondi, dated 478 million years old contain fossilized soft parts.[18][19]

Divergence and extinction

 
Ogygopsis klotzi from the Mt. Stephen Trilobite Beds (Middle Cambrian) near Field, British Columbia, Canada

Trilobites saw great diversification over time.[20] For such a long-lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished and surviving groups diversified to fill ecological niches with comparable or unique adaptations. Generally, trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn-out decline in the Devonian, culminating in the final extinction of the last few survivors at the end of the Permian period.[21]

Evolutionary trends

Principal evolutionary trends from primitive morphologies, such as exemplified by Eoredlichia,[22] include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium (micropygy to isopygy), and development of extreme spinosity in certain groups.[21] Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments.[22] Specific changes to the cephalon are also noted; variable glabella size and shape, position of eyes and facial sutures, and hypostome specialization.[22] Several morphologies appeared independently within different major taxa (e.g. eye reduction or miniaturization).[22]

Effacement, the loss of surface detail in the cephalon, pygidium, or the thoracic furrows, is also a common evolutionary trend. Notable examples of this were the orders Agnostida and Asaphida, and the suborder Illaenina of the Corynexochida. Effacement is believed to be an indication of either a burrowing lifestyle or a pelagic one. Effacement poses a problem for taxonomists since the loss of details (particularly of the glabella) can make the determination of phylogenetic relationships difficult.[23]

Pre-Cambrian

Phylogenetic biogeographic analysis of Early Cambrian Olenellidae and Redlichiidae suggests that a uniform trilobite fauna existed over Laurentia, Gondwana, and Siberia before the tectonic breakup of the supercontinent Pannotia between 600 and 550 million years ago.[1] Tectonic breakup of Pannotia then allowed for the diversification and radiation expressed later in the Cambrian as the distinctive Olenellid province (Laurentia, Siberia, and Baltica) and the separate Redlichid province (Australia, Antarctica, and China).[1][24] Breakup of Pannotia significantly antedates the first appearance of trilobites in the fossil record, supporting a long and cryptic development of trilobites extending perhaps as far back as 700 million years ago or possibly further back in time.[24]

Cambrian

Very shortly after trilobite fossils appeared in the lower Cambrian, they rapidly diversified into the major orders that typified the Cambrian—Redlichiida, Ptychopariida, Agnostida, and Corynexochida. The first major crisis in the trilobite fossil record occurred in the Middle Cambrian; surviving orders developed isopygius or macropygius bodies and developed thicker cuticles, allowing better defense against predators (see Thorax below).[25] The end-Cambrian mass extinction event marked a major change in trilobite fauna; almost all Redlichiida (including the Olenelloidea) and most Late Cambrian stocks became extinct.[21] A continuing decrease in Laurentian continental shelf area[26] is recorded at the same time as the extinctions, suggesting major environmental upheaval.

Notable trilobite genera appearing in the Cambrian include:[27]

Ordovician

 
Cast of Isotelus rex, the largest-known trilobite, from the middle to upper Ordovician of North America

The Early Ordovician is marked by vigorous radiations of articulate brachiopods, bryozoans, bivalves, echinoderms, and graptolites, with many groups appearing in the fossil record for the first time.[21] Although intra-species trilobite diversity seems to have peaked during the Cambrian,[28] trilobites were still active participants in the Ordovician radiation event, with a new fauna taking over from the old Cambrian one.[29] Phacopida and Trinucleioidea are characteristic forms, highly differentiated and diverse, most with uncertain ancestors.[21] The Phacopida and other "new" clades almost certainly had Cambrian forebearers, but the fact that they have avoided detection is a strong indication that novel morphologies were developing very rapidly.[15] Changes within the trilobite fauna during the Ordovician foreshadowed the mass extinction at the end of the Ordovician, allowing many families to continue into the Silurian with little disturbance.[29] Ordovician trilobites were successful at exploiting new environments, notably reefs. The Ordovician mass extinction did not leave the trilobites unscathed; some distinctive and previously successful forms such as the Telephinidae and Agnostida became extinct. The Ordovician marks the last great diversification period amongst the trilobites: very few entirely new patterns of organisation arose post-Ordovician. Later evolution in trilobites was largely a matter of variations upon the Ordovician themes. By the Ordovician mass extinction, vigorous trilobite radiation has stopped and gradual decline is foreshadowed.[21]

Some of the genera of Trilobites appearing in the Ordovician include:[27]

Silurian and Devonian

 
Number of families, Middle Devonian to Upper Permian. Proetida – brown, Phacopida – steel blue, Lichida – clear blue, Harpetida – pink, Odontopleurida – olive and Corynexochida – purple

Most Early Silurian families constitute a subgroup of the Late Ordovician fauna. Few, if any, of the dominant Early Ordovician fauna survived to the end of the Ordovician, yet 74% of the dominant Late Ordovician trilobite fauna survived the Ordovician. Late Ordovician survivors account for all post-Ordovician trilobite groups except the Harpetida.[29]

Silurian and Devonian trilobite assemblages are superficially similar to Ordovician assemblages, dominated by Lichida and Phacopida (including the well-known Calymenina). A number of characteristic forms do not extend far into the Devonian and almost all the remainder were wiped out by a series of dramatic Middle and Late Devonian extinctions.[22] Three orders and all but five families were exterminated by the combination of sea level changes and a break in the redox equilibrium (a meteorite impact has also been suggested as a cause).[22] Only a single order, the Proetida, survived into the Carboniferous.[21]

Genera of trilobites during the Silurian and Devonian periods include:[27]

Carboniferous and Permian

The Proetida survived for millions of years, continued through the Carboniferous period and lasted until the end of the Permian (when the vast majority of species on Earth were wiped out).[21] It is unknown why the order Proetida alone survived the Devonian. The Proetida maintained relatively diverse faunas in both deep and shallow water shelf environments throughout the Carboniferous.[22] For many millions of years the Proetida existed untroubled in their ecological niche.[21] An analogy would be today's crinoids, which mostly exist as deep water species; in the Paleozoic era, vast 'forests' of crinoids lived in shallow near-shore environments.[21]

Some of the genera of trilobites during the Carboniferous and Permian periods include:[27]

Final extinction

Exactly why the trilobites became extinct is not clear; with repeated extinction events (often followed by apparent recovery) throughout the trilobite fossil record, a combination of causes is likely. After the extinction event at the end of the Devonian period, what trilobite diversity remained was bottlenecked into the order Proetida. Decreasing diversity[30] of genera limited to shallow-water shelf habitats coupled with a drastic lowering of sea level (regression) meant that the final decline of trilobites happened shortly before the end of the Permian mass extinction event.[22] With so many marine species involved in the Permian extinction, the end of nearly 300 million successful years for the trilobites would not have been unexpected at the time.[30]

Trilobites have no known direct descendants. Their closest living relatives would be the chelicerates due to both being arachnomorphs. Though horseshoe crabs are often cited as their closest living relatives, they are no closer evolutionarily than other chelicerates.[31]

Fossil distribution

 
Cruziana, fossil trilobite-burrowing trace
 
A trilobite fragment (T) in a thin-section of an Ordovician limestone; E=echinoderm; scale bar is 2 mm
 
Rusophycus, a "resting trace" of a trilobite; Ordovician of southern Ohio. Scale bar is 10 mm.
 
Plate from Barrande's work Système silurien du centre de la Bohême

Trilobites appear to have been primarily marine organisms, since the fossilized remains of trilobites are always found in rocks containing fossils of other salt-water animals such as brachiopods, crinoids, and corals. Some trackways suggest trilobites made at least temporary excursions unto land.[8] Within the marine paleoenvironment, trilobites were found in a broad range from extremely shallow water to very deep water. Trilobites, like brachiopods, crinoids, and corals, are found on all modern continents, and occupied every ancient ocean from which Paleozoic fossils have been collected.[32] The remnants of trilobites can range from the preserved body to pieces of the exoskeleton, which it shed in the process known as ecdysis. In addition, the tracks left behind by trilobites living on the sea floor are often preserved as trace fossils.

There are three main forms of trace fossils associated with trilobites: Rusophycus, Cruziana and Diplichnites—such trace fossils represent the preserved life activity of trilobites active upon the sea floor. Rusophycus, the resting trace, are trilobite excavations involving little or no forward movement and ethological interpretations suggest resting, protection and hunting.[33] Cruziana, the feeding trace, are furrows through the sediment, which are believed to represent the movement of trilobites while deposit feeding.[34] Many of the Diplichnites fossils are believed to be traces made by trilobites walking on the sediment surface.[34] Care must be taken as similar trace fossils are recorded in freshwater[35] and post-Paleozoic deposits,[36] representing non-trilobite origins.

Trilobite fossils are found worldwide, with thousands of known species. Because they appeared quickly in geological time, and moulted like other arthropods, trilobites serve as excellent index fossils, enabling geologists to date the age of the rocks in which they are found. They were among the first fossils to attract widespread attention, and new species are being discovered every year.

 
Fossil hunters look for trilobites and other fossils in Penn Dixie Fossil Park and Nature Preserve.

In the United States, the best open-to-the-public collection of trilobites is located in Hamburg, New York. The shale quarry, informally known as Penn Dixie, stopped mining in the 1960s. The large amounts of trilobites were discovered in the 1970s by Dan Cooper. [37] As a well-known rock collector, he incited scientific and public interest in the location.[38] The fossils are dated to the Givetian (387.2 - 382.7 million years ago) when the Western New York Region was 30 degrees south of the equator and completely covered in water.[39] The site was purchased from Vincent C. Bonerb by the Town of Hamburg with the cooperation of the Hamburg Natural History Society to protect the land from development.[37] In 1994, the quarry became Penn Dixie Fossil Park & Nature Reserve when they received 501(c)3 status and was opened for visitation and collection of trilobite samples. The two most common found samples are ‘’Eldredgeops rana and Greenops.[40]

A famous location for trilobite fossils in the United Kingdom is Wren's Nest, Dudley, in the West Midlands, where Calymene blumenbachii is found in the Silurian Wenlock Group. This trilobite is featured on the town's coat of arms and was named the Dudley Bug or Dudley Locust by quarrymen who once worked the now abandoned limestone quarries. Llandrindod Wells, Powys, Wales, is another famous trilobite location. The well-known Elrathia kingi trilobite is found in abundance in the Cambrian Wheeler Shale of Utah.[41]

Spectacularly preserved trilobite fossils, often showing soft body parts (legs, gills, antennae, etc.) have been found in British Columbia, Canada (the Cambrian Burgess Shale and similar localities); New York, U.S.A. (Ordovician Walcott–Rust quarry, near Russia, and Beecher's Trilobite Bed, near Rome); China (Lower Cambrian Maotianshan Shales near Chengjiang); Germany (the Devonian Hunsrück Slates near Bundenbach) and, much more rarely, in trilobite-bearing strata in Utah (Wheeler Shale and other formations), Ontario, and Manuels River, Newfoundland and Labrador.

Sites in Morocco also yield very well-preserved trilobites, many buried in mudslides alive and so perfectly preserved. An industry has developed around their recovery, leading to controversies about practices in restoral.[42] The variety of eye and upper body forms and fragile protuberances is best known from these samples preserved similarly to bodies in Pompeii.

The French palaeontologist Joachim Barrande (1799–1883) carried out his landmark study of trilobites in the Cambrian, Ordovician and Silurian of Bohemia, publishing the first volume of Système silurien du centre de la Bohême in 1852.

