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Pikaia

Pikaia gracilens is an extinct, primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it became "the most famous early chordate fossil",[1] or "famously known as the earliest described Cambrian chordate".[2] It is estimated to have lived during the latter period of the Cambrian explosion. Since its initial discovery, more than a hundred specimens have been recovered.[3]

Pikaia
Temporal range: Early Cambrian to Middle Cambrian (Stage 3 to Wuliuan), 513–505 Ma
Life reconstruction of Pikaia gracilens
Scientific classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Family: Pikaiidae
Genus: Pikaia
Walcott, 1911
Species:
P. gracilens
Binomial name
Pikaia gracilens
Walcott, 1911

The body structure resembles that of the lancelet and it swam perhaps much like an eel. A notochord and myomeres (segmented blocks of skeletal muscles) span the entire length of the body, and are considered the defining signatures of chordate characters. Its primitive nature is indicated by the body covering, a cuticle, which is characteristic of invertebrates and some protochordates.

The exact phylogenetic position is unclear. Proposed affinities include those of cephalochordata, craniata, or a stem-chordate not closely related to any extant lineage.[4] Popularly but falsely[5] attributed as an ancestor of all vertebrates,[6] or the oldest fish,[7] or the oldest ancestor of humans,[8][9] it is generally viewed as a basal chordate alongside other Cambrian chordates; it is a close relative of vertebrate ancestors[10][11] but it is not an ancestor itself.[2][12]

Discovery edit

 
Fossil of Pikaia gracilens (Syntype USNM PAL 57628)

The fossils of Pikaia gracilens was discovered by Charles Walcott from the Burgess shale member of the Stephen formation in British Columbia, and described it in 1911. He named it after Pika Peak, a mountain in Alberta, Canada. Based on the obvious and regular segmentation of the body, as is the feature of annelids,[13] Walcott classified it as a polychaete worm and created a new family Pikaidae for it. (Princeton palaeontologist Benjamin Franklin Howell changed the name of the family to Pikaiidae in 1962.[14]) Walcott was aware of the limitation of his classification, as he noted: "I am unable to place it within any of the families of the Polychaeta, owing to the absence of parapodia [paired protrusions on the sides of polychaete worms] on the body segments back of the fifth."[15]

University of Cambridge palaeontologist Harry B. Whittington and his student Simon Conway Morris re-examined the Burgess Shale fauna and noted the anatomical details of Pikaia for the first time. The fossil specimens bears features of notochord and muscle blocks that are fundamental structures of chordates, and not of annelids. In 1977, Conway Morris presented a paper that indicated the possible chordate position, without further explanation.[16][17] He and Whittington were convinced that the animal was obviously a chordate, as they wrote in Scientific American in 1979:

Finally, we find among the Burgess Shale fauna one of the earliest-known invertebrate representatives of our own conspicuous corner of the animal kingdom: the chordate phylum... The chordates are represented in the Burgess Shale by the genus Pikaia and the single species P. gracilens.[18]

Conway Morris formally placed P. gracilens among the chordates in a paper in the Annual Review of Ecology and Systematics that same year.[19] However, he provided no structural analyses such as using microscopes to confirm the chordate features. The comparative description only earned a "putative" chordate status.[20] The fossil's chordate nature was received sceptically for several decades.[21][22][23] Only in 2012, when detailed analysis was reported by Conway Morris and Jean-Bernard Caron, that the chordate position became generally accepted.[12][24]

The fossils are found only in a restricted series of horizons in the strata exposed on Fossil Ridge, close to the Yoho National Park. From the same location, other fish-like animal fossils named Metaspriggina were discovered in 1993. Conway Morris identified the animals as another Cambrian chordate.[25] The fossil specimens are preserved in the Smithsonian Institution and the Royal Ontario Museum.[3]

Description edit

Pikaia has a lancelet-like body, tapering at both ends, laterally flat and lacked a well-defined head. It measures an average of about 1+12 inches (3.8 cm) in length. Walcott recorded the longest individuals as 5 cm (2.0 in) in length.[15] Pikaia has a pair of large, antenna-like tentacles on its head that resembles those of invertebrates such as snails.[23] The attachment of the tentacles makes a two-lobed structure of the head.[26] The tentacles may be comparable to those in the present-day hagfish, a jawless chordate.[27] It has a small circular mouth that could be used to eat small food particles in a single bite. There are a series of short appendages on either side of the underside of the head just after the mouth, and their exact nature or function is unknown. The pharynx is associated with six pairs of slits with tiny filaments that could be used for respiratory apparatus.[28] In these ways, it differs from the modern lancelets, which have distinct pharyngeal gill slits on either sides of the pharynx and are used for filter feeding.[29]

A major primitive structure of Pikaia is a cuticle as its body covering. Cuticle is a hard protein layer predominantly found in invertebrates such as arthropods, molluscs, echinoderms and nematodes.[30][31] Unlike a typical cuticle, the cuticle of Pikaia does not have hard extracellular (exoskeleton) protection, and the entirely body is essentially soft-bodied.[32] Although primitive, Pikaia shows the essential prerequisites for vertebrates. When alive, Pikaia was a compressed, leaf-shaped animal with an expanded tail fin; the flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines. The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail.[33]

Pikaia was an active and free swimmer.[15] It likely swam by