Importance

The study of Paleozoic trilobites in the Welsh-English borders by Niles Eldredge was fundamental in formulating and testing punctuated equilibrium as a mechanism of evolution.[43][44][45]

Identification of the 'Atlantic' and 'Pacific' trilobite faunas in North America and Europe[46] implied the closure of the Iapetus Ocean (producing the Iapetus suture),[47] thus providing important supporting evidence for the theory of continental drift.[48][49]

Trilobites have been important in estimating the rate of speciation during the period known as the Cambrian explosion because they are the most diverse group of metazoans known from the fossil record of the early Cambrian.[50][51]

Trilobites are excellent stratigraphic markers of the Cambrian period: researchers who find trilobites with alimentary prosopon, and a micropygium, have found Early Cambrian strata.[52] Most of the Cambrian stratigraphy is based on the use of trilobite marker fossils.[53][54][55]

Trilobites are the state fossils of Ohio (Isotelus), Wisconsin (Calymene celebra) and Pennsylvania (Phacops rana).

Taxonomy

The 10 most commonly recognized trilobite orders are Agnostida, Redlichiida, Corynexochida, Lichida, Odontopleurida, Phacopida, Proetida, Asaphida, Harpetida and Ptychopariida. In 2020, an 11th order, Trinucleida, was proposed to be elevated out of the asaphid superfamily Trinucleioidea.[56] Sometimes the Nektaspida are considered trilobites, but these lack a calcified exoskeleton and eyes. Some scholars have proposed that the order Agnostida is polyphyletic, with the suborder Agnostina representing non-trilobite arthropods unrelated to the suborder Eodiscina. Under this hypothesis, Eodiscina would be elevated to a new order, Eodiscida.

Over 20,000 species of trilobite have been described.[57]

Despite their rich fossil record with thousands of described genera found throughout the world, the taxonomy and phylogeny of trilobites have many uncertainties.[58] Except possibly for the members of the orders Phacopida and Lichida (which first appear during the early Ordovician), nine of the eleven trilobite orders appear prior to the end of the Cambrian. Most scientists believe that order Redlichiida, more specifically its suborder Redlichiina, contains a common ancestor of all other orders, with the possible exception of the Agnostina. While many potential phylogenies are found in the literature, most have suborder Redlichiina giving rise to orders Corynexochida and Ptychopariida during the Lower Cambrian, and the Lichida descending from either the Redlichiida or Corynexochida in the Middle Cambrian. Order Ptychopariida is the most problematic order for trilobite classification. In the 1959 Treatise on Invertebrate Paleontology,[59] what are now members of orders Ptychopariida, Asaphida, Proetida and Harpetida were grouped together as order Ptychopariida; subclass Librostoma was erected in 1990[60] to encompass all of these orders, based on their shared ancestral character of a natant (unattached) hypostome. The most recently recognized of the nine trilobite orders, Harpetida, was erected in 2002.[61] The progenitor of order Phacopida is unclear.

Relationship to other taxa

Once soft-part anatomy had been recovered, the trilobites were originally allied to the Crustacea. A relationship with the Chelicerata, in a clade termed Arachnomorpha (Arachnata), was in vogue for some time,[62] but, a more recent analysis of Panarthropoda suggests the trilobites are one of two major branches of Artiopoda.[31]

Morphology

When trilobites are found, only the exoskeleton is preserved (often in an incomplete state) in all but a handful of locations. A few locations (Lagerstätten) preserve identifiable soft body parts (legs, gills, musculature & digestive tract) and enigmatic traces of other structures (e.g. fine details of eye structure) as well as the exoskeleton.

Trilobites range in length from minute (less than 3 millimetres (0.12 in)) to very large (over 30 centimetres (12 in)), with an average size range of 3–10 cm (1.2–3.9 in). Supposedly the smallest species is Acanthopleurella stipulae with a maximum of 1.5 millimetres (0.059 in).[63] The world's largest-known trilobite specimen, assigned to Isotelus rex is 72 cm (28 in) in length. It was found in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay.[26] However, a partial specimen of the Ordovician trilobite Hungioides bohemicus found in 2009 in Arouca, Portugal is estimated to have measured when complete 86.5 cm (34.1 in) in length.[64][65][66]

 
The trilobite body is divided into three major sections (tagmata): 1 – cephalon; 2 – thorax; 3 – pygidium. Trilobites are so named for the three longitudinal lobes: 4 – right pleural lobe; 5 – axial lobe; 6 – left pleural lobe; the antennae and legs are not shown in these diagrams

The exoskeleton is composed of calcite and calcium phosphate minerals in a lattice of chitin[67] that covers the upper surface (dorsal) of the trilobite and curled round the lower edge to produce a small fringe called the "doublure".Three distinctive tagmata (sections) are present: cephalon (head); thorax (body) and pygidium (tail).

Terminology

As might be expected for a group of animals comprising c. 5,000 genera,[68] the morphology and description of trilobites can be complex. Despite morphological complexity and an unclear position within higher classifications, there are a number of characteristics which distinguish the trilobites from other arthropods: a generally sub-elliptical, dorsal, chitinous exoskeleton divided longitudinally into three distinct lobes (from which the group gets its name); having a distinct, relatively large head shield (cephalon) articulating axially with a thorax comprising articulated transverse segments, the hindmost of which are almost invariably fused to form a tail shield (pygidium). When describing differences between trilobite taxa, the presence, size, and shape of the cephalic features are often mentioned.

During moulting, the exoskeleton generally splits between the head and thorax, which is why so many trilobite fossils are missing one or the other. In most groups facial sutures on the cephalon helped facilitate moulting. Similar to lobsters and crabs, trilobites would have physically "grown" between the moult stage and the hardening of the new exoskeleton.

Cephalon

Morphology of the trilobite cephalon
 
The major subdivisions of the cephalon
 
The subdivisions can be further broken down into different areas used in describing trilobite cephalic morphology. 1 – preocular area; 2 – palpebral area; 3 – postocular area; 4 – posterolateral projection; 5 – occipital ring; 6 – glabella; 7 – posterior area; 8 – lateral border; 9 – librigenal area; 10 – preglabellar area

A trilobite's cephalon, or head section, is highly variable with a lot of morphological complexity. The glabella forms a dome underneath which sat the "crop" or "stomach". Generally the exoskeleton has few distinguishing ventral features, but the cephalon often preserves muscle attachment scars and occasionally the hypostome, a small rigid plate comparable to the ventral plate in other arthropods. A toothless mouth and stomach sat upon the hypostome with the mouth facing backward at the rear edge of the hypostome.

Hypostome morphology is highly variable; sometimes supported by an un-mineralised membrane (natant), sometimes fused onto the anterior doublure with an outline very similar to the glabella above (conterminant) or fused to the anterior doublure with an outline significantly different from the glabella (impendent). Many variations in shape and placement of the hypostome have been described.[60] The size of the glabella and the lateral fringe of the cephalon, together with hypostome variation, have been linked to different lifestyles, diets and specific ecological niches.[9]

The anterior and lateral fringe of the cephalon is greatly enlarged in the Harpetida, in other species a bulge in the pre-glabellar area is preserved that suggests a brood pouch.[69] Highly complex compound eyes are another obvious feature of the cephalon.

Facial sutures

Facial or cephalic sutures are the natural fracture lines in the cephalon of trilobites. Their function was to assist the trilobite in shedding its old exoskeleton during ecdysis (or molting).[70]

All species assigned to the suborder Olenellina, that became extinct at the very end of the Early Cambrian (like Fallotaspis, Nevadia, Judomia, and Olenellus) lacked facial sutures. They are believed to have never developed facial sutures, having pre-dated their evolution. Because of this (along with other primitive characteristics), they are thought to be the earliest ancestors of later trilobites.[71][1]

Some other later trilobites also lost facial sutures secondarily.[71] The type of sutures found in different species are used extensively in the taxonomy and phylogeny of trilobites.[72]

Dorsal sutures

The dorsal surface of the trilobite cephalon (the frontmost tagma, or the 'head') can be divided into two regions—the cranidium and the librigena ("free cheeks"). The cranidium can be further divided into the glabella (the central lobe in the cephalon) and the fixigena ("fixed cheeks").[73] The facial sutures lie along the anterior edge, at the division between the cranidium and the librigena.

Trilobite facial sutures on the dorsal side can be roughly divided into five main types according to where the sutures end relative to the genal angle (the edges where the side and rear margins of the cephalon converge).[74]

  • Absent – Facial sutures are lacking in the Olenellina. This is considered a primitive state, and is always combined with the presence of eyes.
  • Proparian – The facial suture ends in front of the genal angle, along the lateral margin.[73] Example genera showing this type of suture include Dalmanites of Phacopina (Phacopida) and Ekwipagetia of Eodiscina (Agnostida).
  • Gonatoparian – The facial suture ends at the tip of the genal angle.[75] Example genera showing this type of suture include Calymene and Trimerus of Calymenina (Phacopida).[72]
  • Opisthoparian – The facial suture ends at the posterior margin of the cephalon.[72] Example genera showing this type of suture include Peltura of Olenina (Ptychopariida) and Bumastus of Illaenina (Corynexochida). This is the most common type of facial suture.[72]
  • Hypoparian or marginal – In some trilobites, dorsal sutures may be secondarily lost. Several exemplary time series of species show the "migration" of the dorsal suture until it coincides with the margins of the cephalon.[73] As the visual surface of the eye is on the diminishing free cheek (or librigena), the number of lenses tends to go down, and eventually the eye disappears. The loss of dorsal sutures may arise from the proparian state, such as in some Eodiscina like Weymouthia, all Agnostina, and some Phacopina such as Ductina. The marginal sutures exhibited by the harpetids and trinucleioids, are derived from opisthoparian sutures.[76] On the other hand, blindness is not always accompanied by the loss of facial sutures.
 

The primitive state of the dorsal sutures is proparian. Opisthoparian sutures have developed several times independently. There are no examples of proparian sutures developing in taxa with opisthoparian ancestry. Trilobites that exhibit opisthoparian sutures as adults commonly have proparian sutures as instars (known exceptions being Yunnanocephalus and Duyunaspis).[77] Hypoparian sutures have also arisen independently in several groups of trilobites.

The course of the facial sutures from the front of the visual surface varies at least as strongly as it does in the rear, but the lack of a clear reference point similar to the genal angle makes it difficult to categorize. One of the more pronounced states is that the front of the facial sutures do not cut the lateral or frontal border on its own, but coincide in front of the glabella, and cut the frontal border at the midline. This is, inter alia, the case in the Asaphida. Even more pronounced is the situation that the frontal branches of the facial sutures end in each other, resulting in yoked free cheeks. This is known in Triarthrus, and in the Phacopidae, but in that family the facial sutures are not functional, as can be concluded from the fact that free cheeks are not found separated from the cranidium.

There are also two types of sutures in the dorsal surface connected to the compound eyes of trilobites.[72][78] They are:

  • Ocular sutures – are sutures surrounding the edges of the compound eye. Trilobites with these sutures lose the entire surface of the eyes when molting. It is common among Cambrian trilobites.
  • Palpebral sutures – are sutures which form part of the dorsal facial suture running along the top edges of the compound eye.
Ventral sutures
 

Dorsal facial sutures continue downward to the ventral side of the cephalon where they become the Connective sutures that divide the doublure. The following are the types of ventral sutures.[78]

  • Connective sutures – are the sutures that continue from the facial sutures past the front margin of the cephalon.
  • Rostral suture – is only present when the trilobite possesses a rostrum (or rostral plate). It connects the rostrum to the front part of the dorsal cranidium.
  • Hypostomal suture – separates the hypostome from the doublure when the hypostome is of the attached type. It is absent when the hypostome is free-floating (i.e. natant). it is also absent in some coterminant hypostomes where the hypostome is fused to the doublure.
  • Median suture – exhibited by asaphid trilobites, they are formed when instead of becoming connective sutures, the two dorsal sutures converge at a point in front of the cephalon then divide straight down the center of the doublure.

Rostrum

The rostrum (or the rostral plate) is a distinct part of the doublure located at the front of the cephalon. It is separated from the rest of the doublure by the rostral suture.

During molting in trilobites like Paradoxides, the rostrum is used to anchor the front part of the trilobite as the cranidium separates from the librigena. The opening created by the arching of the body provides an exit for the molting trilobite.

It is absent in some trilobites like Lachnostoma.

Hypostome

 
Illustration of the three types of hypostome. Doublure is shown in light gray, the inside surface of the cephalon in dark gray, and the hypostome in light blue. The glabella is outlined in red broken lines.
 
Asaphus expansus ventral side prepared, showing the attachment of the hypostome

The hypostome is the hard mouthpart of the trilobite found on the ventral side of the cephalon typically below the glabella. The hypostome can be classified into three types based on whether they are permanently attached to the rostrum or not and whether they are aligned to the anterior dorsal tip of the glabella.

  • Natant – Hypostome not attached to doublure. Aligned with front edge of glabella.
  • Conterminant – Hypostome attached to rostral plate of doublure. Aligned with front edge of glabella.
  • Impendent – Hypostome attached to rostral plate but not aligned to glabella.

Thorax

The thorax is a series of articulated segments that lie between the cephalon and pygidium. The number of segments varies between 2 and 103[79] with most species in the 2 to 16 range.[63]

Each segment consists of the central axial ring and the outer pleurae, which protected the limbs and gills. The pleurae are sometimes abbreviated or extended to form long spines. Apodemes are bulbous projections on the ventral surface of the exoskeleton to which most leg muscles attached, although some leg muscles attached directly to the exoskeleton.[80] Determining a junction between thorax and pygidium can be difficult and many segment counts suffer from this problem.[63]

Volvation

 
An enrolled phacopid trilobite Flexiclaymene meeki from the Upper Ordovician of Ohio

Trilobite fossils are often found "enrolled" (curled up) like modern pill bugs for protection; evidence suggests enrollment ("volvation") helped protect against the inherent weakness of the arthropod cuticle that was exploited by anomalocarid predators.[25]

Some trilobites achieved a fully closed capsule (e.g. Phacops), while others with long pleural spines (e.g. Selenopeltis) left a gap at the sides or those with a small pygidium (e.g. Paradoxides) left a gap between the cephalon and pygidium.[63] In Phacops, the pleurae overlap a smooth bevel (facet) allowing a close seal with the doublure.[80] The doublure carries a Panderian notch or protuberance on each segment to prevent over rotation and achieve a good seal.[80] Even in an agnostid, with only 2 articulating thoracic segments, the process of enrollment required a complex musculature to contract the exoskeleton and return to the flat condition.[81]

Pygidium

The pygidium is formed from a number of segments and the telson fused together. Segments in the pygidium are similar to the thoracic segments (bearing biramous limbs) but are not articulated. Trilobites can be described based on the pygidium being micropygous (pygidium smaller than cephalon), subisopygous (pygidium sub equal to cephalon), isopygous (pygidium equal in size to cephalon), or macropygous (pygidium larger than cephalon).

Prosopon (surface sculpture)

 
Koneprusia brutoni, an example of a species with elaborate spines from the Devonian Hamar Laghdad Formation, Alnif, Morocco

Trilobite exoskeletons show a variety of small-scale structures collectively called prosopon. Prosopon does not include large scale extensions of the cuticle (e.g. hollow pleural spines) but to finer scale features, such as ribbing, domes, pustules, pitting, ridging and perforations. The exact purpose of the prosopon is not resolved but suggestions include structural strengthening, sensory pits or hairs, preventing predator attacks and maintaining aeration while enrolled.[63] In one example, alimentary ridge networks (easily visible in Cambrian trilobites) might have been either digestive or respiratory tubes in the cephalon and other regions.[21]

Spines

Some trilobites such as those of the order Lichida evolved elaborate spiny forms, from the Ordovician until the end of the Devonian period. Examples of these specimens have been found in the Hamar Laghdad Formation of Alnif in Morocco. There is a serious counterfeiting and fakery problem with much of the Moroccan material that is offered commercially. Spectacular spined trilobites have also been found in western Russia; Oklahoma, USA; and Ontario, Canada.

Some trilobites had horns on their heads similar to several modern beetles. Based on the size, location, and shape of the horns it has been suggested that these horns may have been used to combat for mates. Horns were widespread in the family Raphiophoridae (Asaphida).[82] Another function of these spines was protection from predators. When enrolled, trilobite spines offered additional protection. This conclusion is likely to be applicable to other trilobites as well, such as in the Phacopid trilobite genus Walliserops, that developed spectacular tridents.[83]

Soft body parts

 
Life reconstruction of Triarthrus eatoni based on preserved soft body parts

Only 21 or so species are described from which soft body parts are preserved,[80][84] so some features (e.g. the posterior antenniform cerci preserved only in Olenoides serratus)[85] remain difficult to assess in the wider picture.[86]

Appendages

 
Drawing of a biramous leg of Agnostus pisiformis
 
Ventral side of Triarthrus eatoni, showing antennas and biramous legs preserved in pyrite

Trilobites had a single pair of preoral antennae and otherwise undifferentiated biramous limbs (2, 3 or 4 cephalic pairs, followed by one pair per thoracic segment and some pygidium pairs).[80][84] Each endopodite (walking leg) had 6 or 7 segments,[84] homologous to other early arthropods.[86] Endopodites are attached to the coxa, which also bore a feather-like exopodite, or gill branch, which was used for respiration and, in some species, swimming.[86] A 2021 study found that the upper limb branch of trilobites is a "well-developed gill" that oxygenates the hemolymph, comparable to the book gill in modern horseshoe crab Limulus. In Olenoides, the partially articulated junction with the body is distinct from the exopods of Chelicerata or Crustacea.[87][88] The inside of the coxa (or gnathobase) carries spines, probably to process prey items.[89] The last exopodite segment usually had claws or spines.[80] Many examples of hairs on the legs suggest adaptations for feeding (as for the gnathobases) or sensory organs to help with walking.[86]

Digestive tract

The toothless mouth of trilobites was situated on the rear edge of the hypostome (facing backward), in front of the legs attached to the cephalon. The mouth is linked by a small esophagus to the stomach that lay forward of the mouth, below the glabella. The "intestine" led backward from there to the pygidium.[80] The "feeding limbs" attached to the cephalon are thought to have fed food into the mouth, possibly "slicing" the food on the hypostome and/or gnathobases first. Alternative lifestyles are suggested, with the cephalic legs used to disturb the sediment to make food available. A large glabella, (implying a large stomach), coupled with an impendent hypostome has been used as evidence of more complex food sources, i.e. possibly a carnivorous lifestyle.[9]

Internal organs

While there is direct and implied evidence for the presence and location of the mouth, stomach and digestive tract (see above) the presence of heart, brain and liver are only implied (although "present" in many reconstructions) with little direct geological evidence.[86]

Musculature

Although rarely preserved, long lateral muscles extended from the cephalon to midway down the pygidium, attaching to the axial rings allowing enrollment while separate muscles on the legs tucked them out of the way.[80]

Sensory organs

Many trilobites had complex eyes; they also had a pair of antennae. Some trilobites were blind, probably living too deep in the sea for light to reach them. As such, they became secondarily blind in this branch of trilobite evolution. Other trilobites (e.g., Phacops rana and Erbenochile erbeni) had large eyes that were for use in well lit, predator-filled waters.

Antennae

 
An exceptionally well preserved Olenoides serratus from the Burgess Shale. The antennae and legs are preserved as reflective carbon film.

The pair of antennae suspected in most trilobites (and preserved in a few examples) were highly flexible to allow them to be retracted when the trilobite was enrolled. One species (Olenoides serratus) preserves antenna-like cerci, which project from the rear of the trilobite.[85]

Eyes

 
Unknown Phacops sp. eye

Even the earliest trilobites had complex, compound eyes with lenses made of calcite (a characteristic of all trilobite eyes), confirming that the eyes of arthropods and probably other animals could have developed before the Cambrian.[90] Improving eyesight of both predator and prey in marine environments has been suggested as one of the evolutionary pressures furthering an apparent rapid development of new life forms during what is known as the Cambrian explosion.[91]

Trilobite eyes were typically compound, with each lens being an elongated prism.[92] The number of lenses in such an eye varied: some trilobites had only one, while some had thousands of lenses in a single eye. In compound eyes, the lenses were typically arranged hexagonally.[21] The fossil record of trilobite eyes is complete enough that their evolution can be studied through time, which compensates to some extent for the lack of preservation of soft internal parts.[93]

Lenses of trilobites' eyes were made of calcite (calcium carbonate, CaCO3). Pure forms of calcite are transparent, and some trilobites used crystallographically oriented, clear calcite crystals to form each lens of each eye.[94] Rigid calcite lenses would have been unable to accommodate to a change of focus like the soft lens in a human eye would; in some trilobites, the calcite formed an internal doublet structure,[95] giving superb depth of field and minimal spherical aberration, according to optical principles discovered by French scientist René Descartes and Dutch physicist Christiaan Huygens in the 17th century.[92][95] A living species with similar lenses is the brittle star Ophiocoma wendtii.[96]

In other trilobites, with a Huygens interface apparently missing, a gradient-index lens is invoked with the refractive index of the lens changing toward the center.[97]

Sublensar sensory structures have been found in the eyes of some phacopid trilobites.[98] The structures consist of what appear to be several sensory cells surrounding a rhadomeric structure, resembling closely the sublensar structures found in the eyes of many modern arthropod apposition eyes, especially Limulus, a genus of horseshoe crabs.[98]

  • Holochroal eyes had a great number (sometimes over 15,000) of small (30–100 μm, rarely larger)[93] lenses. Lenses were hexagonally close packed, touching each other, with a single corneal membrane covering all lenses.[94] Each lens was in direct contact with adjacent lenses. Holochroal eyes are the ancestral eye of trilobites, and are by far the most common, found in all orders except the Agnostida, and through the entirety of the Trilobites' existence.[93] Little is known of the early history of holochroal eyes; Lower and Middle Cambrian trilobites rarely preserve the visual surface.[93] The spatial resolving power of grated eyes (such as holochroal eyes) is dependent on light intensity, circular motion, receptor density, registered light angle, and the extent to which the signal of individual rhabdoms are neurally combined. This implies that lenses need to be larger under low light conditions (such as for Pricyclopyge, when comparing it to Carolinites), and for fast moving predators and prey. As the circular velocity caused by the forward speed of an animal itself is much higher for the ommatidia directed perpendicular to the movement, fast-moving trilobites (such as Carolinites) have eyes flattened from the side and more curved were ommatia are directed to the front or back. Thus eye morphology can be used to make assumptions about the ecosystem of trilobites.[99]
     
    The schizochroal eye of Erbenochile erbenii; the eye shade is unequivocal evidence that some trilobites were diurnal[100]
  • Schizochroal eyes typically had fewer (around 700), larger lenses than holochroal eyes and are found only in Phacopina. Each lens had a cornea, and adjacent lenses were separated by thick interlensar cuticle, known as sclera. Schizochroal eyes appear quite suddenly in the early Ordovician, and were presumably derived from a holochroal ancestor.[93] Field of view (all-around vision), eye placement and coincidental development of more efficient enrollment mechanisms point to the eye as a more defensive "early warning" system than directly aiding in the hunt for food.[93] Modern eyes that are functionally equivalent to the schizochroal eye were not thought to exist,[94] but are found in the modern insect species Xenos peckii.[101]
  • Abathochroal eyes are found only in Cambrian Eodiscina, and have around 70 small separate lenses that had individual cornea.[102] The sclera was separate from the cornea, and was not as thick as the sclera in schizochroal eyes.[94] Although well preserved examples are sparse in the early fossil record, abathochroal eyes have been recorded in the lower Cambrian, making them among the oldest known.[94] Environmental conditions seem to have resulted in the later loss of visual organs in many Eodiscina.[94]

Secondary blindness is not uncommon, particularly in long lived groups such as the Agnostida and Trinucleioidea. In Proetida and Phacopina from western Europe and particularly Tropidocoryphinae from France (where there is good stratigraphic control), there are well studied trends showing progressive eye reduction between closely related species that eventually leads to blindness.[94]

Several other structures on trilobites have been explained as photo-receptors.[94] Of particular interest are "macula", the small areas of thinned cuticle on the underside of the hypostome. In some trilobites macula are suggested to function as simple "ventral eyes" that could have detected night and day or allowed a trilobite to navigate while swimming (or turned) upside down.[97]

Sensory pits

 
Pitted fringes on the cephalon of the trinucleoidean Lloydolithus lloydi

There are several types of prosopon that have been suggested as sensory apparatus collecting chemical or vibrational signals. The connection between large pitted fringes on the cephalon of Harpetida and Trinucleoidea with corresponding small or absent eyes makes for an interesting possibility of the fringe as a "compound ear".[94]

Development

 
Five Stages in the development of Sao hirsuta

Trilobites grew through successive moult stages called instars, in which existing segments increased in size and new trunk segments appeared at a sub-terminal generative zone during the anamorphic phase of development. This was followed by the epimorphic developmental phase, in which the animal continued to grow and moult, but no new trunk segments were expressed in the exoskeleton. The combination of anamorphic and epimorphic growth constitutes the hemianamorphic developmental mode that is common among many living arthropods.[103]

Trilobite development was unusual in the way in which articulations developed between segments, and changes in the development of articulation gave rise to the conventionally recognized developmental phases of the trilobite life cycle (divided into 3 stages), which are not readily compared with those of other arthropods. Actual growth and change in external form of the trilobite would have occurred when the trilobite was soft shelled, following moulting and before the next exoskeleton hardened.[104]

 
Elrathia kingii growth series with holaspids ranging from 16.2 mm to 39.8 mm in length

Trilobite larvae are known from the Cambrian to the Carboniferous[105] and from all sub-orders.[104][106] As instars from closely related taxa are more similar than instars from distantly related taxa, trilobite larvae provide morphological information important in evaluating high-level phylogenetic relationships among trilobites.[104]

Despite the absence of supporting fossil evidence, their similarity to living arthropods has led to the belief that trilobites multiplied sexually and produced eggs.[104][107] Some species may have kept eggs or larvae in a brood pouch forward of the glabella,[69] particularly when the ecological niche was challenging to larvae.[10] Size and morphology of the first calcified stage are highly variable between (but not within) trilobite taxa, suggesting some trilobites passed through more growth within the egg than others. Early developmental stages prior to calcification of the exoskeleton are a possibility (suggested for fallotaspids),[15] but so is calcification and hatching coinciding.[104]

The earliest post-embryonic trilobite growth stage known with certainty are the "protaspid" stages (anamorphic phase).[104] Starting with an indistinguishable proto-cephalon and proto-pygidium (anaprotaspid) a number of changes occur ending with a transverse furrow separating the proto-cephalon and proto-pygidium (metaprotaspid) that can continue to add segments. Segments are added at the posterior part of the pygidium, but all segments remain fused together.[104][106]

The "meraspid" stages (anamorphic phase) are marked by the appearance of an articulation between the head and the fused trunk. Prior to the onset of the first meraspid stage the animal had a two-part structure—the head and the plate of fused trunk segments, the pygidium. During the meraspid stages, new segments appeared near the rear of the pygidium as well as additional articulations developing at the front of the pygidium, releasing freely articulating segments into the thorax. Segments are generally added one per moult (although two per moult and one every alternate moult are also recorded), with number of stages equal to the number of thoracic segments. A substantial amount of growth, from less than 25% up to 30%–40%, probably took place in the meraspid stages.[104]

The "holaspid" stages (epimorphic phase) commence when a stable, mature number of segments has been released into the thorax. Moulting continued during the holaspid stages, with no changes in thoracic segment number.[104] Some trilobites are suggested to have continued moulting and growing throughout the life of the individual, albeit at a slower rate on reaching maturity.

Some trilobites showed a marked transition in morphology at one particular instar, which has been called "trilobite metamorphosis". Radical change in morphology is linked to the loss or gain of distinctive features that mark a change in mode of life.[108] A change in lifestyle during development has significance in terms of evolutionary pressure, as the trilobite could pass through several ecological niches on the way to adult development and changes would strongly affect survivorship and dispersal of trilobite taxa.[104] It is worth noting that trilobites with all protaspid stages solely planktonic and later meraspid stages benthic (e.g. asaphids) failed to last through the Ordovician extinctions, while trilobites that were planktonic for only the first protaspid stage before metamorphosing into benthic forms survived (e.g. lichids, phacopids).[108] Pelagic larval life-style proved ill-adapted to the rapid onset of global climatic cooling and loss of tropical shelf habitats during the Ordovician.[26]

History of usage and research

 
Drawing of Ogygiocarella debuchii by Rev. Edward Lhwyd, made in 1698

Rev. Edward Lhwyd published in 1698 in The Philosophical Transactions of the Royal Society, the oldest scientific journal in the English language, part of his letter "Concerning Several Regularly Figured Stones Lately Found by Him", that was accompanied by a page of etchings of fossils.[109] One of his etchings depicted a trilobite he found near Llandeilo, probably on the grounds of Lord Dynefor's castle, he described as "… the skeleton of some flat Fish …".[2]

The discovery of Calymene blumenbachii (the Dudley locust) in 1749 by Charles Lyttleton, could be identified as the beginning of trilobite research. Lyttleton submitted a letter to the Royal Society of London in 1750 concerning a "petrified insect" he found in the "limestone pits at Dudley". In 1754, Manuel Mendez da Costa proclaimed that the Dudley locust was not an insect, but instead belonged to "the crustaceous tribe of animals". He proposed to call the Dudley specimens Pediculus marinus major trilobos (large trilobed marine louse), a name which lasted well into the 1800s. German naturalist Johann Walch, who executed the first inclusive study of this group, proposed the use of the name "trilobite". He considered it appropriate to derive the name from the unique three-lobed character of the central axis and a pleural zone to each side.[110]

Written descriptions of trilobites date possibly from the third century BC and definitely from the fourth century AD. The Spanish geologists Eladio Liñán and Rodolfo Gozalo argue that some of the fossils described in Greek and Latin lapidaries as scorpion stone, beetle stone, and ant stone, refer to trilobite fossils. Less ambiguous references to trilobite fossils can be found in Chinese sources. Fossils from the Kushan formation of northeastern China were prized as inkstones and decorative pieces.[109]

In the New World, American fossil hunters found plentiful deposits of Elrathia kingi in western Utah in the 1860s. Until the early 1900s, the Ute Native Americans of Utah wore these trilobites, which they called pachavee (little water bug), as amulets.[111][112] A hole was bored in the head and the fossil was worn on a string.[111] According to the Ute themselves, trilobite necklaces protect against bullets and diseases such as diphtheria.[111][112] In 1931, Frank Beckwith uncovered evidence of the Ute use of trilobites. Travelling through the badlands, he photographed two petroglyphs that most likely represent trilobites. On the same trip he examined a burial, of unknown age, with a drilled trilobite fossil lying in the chest cavity of the interred. Since then, trilobite amulets have been found all over the Great Basin, as well as in British Columbia and Australia.[109]

In the 1880s, archaeologists discovered in the Grotte du Trilobite (Caves of Arcy-sur-Cure, Yonne, France) a much-handled trilobite fossil that had been drilled as if to be worn as a pendant. The occupation stratum in which the trilobite was found has been dated as 15,000 years old. Because the pendant was handled so much, the species of trilobite cannot be determined. This type of trilobite is not found around Yonne, so it may have been highly prized and traded from elsewhere.[109]

See also

  Media related to Trilobita at Wikimedia Commons   Data related to Trilobita at Wikispecies

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Bibliography

External links

  • Gon III, Sam, A Guide to the Orders of Trilobite. (A site with information covering trilobites from all angles. Includes many line drawings and photographs.)
  • The Virtual Fossil Museum – Class Trilobita – including extensive photographs organized by taxonomy and locality.
  • Western Trilobites Association
  • – a collection of photographs of trilobite fossils
  • Information on fraudulently made trilobites 2013-01-21 at the Wayback Machine
  • The Paleontological Society
  • Chisholm, Hugh, ed. (1911). "Trilobites" . Encyclopædia Britannica (11th ed.). Cambridge University Press.
  • When Trilobites Ruled the Earth
  • American Museum of Natural History Trilobite website
  • Trilobites in Houston Museum of Natural Sciences
  • A nice, copyrighted photo of a trilobite

trilobite, other, uses, disambiguation, confused, with, trilobozoa, meaning, three, lobes, extinct, marine, arthropods, that, form, class, trilobita, form, earliest, known, groups, arthropods, first, appearance, trilobites, fossil, record, defines, base, atdab. For other uses see Trilobite disambiguation Not to be confused with Trilobozoa Trilobites ˈ t r aɪ l e ˌ b aɪ t s ˈ t r ɪ l e 4 5 6 meaning three lobes are extinct marine arthropods that form the class Trilobita Trilobites form one of the earliest known groups of arthropods The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period 521 million years ago and they flourished throughout the lower Paleozoic before slipping into a long decline when during the Devonian all trilobite orders except the Proetida died out The last extant trilobites finally disappeared in the mass extinction at the end of the Permian about 252 million years ago Trilobites were among the most successful of all early animals existing in oceans for almost 270 million years with over 22 000 species having been described TrilobiteTemporal range Early Cambrian 1 2 Late Permian 521 251 9 Ma PreꞒ Ꞓ O S D C P T J K Pg NKainops invius early DevonianParadoxides sp late CambrianScientific classificationKingdom AnimaliaPhylum Arthropoda unranked ArtiopodaSubphylum TrilobitomorphaClass TrilobitaWalch 1771 3 OrdersAgnostida Asaphida Corynexochida Harpetida Redlichiida Lichida Odontopleurida Phacopida Proetida PtychopariidaBy the time trilobites first appeared in the fossil record they were already highly diversified and geographically dispersed Because trilobites had wide diversity and an easily fossilized exoskeleton they left an extensive fossil record The study of their fossils has facilitated important contributions to biostratigraphy paleontology evolutionary biology and plate tectonics Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha equivalent to the Arachnata 7 although several alternative taxonomies are found in the literature More recently they have been placed within the Artiopoda which includes many organisms that are morphologically similar to trilobites but are largely unmineralised Trilobites evolved into many ecological niches some moved over the seabed as predators scavengers or filter feeders and some swam feeding on plankton Some even crawled onto land 8 Most lifestyles expected of modern marine arthropods are seen in trilobites with the possible exception of parasitism where scientific debate continues 9 Some trilobites particularly the family Olenidae are even thought to have evolved a symbiotic relationship with sulfur eating bacteria from which they derived food 10 The largest trilobites were more than 45 centimetres 18 in long and may have weighed as much as 4 5 kilograms 9 9 lb 11 Contents 1 Evolution 1 1 Trilobite relatives 1 1 1 Fossil record of early trilobites 1 2 Divergence and extinction 1 2 1 Evolutionary trends 1 2 2 Pre Cambrian 1 2 3 Cambrian 1 2 4 Ordovician 1 2 5 Silurian and Devonian 1 2 6 Carboniferous and Permian 1 2 7 Final extinction 2 Fossil distribution 3 Importance 4 Taxonomy 4 1 Relationship to other taxa 5 Morphology 5 1 Terminology 5 2 Cephalon 5 2 1 Facial sutures 5 2 1 1 Dorsal sutures 5 2 1 2 Ventral sutures 5 2 2 Rostrum 5 2 3 Hypostome 5 3 Thorax 5 4 Volvation 5 5 Pygidium 5 6 Prosopon surface sculpture 5 7 Spines 6 Soft body parts 6 1 Appendages 6 2 Digestive tract 6 3 Internal organs 6 4 Musculature 7 Sensory organs 7 1 Antennae 7 2 Eyes 7 3 Sensory pits 8 Development 9 History of usage and research 10 See also 11 References 12 Bibliography 13 External linksEvolution EditTrilobite relatives Edit Trilobites belong to the Artiopoda a group of extinct arthropods generally morphologically similar to trilobites but aside from trilobites lacking mineralised exoskeletons Because of their lack of mineralised exoskeletons non trilobite artipodans are typically only found in exceptionally preserved deposits mostly during the Cambrian period The exact relationships of artiopods to other arthropods is uncertain They have been considered closely related to chelicerates which include horseshoe crabs and arachnids as part of a clade called Arachnomorpha while others consider them to be more closely related to Mandibulata which contains insects crustaceans and myriapods as part of a clade called Antennulata 12 Fossil record of early trilobites Edit Redlichiida such as this Paradoxides may represent the ancestral trilobites Meroperix from the Silurian of Wisconsin Walliserops trifurcatus from Jebel Oufatene mountain near Fezzou Morocco The earliest trilobites known from the fossil record are redlichiids and ptychopariid bigotinids dated to around 520 million years ago 1 2 Contenders for the earliest trilobites include Profallotaspis jakutensis Siberia Fritzaspis spp western USA Hupetina antiqua Morocco 13 and Serrania gordaensis Spain 14 All trilobites are thought to have originated in present day Siberia with subsequent distribution and radiation from this location 1 All Olenellina lack facial sutures see below and this is thought to represent the original state The earliest sutured trilobite found so far Lemdadella occurs almost at the same time as the earliest Olenellina suggesting the trilobites origin lies before the start of the Atdabanian but without leaving fossils 1 Other groups show secondary lost facial sutures such as all Agnostina and some Phacopina Another common feature of the Olenellina also suggests this suborder to be the ancestral trilobite stock early protaspid stages have not been found supposedly because these were not calcified and this also is supposed to represent the original state 15 Earlier trilobites may be found and could shed more light on the origin of trilobites 1 16 17 Three specimens of a trilobite from Morocco Megistaspis hammondi dated 478 million years old contain fossilized soft parts 18 19 Divergence and extinction Edit Ogygopsis klotzi from the Mt Stephen Trilobite Beds Middle Cambrian near Field British Columbia Canada Trilobites saw great diversification over time 20 For such a long lasting group of animals it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished and surviving groups diversified to fill ecological niches with comparable or unique adaptations Generally trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn out decline in the Devonian culminating in the final extinction of the last few survivors at the end of the Permian period 21 Evolutionary trends Edit Principal evolutionary trends from primitive morphologies such as exemplified by Eoredlichia 22 include the origin of new types of eyes improvement of enrollment and articulation mechanisms increased size of pygidium micropygy to isopygy and development of extreme spinosity in certain groups 21 Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments 22 Specific changes to the cephalon are also noted variable glabella size and shape position of eyes and facial sutures and hypostome specialization 22 Several morphologies appeared independently within different major taxa e g eye reduction or miniaturization 22 Effacement the loss of surface detail in the cephalon pygidium or the thoracic furrows is also a common evolutionary trend Notable examples of this were the orders Agnostida and Asaphida and the suborder Illaenina of the Corynexochida Effacement is believed to be an indication of either a burrowing lifestyle or a pelagic one Effacement poses a problem for taxonomists since the loss of details particularly of the glabella can make the determination of phylogenetic relationships difficult 23 Pre Cambrian Edit Phylogenetic biogeographic analysis of Early Cambrian Olenellidae and Redlichiidae suggests that a uniform trilobite fauna existed over Laurentia Gondwana and Siberia before the tectonic breakup of the supercontinent Pannotia between 600 and 550 million years ago 1 Tectonic breakup of Pannotia then allowed for the diversification and radiation expressed later in the Cambrian as the distinctive Olenellid province Laurentia Siberia and Baltica and the separate Redlichid province Australia Antarctica and China 1 24 Breakup of Pannotia significantly antedates the first appearance of trilobites in the fossil record supporting a long and cryptic development of trilobites extending perhaps as far back as 700 million years ago or possibly further back in time 24 Cambrian Edit Very shortly after trilobite fossils appeared in the lower Cambrian they rapidly diversified into the major orders that typified the Cambrian Redlichiida Ptychopariida Agnostida and Corynexochida The first major crisis in the trilobite fossil record occurred in the Middle Cambrian surviving orders developed isopygius or macropygius bodies and developed thicker cuticles allowing better defense against predators see Thorax below 25 The end Cambrian mass extinction event marked a major change in trilobite fauna almost all Redlichiida including the Olenelloidea and most Late Cambrian stocks became extinct 21 A continuing decrease in Laurentian continental shelf area 26 is recorded at the same time as the extinctions suggesting major environmental upheaval Notable trilobite genera appearing in the Cambrian include 27 Abadiella Lower Cambrian Buenellus Lower Cambrian Judomia Lower Cambrian Olenellus Lower Cambrian Ellipsocephalus Middle Cambrian Elrathia Middle Cambrian Paradoxides Middle Cambrian Peronopsis Middle Cambrian Xiuqiella Middle Cambrian Yiliangella Middle Cambrian Yiliangellina Middle Cambrian Olenus Late Cambrian Ordovician Edit Cast of Isotelus rex the largest known trilobite from the middle to upper Ordovician of North America Cheirurus sp middle Ordovician age Volkhov River Russia The Early Ordovician is marked by vigorous radiations of articulate brachiopods bryozoans bivalves echinoderms and graptolites with many groups appearing in the fossil record for the first time 21 Although intra species trilobite diversity seems to have peaked during the Cambrian 28 trilobites were still active participants in the Ordovician radiation event with a new fauna taking over from the old Cambrian one 29 Phacopida and Trinucleioidea are characteristic forms highly differentiated and diverse most with uncertain ancestors 21 The Phacopida and other new clades almost certainly had Cambrian forebearers but the fact that they have avoided detection is a strong indication that novel morphologies were developing very rapidly 15 Changes within the trilobite fauna during the Ordovician foreshadowed the mass extinction at the end of the Ordovician allowing many families to continue into the Silurian with little disturbance 29 Ordovician trilobites were successful at exploiting new environments notably reefs The Ordovician mass extinction did not leave the trilobites unscathed some distinctive and previously successful forms such as the Telephinidae and Agnostida became extinct The Ordovician marks the last great diversification period amongst the trilobites very few entirely new patterns of organisation arose post Ordovician Later evolution in trilobites was largely a matter of variations upon the Ordovician themes By the Ordovician mass extinction vigorous trilobite radiation has stopped and gradual decline is foreshadowed 21 Some of the genera of Trilobites appearing in the Ordovician include 27 Cyclopyge Early to Late Ordovician Selenopeltis Early to Late Ordovician Parabolina Early Ordovician Cheirurus Middle Ordovician Eodalmanitina Middle Ordovician Trinucleus Middle Ordovician Triarthrus Late Ordovician Silurian and Devonian Edit Number of families Middle Devonian to Upper Permian Proetida brown Phacopida steel blue Lichida clear blue Harpetida pink Odontopleurida olive and Corynexochida purpleMost Early Silurian families constitute a subgroup of the Late Ordovician fauna Few if any of the dominant Early Ordovician fauna survived to the end of the Ordovician yet 74 of the dominant Late Ordovician trilobite fauna survived the Ordovician Late Ordovician survivors account for all post Ordovician trilobite groups except the Harpetida 29 Silurian and Devonian trilobite assemblages are superficially similar to Ordovician assemblages dominated by Lichida and Phacopida including the well known Calymenina A number of characteristic forms do not extend far into the Devonian and almost all the remainder were wiped out by a series of dramatic Middle and Late Devonian extinctions 22 Three orders and all but five families were exterminated by the combination of sea level changes and a break in the redox equilibrium a meteorite impact has also been suggested as a cause 22 Only a single order the Proetida survived into the Carboniferous 21 Genera of trilobites during the Silurian and Devonian periods include 27 Dalmanites Early to Late Silurian Calymene Silurian Encrinurus Silurian Exallaspis Middle to Late Silurian Paralejurus Early Devonian Lioharpes Middle Devonian Phacops Middle to Late Devonian Carboniferous and Permian Edit The Proetida survived for millions of years continued through the Carboniferous period and lasted until the end of the Permian when the vast majority of species on Earth were wiped out 21 It is unknown why the order Proetida alone survived the Devonian The Proetida maintained relatively diverse faunas in both deep and shallow water shelf environments throughout the Carboniferous 22 For many millions of years the Proetida existed untroubled in their ecological niche 21 An analogy would be today s crinoids which mostly exist as deep water species in the Paleozoic era vast forests of crinoids lived in shallow near shore environments 21 Some of the genera of trilobites during the Carboniferous and Permian periods include 27 Archegonus Early to Middle Carboniferous Hesslerides Middle Carboniferous Endops Middle Permian Triproetus Late Carboniferous to Early Permian Ditomopyge Late Carboniferous to Late Permian Final extinction Edit Exactly why the trilobites became extinct is not clear with repeated extinction events often followed by apparent recovery throughout the trilobite fossil record a combination of causes is likely After the extinction event at the end of the Devonian period what trilobite diversity remained was bottlenecked into the order Proetida Decreasing diversity 30 of genera limited to shallow water shelf habitats coupled with a drastic lowering of sea level regression meant that the final decline of trilobites happened shortly before the end of the Permian mass extinction event 22 With so many marine species involved in the Permian extinction the end of nearly 300 million successful years for the trilobites would not have been unexpected at the time 30 Trilobites have no known direct descendants Their closest living relatives would be the chelicerates due to both being arachnomorphs Though horseshoe crabs are often cited as their closest living relatives they are no closer evolutionarily than other chelicerates 31 Fossil distribution Edit Cruziana fossil trilobite burrowing trace A trilobite fragment T in a thin section of an Ordovician limestone E echinoderm scale bar is 2 mm Rusophycus a resting trace of a trilobite Ordovician of southern Ohio Scale bar is 10 mm Plate from Barrande s work Systeme silurien du centre de la Boheme Trilobites appear to have been primarily marine organisms since the fossilized remains of trilobites are always found in rocks containing fossils of other salt water animals such as brachiopods crinoids and corals Some trackways suggest trilobites made at least temporary excursions unto land 8 Within the marine paleoenvironment trilobites were found in a broad range from extremely shallow water to very deep water Trilobites like brachiopods crinoids and corals are found on all modern continents and occupied every ancient ocean from which Paleozoic fossils have been collected 32 The remnants of trilobites can range from the preserved body to pieces of the exoskeleton which it shed in the process known as ecdysis In addition the tracks left behind by trilobites living on the sea floor are often preserved as trace fossils There are three main forms of trace fossils associated with trilobites Rusophycus Cruziana and Diplichnites such trace fossils represent the preserved life activity of trilobites active upon the sea floor Rusophycus the resting trace are trilobite excavations involving little or no forward movement and ethological interpretations suggest resting protection and hunting 33 Cruziana the feeding trace are furrows through the sediment which are believed to represent the movement of trilobites while deposit feeding 34 Many of the Diplichnites fossils are believed to be traces made by trilobites walking on the sediment surface 34 Care must be taken as similar trace fossils are recorded in freshwater 35 and post Paleozoic deposits 36 representing non trilobite origins Trilobite fossils are found worldwide with thousands of known species Because they appeared quickly in geological time and moulted like other arthropods trilobites serve as excellent index fossils enabling geologists to date the age of the rocks in which they are found They were among the first fossils to attract widespread attention and new species are being discovered every year Fossil hunters look for trilobites and other fossils in Penn Dixie Fossil Park and Nature Preserve In the United States the best open to the public collection of trilobites is located in Hamburg New York The shale quarry informally known as Penn Dixie stopped mining in the 1960s The large amounts of trilobites were discovered in the 1970s by Dan Cooper 37 As a well known rock collector he incited scientific and public interest in the location 38 The fossils are dated to the Givetian 387 2 382 7 million years ago when the Western New York Region was 30 degrees south of the equator and completely covered in water 39 The site was purchased from Vincent C Bonerb by the Town of Hamburg with the cooperation of the Hamburg Natural History Society to protect the land from development 37 In 1994 the quarry became Penn Dixie Fossil Park amp Nature Reserve when they received 501 c 3 status and was opened for visitation and collection of trilobite samples The two most common found samples are Eldredgeops ranaandGreenops 40 A famous location for trilobite fossils in the United Kingdom is Wren s Nest Dudley in the West Midlands where Calymene blumenbachii is found in the Silurian Wenlock Group This trilobite is featured on the town s coat of arms and was named the Dudley Bug or Dudley Locust by quarrymen who once worked the now abandoned limestone quarries Llandrindod Wells Powys Wales is another famous trilobite location The well known Elrathia kingi trilobite is found in abundance in the Cambrian Wheeler Shale of Utah 41 Spectacularly preserved trilobite fossils often showing soft body parts legs gills antennae etc have been found in British Columbia Canada the Cambrian Burgess Shale and similar localities New York U S A Ordovician Walcott Rust quarry near Russia and Beecher s Trilobite Bed near Rome China Lower Cambrian Maotianshan Shales near Chengjiang Germany the Devonian Hunsruck Slates near Bundenbach and much more rarely in trilobite bearing strata in Utah Wheeler Shale and other formations Ontario and Manuels River Newfoundland and Labrador Sites in Morocco also yield very well preserved trilobites many buried in mudslides alive and so perfectly preserved An industry has developed around their recovery leading to controversies about practices in restoral 42 The variety of eye and upper body forms and fragile protuberances is best known from these samples preserved similarly to bodies in Pompeii The French palaeontologist Joachim Barrande 1799 1883 carried out his landmark study of trilobites in the Cambrian Ordovician and Silurian of Bohemia publishing the first volume of Systeme silurien du centre de la Boheme in 1852 Importance EditThe study of Paleozoic trilobites in the Welsh English borders by Niles Eldredge was fundamental in formulating and testing punctuated equilibrium as a mechanism of evolution 43 44 45 Identification of the Atlantic and Pacific trilobite faunas in North America and Europe 46 implied the closure of the Iapetus Ocean producing the Iapetus suture 47 thus providing important supporting evidence for the theory of continental drift 48 49 Trilobites have been important in estimating the rate of speciation during the period known as the Cambrian explosion because they are the most diverse group of metazoans known from the fossil record of the early Cambrian 50 51 Trilobites are excellent stratigraphic markers of the Cambrian period researchers who find trilobites with alimentary prosopon and a micropygium have found Early Cambrian strata 52 Most of the Cambrian stratigraphy is based on the use of trilobite marker fossils 53 54 55 Trilobites are the state fossils of Ohio Isotelus Wisconsin Calymene celebra and Pennsylvania Phacops rana Taxonomy EditSee also List of trilobite genera The 10 most commonly recognized trilobite orders are Agnostida Redlichiida Corynexochida Lichida Odontopleurida Phacopida Proetida Asaphida Harpetida and Ptychopariida In 2020 an 11th order Trinucleida was proposed to be elevated out of the asaphid superfamily Trinucleioidea 56 Sometimes the Nektaspida are considered trilobites but these lack a calcified exoskeleton and eyes Some scholars have proposed that the order Agnostida is polyphyletic with the suborder Agnostina representing non trilobite arthropods unrelated to the suborder Eodiscina Under this hypothesis Eodiscina would be elevated to a new order Eodiscida Over 20 000 species of trilobite have been described 57 Despite their rich fossil record with thousands of described genera found throughout the world the taxonomy and phylogeny of trilobites have many uncertainties 58 Except possibly for the members of the orders Phacopida and Lichida which first appear during the early Ordovician nine of the eleven trilobite orders appear prior to the end of the Cambrian Most scientists believe that order Redlichiida more specifically its suborder Redlichiina contains a common ancestor of all other orders with the possible exception of the Agnostina While many potential phylogenies are found in the literature most have suborder Redlichiina giving rise to orders Corynexochida and Ptychopariida during the Lower Cambrian and the Lichida descending from either the Redlichiida or Corynexochida in the Middle Cambrian Order Ptychopariida is the most problematic order for trilobite classification In the 1959 Treatise on Invertebrate Paleontology 59 what are now members of orders Ptychopariida Asaphida Proetida and Harpetida were grouped together as order Ptychopariida subclass Librostoma was erected in 1990 60 to encompass all of these orders based on their shared ancestral character of a natant unattached hypostome The most recently recognized of the nine trilobite orders Harpetida was erected in 2002 61 The progenitor of order Phacopida is unclear Relationship to other taxa Edit Once soft part anatomy had been recovered the trilobites were originally allied to the Crustacea A relationship with the Chelicerata in a clade termed Arachnomorpha Arachnata was in vogue for some time 62 but a more recent analysis of Panarthropoda suggests the trilobites are one of two major branches of Artiopoda 31 Morphology EditWhen trilobites are found only the exoskeleton is preserved often in an incomplete state in all but a handful of locations A few locations Lagerstatten preserve identifiable soft body parts legs gills musculature amp digestive tract and enigmatic traces of other structures e g fine details of eye structure as well as the exoskeleton Trilobites range in length from minute less than 3 millimetres 0 12 in to very large over 30 centimetres 12 in with an average size range of 3 10 cm 1 2 3 9 in Supposedly the smallest species is Acanthopleurella stipulae with a maximum of 1 5 millimetres 0 059 in 63 The world s largest known trilobite specimen assigned to Isotelus rex is 72 cm 28 in in length It was found in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay 26 However a partial specimen of the Ordovician trilobite Hungioides bohemicus found in 2009 in Arouca Portugal is estimated to have measured when complete 86 5 cm 34 1 in in length 64 65 66 The trilobite body is divided into three major sections tagmata 1 cephalon 2 thorax 3 pygidium Trilobites are so named for the three longitudinal lobes 4 right pleural lobe 5 axial lobe 6 left pleural lobe the antennae and legs are not shown in these diagrams The exoskeleton is composed of calcite and calcium phosphate minerals in a lattice of chitin 67 that covers the upper surface dorsal of the trilobite and curled round the lower edge to produce a small fringe called the doublure Three distinctive tagmata sections are present cephalon head thorax body and pygidium tail Terminology Edit As might be expected for a group of animals comprising c 5 000 genera 68 the morphology and description of trilobites can be complex Despite morphological complexity and an unclear position within higher classifications there are a number of characteristics which distinguish the trilobites from other arthropods a generally sub elliptical dorsal chitinous exoskeleton divided longitudinally into three distinct lobes from which the group gets its name having a distinct relatively large head shield cephalon articulating axially with a thorax comprising articulated transverse segments the hindmost of which are almost invariably fused to form a tail shield pygidium When describing differences between trilobite taxa the presence size and shape of the cephalic features are often mentioned During moulting the exoskeleton generally splits between the head and thorax which is why so many trilobite fossils are missing one or the other In most groups facial sutures on the cephalon helped facilitate moulting Similar to lobsters and crabs trilobites would have physically grown between the moult stage and the hardening of the new exoskeleton Cephalon Edit See also Cephalon arthropod anatomy Morphology of the trilobite cephalon The major subdivisions of the cephalon The subdivisions can be further broken down into different areas used in describing trilobite cephalic morphology 1 preocular area 2 palpebral area 3 postocular area 4 posterolateral projection 5 occipital ring 6 glabella 7 posterior area 8 lateral border 9 librigenal area 10 preglabellar area A trilobite s cephalon or head section is highly variable with a lot of morphological complexity The glabella forms a dome underneath which sat the crop or stomach Generally the exoskeleton has few distinguishing ventral features but the cephalon often preserves muscle attachment scars and occasionally the hypostome a small rigid plate comparable to the ventral plate in other arthropods A toothless mouth and stomach sat upon the hypostome with the mouth facing backward at the rear edge of the hypostome Hypostome morphology is highly variable sometimes supported by an un mineralised membrane natant sometimes fused onto the anterior doublure with an outline very similar to the glabella above conterminant or fused to the anterior doublure with an outline significantly different from the glabella impendent Many variations in shape and placement of the hypostome have been described 60 The size of the glabella and the lateral fringe of the cephalon together with hypostome variation have been linked to different lifestyles diets and specific ecological niches 9 The anterior and lateral fringe of the cephalon is greatly enlarged in the Harpetida in other species a bulge in the pre glabellar area is preserved that suggests a brood pouch 69 Highly complex compound eyes are another obvious feature of the cephalon Facial sutures Edit Facial or cephalic sutures are the natural fracture lines in the cephalon of trilobites Their function was to assist the trilobite in shedding its old exoskeleton during ecdysis or molting 70 All species assigned to the suborder Olenellina that became extinct at the very end of the Early Cambrian like Fallotaspis Nevadia Judomia and Olenellus lacked facial sutures They are believed to have never developed facial sutures having pre dated their evolution Because of this along with other primitive characteristics they are thought to be the earliest ancestors of later trilobites 71 1 Some other later trilobites also lost facial sutures secondarily 71 The type of sutures found in different species are used extensively in the taxonomy and phylogeny of trilobites 72 Dorsal sutures Edit The dorsal surface of the trilobite cephalon the frontmost tagma or the head can be divided into two regions the cranidium and the librigena free cheeks The cranidium can be further divided into the glabella the central lobe in the cephalon and the fixigena fixed cheeks 73 The facial sutures lie along the anterior edge at the division between the cranidium and the librigena Trilobite facial sutures on the dorsal side can be roughly divided into five main types according to where the sutures end relative to the genal angle the edges where the side and rear margins of the cephalon converge 74 Absent Facial sutures are lacking in the Olenellina This is considered a primitive state and is always combined with the presence of eyes Proparian The facial suture ends in front of the genal angle along the lateral margin 73 Example genera showing this type of suture include Dalmanites of Phacopina Phacopida and Ekwipagetia of Eodiscina Agnostida Gonatoparian The facial suture ends at the tip of the genal angle 75 Example genera showing this type of suture include Calymene and Trimerus of Calymenina Phacopida 72 Opisthoparian The facial suture ends at the posterior margin of the cephalon 72 Example genera showing this type of suture include Peltura of Olenina Ptychopariida and Bumastus of Illaenina Corynexochida This is the most common type of facial suture 72 Hypoparian or marginal In some trilobites dorsal sutures may be secondarily lost Several exemplary time series of species show the migration of the dorsal suture until it coincides with the margins of the cephalon 73 As the visual surface of the eye is on the diminishing free cheek or librigena the number of lenses tends to go down and eventually the eye disappears The loss of dorsal sutures may arise from the proparian state such as in some Eodiscina like Weymouthia all Agnostina and some Phacopina such as Ductina The marginal sutures exhibited by the harpetids and trinucleioids are derived from opisthoparian sutures 76 On the other hand blindness is not always accompanied by the loss of facial sutures The primitive state of the dorsal sutures is proparian Opisthoparian sutures have developed several times independently There are no examples of proparian sutures developing in taxa with opisthoparian ancestry Trilobites that exhibit opisthoparian sutures as adults commonly have proparian sutures as instars known exceptions being Yunnanocephalus and Duyunaspis 77 Hypoparian sutures have also arisen independently in several groups of trilobites The course of the facial sutures from the front of the visual surface varies at least as strongly as it does in the rear but the lack of a clear reference point similar to the genal angle makes it difficult to categorize One of the more pronounced states is that the front of the facial sutures do not cut the lateral or frontal border on its own but coincide in front of the glabella and cut the frontal border at the midline This is inter alia the case in the Asaphida Even more pronounced is the situation that the frontal branches of the facial sutures end in each other resulting in yoked free cheeks This is known in Triarthrus and in the Phacopidae but in that family the facial sutures are not functional as can be concluded from the fact that free cheeks are not found separated from the cranidium There are also two types of sutures in the dorsal surface connected to the compound eyes of trilobites 72 78 They are Ocular sutures are sutures surrounding the edges of the compound eye Trilobites with these sutures lose the entire surface of the eyes when molting It is common among Cambrian trilobites Palpebral sutures are sutures which form part of the dorsal facial suture running along the top edges of the compound eye Ventral sutures Edit Dorsal facial sutures continue downward to the ventral side of the cephalon where they become the Connective sutures that divide the doublure The following are the types of ventral sutures 78 Connective sutures are the sutures that continue from the facial sutures past the front margin of the cephalon Rostral suture is only present when the trilobite possesses a rostrum or rostral plate It connects the rostrum to the front part of the dorsal cranidium Hypostomal suture separates the hypostome from the doublure when the hypostome is of the attached type It is absent when the hypostome is free floating i e natant it is also absent in some coterminant hypostomes where the hypostome is fused to the doublure Median suture exhibited by asaphid trilobites they are formed when instead of becoming connective sutures the two dorsal sutures converge at a point in front of the cephalon then divide straight down the center of the doublure Rostrum Edit The rostrum or the rostral plate is a distinct part of the doublure located at the front of the cephalon It is separated from the rest of the doublure by the rostral suture During molting in trilobites like Paradoxides the rostrum is used to anchor the front part of the trilobite as the cranidium separates from the librigena The opening created by the arching of the body provides an exit for the molting trilobite It is absent in some trilobites like Lachnostoma Hypostome Edit Further information Hypostome trilobite Illustration of the three types of hypostome Doublure is shown in light gray the inside surface of the cephalon in dark gray and the hypostome in light blue The glabella is outlined in red broken lines Asaphus expansus ventral side prepared showing the attachment of the hypostomeThe hypostome is the hard mouthpart of the trilobite found on the ventral side of the cephalon typically below the glabella The hypostome can be classified into three types based on whether they are permanently attached to the rostrum or not and whether they are aligned to the anterior dorsal tip of the glabella Natant Hypostome not attached to doublure Aligned with front edge of glabella Conterminant Hypostome attached to rostral plate of doublure Aligned with front edge of glabella Impendent Hypostome attached to rostral plate but not aligned to glabella Thorax Edit The thorax is a series of articulated segments that lie between the cephalon and pygidium The number of segments varies between 2 and 103 79 with most species in the 2 to 16 range 63 Each segment consists of the central axial ring and the outer pleurae which protected the limbs and gills The pleurae are sometimes abbreviated or extended to form long spines Apodemes are bulbous projections on the ventral surface of the exoskeleton to which most leg muscles attached although some leg muscles attached directly to the exoskeleton 80 Determining a junction between thorax and pygidium can be difficult and many segment counts suffer from this problem 63 Volvation Edit An enrolled phacopid trilobite Flexiclaymene meeki from the Upper Ordovician of Ohio Trilobite fossils are often found enrolled curled up like modern pill bugs for protection evidence suggests enrollment volvation helped protect against the inherent weakness of the arthropod cuticle that was exploited by anomalocarid predators 25 Some trilobites achieved a fully closed capsule e g Phacops while others with long pleural spines e g Selenopeltis left a gap at the sides or those with a small pygidium e g Paradoxides left a gap between the cephalon and pygidium 63 In Phacops the pleurae overlap a smooth bevel facet allowing a close seal with the doublure 80 The doublure carries a Panderian notch or protuberance on each segment to prevent over rotation and achieve a good seal 80 Even in an agnostid with only 2 articulating thoracic segments the process of enrollment required a complex musculature to contract the exoskeleton and return to the flat condition 81 Pygidium Edit The pygidium is formed from a number of segments and the telson fused together Segments in the pygidium are similar to the thoracic segments bearing biramous limbs but are not articulated Trilobites can be described based on the pygidium being micropygous pygidium smaller than cephalon subisopygous pygidium sub equal to cephalon isopygous pygidium equal in size to cephalon or macropygous pygidium larger than cephalon Prosopon surface sculpture Edit Koneprusia brutoni an example of a species with elaborate spines from the Devonian Hamar Laghdad Formation Alnif Morocco Trilobite exoskeletons show a variety of small scale structures collectively called prosopon Prosopon does not include large scale extensions of the cuticle e g hollow pleural spines but to finer scale features such as ribbing domes pustules pitting ridging and perforations The exact purpose of the prosopon is not resolved but suggestions include structural strengthening sensory pits or hairs preventing predator attacks and maintaining aeration while enrolled 63 In one example alimentary ridge networks easily visible in Cambrian trilobites might have been either digestive or respiratory tubes in the cephalon and other regions 21 Spines Edit Some trilobites such as those of the order Lichida evolved elaborate spiny forms from the Ordovician until the end of the Devonian period Examples of these specimens have been found in the Hamar Laghdad Formation of Alnif in Morocco There is a serious counterfeiting and fakery problem with much of the Moroccan material that is offered commercially Spectacular spined trilobites have also been found in western Russia Oklahoma USA and Ontario Canada Some trilobites had horns on their heads similar to several modern beetles Based on the size location and shape of the horns it has been suggested that these horns may have been used to combat for mates Horns were widespread in the family Raphiophoridae Asaphida 82 Another function of these spines was protection from predators When enrolled trilobite spines offered additional protection This conclusion is likely to be applicable to other trilobites as well such as in the Phacopid trilobite genus Walliserops that developed spectacular tridents 83 Soft body parts Edit Life reconstruction of Triarthrus eatoni based on preserved soft body partsOnly 21 or so species are described from which soft body parts are preserved 80 84 so some features e g the posterior antenniform cerci preserved only in Olenoides serratus 85 remain difficult to assess in the wider picture 86 Appendages Edit Drawing of a biramous leg of Agnostus pisiformis Ventral side of Triarthrus eatoni showing antennas and biramous legs preserved in pyrite Trilobites had a single pair of preoral antennae and otherwise undifferentiated biramous limbs 2 3 or 4 cephalic pairs followed by one pair per thoracic segment and some pygidium pairs 80 84 Each endopodite walking leg had 6 or 7 segments 84 homologous to other early arthropods 86 Endopodites are attached to the coxa which also bore a feather like exopodite or gill branch which was used for respiration and in some species swimming 86 A 2021 study found that the upper limb branch of trilobites is a well developed gill that oxygenates the hemolymph comparable to the book gill in modern horseshoe crab Limulus In Olenoides the partially articulated junction with the body is distinct from the exopods of Chelicerata or Crustacea 87 88 The inside of the coxa or gnathobase carries spines probably to process prey items 89 The last exopodite segment usually had claws or spines 80 Many examples of hairs on the legs suggest adaptations for feeding as for the gnathobases or sensory organs to help with walking 86 Digestive tract Edit The toothless mouth of trilobites was situated on the rear edge of the hypostome facing backward in front of the legs attached to the cephalon The mouth is linked by a small esophagus to the stomach that lay forward of the mouth below the glabella The intestine led backward from there to the pygidium 80 The feeding limbs attached to the cephalon are thought to have fed food into the mouth possibly slicing the food on the hypostome and or gnathobases first Alternative lifestyles are suggested with the cephalic legs used to disturb the sediment to make food available A large glabella implying a large stomach coupled with an impendent hypostome has been used as evidence of more complex food sources i e possibly a carnivorous lifestyle 9 Internal organs Edit While there is direct and implied evidence for the presence and location of the mouth stomach and digestive tract see above the presence of heart brain and liver are only implied although present in many reconstructions with little direct geological evidence 86 Musculature Edit Although rarely preserved long lateral muscles extended from the cephalon to midway down the pygidium attaching to the axial rings allowing enrollment while separate muscles on the legs tucked them out of the way 80 Sensory organs EditMany trilobites had complex eyes they also had a pair of antennae Some trilobites were blind probably living too deep in the sea for light to reach them As such they became secondarily blind in this branch of trilobite evolution Other trilobites e g Phacops rana and Erbenochile erbeni had large eyes that were for use in well lit predator filled waters Antennae Edit An exceptionally well preserved Olenoides serratus from the Burgess Shale The antennae and legs are preserved as reflective carbon film The pair of antennae suspected in most trilobites and preserved in a few examples were highly flexible to allow them to be retracted when the trilobite was enrolled One species Olenoides serratus preserves antenna like cerci which project from the rear of the trilobite 85 Eyes Edit Unknown Phacops sp eye Even the earliest trilobites had complex compound eyes with lenses made of calcite a characteristic of all trilobite eyes confirming that the eyes of arthropods and probably other animals could have developed before the Cambrian 90 Improving eyesight of both predator and prey in marine environments has been suggested as one of the evolutionary pressures furthering an apparent rapid development of new life forms during what is known as the Cambrian explosion 91 Trilobite eyes were typically compound with each lens being an elongated prism 92 The number of lenses in such an eye varied some trilobites had only one while some had thousands of lenses in a single eye In compound eyes the lenses were typically arranged hexagonally 21 The fossil record of trilobite eyes is complete enough that their evolution can be studied through time which compensates to some extent for the lack of preservation of soft internal parts 93 Lenses of trilobites eyes were made of calcite calcium carbonate CaCO3 Pure forms of calcite are transparent and some trilobites used crystallographically oriented clear calcite crystals to form each lens of each eye 94 Rigid calcite lenses would have been unable to accommodate to a change of focus like the soft lens in a human eye would in some trilobites the calcite formed an internal doublet structure 95 giving superb depth of field and minimal spherical aberration according to optical principles discovered by French scientist Rene Descartes and Dutch physicist Christiaan Huygens in the 17th century 92 95 A living species with similar lenses is the brittle star Ophiocoma wendtii 96 In other trilobites with a Huygens interface apparently missing a gradient index lens is invoked with the refractive index of the lens changing toward the center 97 Sublensar sensory structures have been found in the eyes of some phacopid trilobites 98 The structures consist of what appear to be several sensory cells surrounding a rhadomeric structure resembling closely the sublensar structures found in the eyes of many modern arthropod apposition eyes especially Limulus a genus of horseshoe crabs 98 Holochroal eyes had a great number sometimes over 15 000 of small 30 100 mm rarely larger 93 lenses Lenses were hexagonally close packed touching each other with a single corneal membrane covering all lenses 94 Each lens was in direct contact with adjacent lenses Holochroal eyes are the ancestral eye of trilobites and are by far the most common found in all orders except the Agnostida and through the entirety of the Trilobites existence 93 Little is known of the early history of holochroal eyes Lower and Middle Cambrian trilobites rarely preserve the visual surface 93 The spatial resolving power of grated eyes such as holochroal eyes is dependent on light intensity circular motion receptor density registered light angle and the extent to which the signal of individual rhabdoms are neurally combined This implies that lenses need to be larger under low light conditions such as for Pricyclopyge when comparing it to Carolinites and for fast moving predators and prey As the circular velocity caused by the forward speed of an animal itself is much higher for the ommatidia directed perpendicular to the movement fast moving trilobites such as Carolinites have eyes flattened from the side and more curved were ommatia are directed to the front or back Thus eye morphology can be used to make assumptions about the ecosystem of trilobites 99 The schizochroal eye of Erbenochile erbenii the eye shade is unequivocal evidence that some trilobites were diurnal 100 Schizochroal eyes typically had fewer around 700 larger lenses than holochroal eyes and are found only in Phacopina Each lens had a cornea and adjacent lenses were separated by thick interlensar cuticle known as sclera Schizochroal eyes appear quite suddenly in the early Ordovician and were presumably derived from a holochroal ancestor 93 Field of view all around vision eye placement and coincidental development of more efficient enrollment mechanisms point to the eye as a more defensive early warning system than directly aiding in the hunt for food 93 Modern eyes that are functionally equivalent to the schizochroal eye were not thought to exist 94 but are found in the modern insect species Xenos peckii 101 Abathochroal eyes are found only in Cambrian Eodiscina and have around 70 small separate lenses that had individual cornea 102 The sclera was separate from the cornea and was not as thick as the sclera in schizochroal eyes 94 Although well preserved examples are sparse in the early fossil record abathochroal eyes have been recorded in the lower Cambrian making them among the oldest known 94 Environmental conditions seem to have resulted in the later loss of visual organs in many Eodiscina 94 Secondary blindness is not uncommon particularly in long lived groups such as the Agnostida and Trinucleioidea In Proetida and Phacopina from western Europe and particularly Tropidocoryphinae from France where there is good stratigraphic control there are well studied trends showing progressive eye reduction between closely related species that eventually leads to blindness 94 Several other structures on trilobites have been explained as photo receptors 94 Of particular interest are macula the small areas of thinned cuticle on the underside of the hypostome In some trilobites macula are suggested to function as simple ventral eyes that could have detected night and day or allowed a trilobite to navigate while swimming or turned upside down 97 Sensory pits Edit Pitted fringes on the cephalon of the trinucleoidean Lloydolithus lloydiThere are several types of prosopon that have been suggested as sensory apparatus collecting chemical or vibrational signals The connection between large pitted fringes on the cephalon of Harpetida and Trinucleoidea with corresponding small or absent eyes makes for an interesting possibility of the fringe as a compound ear 94 Development Edit Five Stages in the development of Sao hirsuta Trilobites grew through successive moult stages called instars in which existing segments increased in size and new trunk segments appeared at a sub terminal generative zone during the anamorphic phase of development This was followed by the epimorphic developmental phase in which the animal continued to grow and moult but no new trunk segments were expressed in the exoskeleton The combination of anamorphic and epimorphic growth constitutes the hemianamorphic developmental mode that is common among many living arthropods 103 Trilobite development was unusual in the way in which articulations developed between segments and changes in the development of articulation gave rise to the conventionally recognized developmental phases of the trilobite life cycle divided into 3 stages which are not readily compared with those of other arthropods Actual growth and change in external form of the trilobite would have occurred when the trilobite was soft shelled following moulting and before the next exoskeleton hardened 104 Elrathia kingii growth series with holaspids ranging from 16 2 mm to 39 8 mm in length Trilobite larvae are known from the Cambrian to the Carboniferous 105 and from all sub orders 104 106 As instars from closely related taxa are more similar than instars from distantly related taxa trilobite larvae provide morphological information important in evaluating high level phylogenetic relationships among trilobites 104 Despite the absence of supporting fossil evidence their similarity to living arthropods has led to the belief that trilobites multiplied sexually and produced eggs 104 107 Some species may have kept eggs or larvae in a brood pouch forward of the glabella 69 particularly when the ecological niche was challenging to larvae 10 Size and morphology of the first calcified stage are highly variable between but not within trilobite taxa suggesting some trilobites passed through more growth within the egg than others Early developmental stages prior to calcification of the exoskeleton are a possibility suggested for fallotaspids 15 but so is calcification and hatching coinciding 104 The earliest post embryonic trilobite growth stage known with certainty are the protaspid stages anamorphic phase 104 Starting with an indistinguishable proto cephalon and proto pygidium anaprotaspid a number of changes occur ending with a transverse furrow separating the proto cephalon and proto pygidium metaprotaspid that can continue to add segments Segments are added at the posterior part of the pygidium but all segments remain fused together 104 106 The meraspid stages anamorphic phase are marked by the appearance of an articulation between the head and the fused trunk Prior to the onset of the first meraspid stage the animal had a two part structure the head and the plate of fused trunk segments the pygidium During the meraspid stages new segments appeared near the rear of the pygidium as well as additional articulations developing at the front of the pygidium releasing freely articulating segments into the thorax Segments are generally added one per moult although two per moult and one every alternate moult are also recorded with number of stages equal to the number of thoracic segments A substantial amount of growth from less than 25 up to 30 40 probably took place in the meraspid stages 104 The holaspid stages epimorphic phase commence when a stable mature number of segments has been released into the thorax Moulting continued during the holaspid stages with no changes in thoracic segment number 104 Some trilobites are suggested to have continued moulting and growing throughout the life of the individual albeit at a slower rate on reaching maturity Some trilobites showed a marked transition in morphology at one particular instar which has been called trilobite metamorphosis Radical change in morphology is linked to the loss or gain of distinctive features that mark a change in mode of life 108 A change in lifestyle during development has significance in terms of evolutionary pressure as the trilobite could pass through several ecological niches on the way to adult development and changes would strongly affect survivorship and dispersal of trilobite taxa 104 It is worth noting that trilobites with all protaspid stages solely planktonic and later meraspid stages benthic e g asaphids failed to last through the Ordovician extinctions while trilobites that were planktonic for only the first protaspid stage before metamorphosing into benthic forms survived e g lichids phacopids 108 Pelagic larval life style proved ill adapted to the rapid onset of global climatic cooling and loss of tropical shelf habitats during the Ordovician 26 History of usage and research Edit Drawing of Ogygiocarella debuchii by Rev Edward Lhwyd made in 1698Rev Edward Lhwyd published in 1698 in The Philosophical Transactions of the Royal Society the oldest scientific journal in the English language part of his letter Concerning Several Regularly Figured Stones Lately Found by Him that was accompanied by a page of etchings of fossils 109 One of his etchings depicted a trilobite he found near Llandeilo probably on the grounds of Lord Dynefor s castle he described as the skeleton of some flat Fish 2 The discovery of Calymene blumenbachii the Dudley locust in 1749 by Charles Lyttleton could be identified as the beginning of trilobite research Lyttleton submitted a letter to the Royal Society of London in 1750 concerning a petrified insect he found in the limestone pits at Dudley In 1754 Manuel Mendez da Costa proclaimed that the Dudley locust was not an insect but instead belonged to the crustaceous tribe of animals He proposed to call the Dudley specimens Pediculus marinus major trilobos large trilobed marine louse a name which lasted well into the 1800s German naturalist Johann Walch who executed the first inclusive study of this group proposed the use of the name trilobite He considered it appropriate to derive the name from the unique three lobed character of the central axis and a pleural zone to each side 110 Written descriptions of trilobites date possibly from the third century BC and definitely from the fourth century AD The Spanish geologists Eladio Linan and Rodolfo Gozalo argue that some of the fossils described in Greek and Latin lapidaries as scorpion stone beetle stone and ant stone refer to trilobite fossils Less ambiguous references to trilobite fossils can be found in Chinese sources Fossils from the Kushan formation of northeastern China were prized as inkstones and decorative pieces 109 In the New World American fossil hunters found plentiful deposits of Elrathia kingi in western Utah in the 1860s Until the early 1900s the Ute Native Americans of Utah wore these trilobites which they called pachavee little water bug as amulets 111 112 A hole was bored in the head and the fossil was worn on a string 111 According to the Ute themselves trilobite necklaces protect against bullets and diseases such as diphtheria 111 112 In 1931 Frank Beckwith uncovered evidence of the Ute use of trilobites Travelling through the badlands he photographed two petroglyphs that most likely represent trilobites On the same trip he examined a burial of unknown age with a drilled trilobite fossil lying in the chest cavity of the interred Since then trilobite amulets have been found all over the Great Basin as well as in British Columbia and Australia 109 In the 1880s archaeologists discovered in the Grotte du Trilobite Caves of Arcy sur Cure Yonne France a much handled trilobite fossil that had been drilled as if to be worn as a pendant The occupation stratum in which the trilobite was found has been dated as 15 000 years old Because the pendant was handled so much the species of trilobite cannot be determined This type of trilobite is not found around Yonne so it may have been highly prized and traded from elsewhere 109 See also Edit Paleontology portal Arthropods portalNotostraca Horseshoe crab Isopoda List of trilobite genera History of life Media related to Trilobita at Wikimedia Commons Data related to Trilobita at WikispeciesReferences Edit a b c d e f g h B S Lieberman 2002 Phylogenetic analysis of some basal early Cambrian trilobites the biogeographic origins of the eutrilobita and the timing of the Cambrian radiation Journal of Paleontology 4 ed 76 4 692 708 doi 10 1666 0022 3360 2002 076 lt 0692 PAOSBE gt 2 0 CO 2 a b c Fortey Richard 2000 Trilobite Eyewitness to Evolution 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Aubril R Feist R 2005 First Carboniferous protaspid larvae Trilobita Journal of Paleontology 79 4 702 718 doi 10 1666 0022 3360 2005 079 0702 FCPLT 2 0 CO 2 S2CID 130584215 a b Rudy Lerosey Aubril The Ontogeny of Trilobites Archived from the original on October 27 2009 Retrieved November 8 2010 Zhang X Pratt B 1994 Middle Cambrian arthropod embryos with blastomeres Science 266 5185 637 9 Bibcode 1994Sci 266 637Z doi 10 1126 science 266 5185 637 PMID 17793458 S2CID 35620499 a b Chatterton B D E Speyer S E 1989 Larval ecology life history strategies and patterns of extinction and survivorship among Ordovician trilobites Paleobiology 15 2 118 132 doi 10 1017 S0094837300009313 S2CID 89379920 a b c d John J McKay 2011 11 22 The first trilobite OhioLINK ETD Center Retrieved 3 October 2012 Alex J Chestnut Using morphometrics phylogenetic systematics and parsimony analysis to gain insight into the evolutionary affinities of the Calymenidae Trilobita OhioLINK ETD Center Archived from the original on March 31 2012 Retrieved August 21 2011 a b c Joleen Robinson October 1970 Tracking the Trilobites Desert Magazine a b van der Geer Alexandra Dermitzakis Michael 2010 Fossils in pharmacy from snake eggs to Saint s bones an overview PDF Hellenic Journal of Geosciences 45 323 332 Bibliography EditFortey Richard 2000 Trilobite Eyewitness to Evolution New York Vintage Books ISBN 978 0 375 70621 9 Lawrence Pete 2014 Trilobites of the World An atlas of 1000 photographs Manchester Siri Scientific Press ISBN 978 0 9574530 3 6 archived from the original on 2014 03 08 retrieved 2014 04 15 Levi Setti Riccardo 2014 The Trilobite Book A Visual Journey Chicago University of Chicago Press ISBN 978 0 226 12441 4External links EditGon III Sam A Guide to the Orders of Trilobite A site with information covering trilobites from all angles Includes many line drawings and photographs The Virtual Fossil Museum Class Trilobita including extensive photographs organized by taxonomy and locality Western Trilobites Association Kevin s Trilobite Gallery a collection of photographs of trilobite fossils Information on fraudulently made trilobites Archived 2013 01 21 at the Wayback Machine The Paleontological Society Chisholm Hugh ed 1911 Trilobites Encyclopaedia Britannica 11th ed Cambridge University Press When Trilobites Ruled the Earth American Museum of Natural History Trilobite website Trilobites in Houston Museum of Natural Sciences A nice copyrighted photo of a trilobite Retrieved from https en wikipedia org w index php title Trilobite amp oldid 1133399768, wikipedia, wiki, book, books, library,

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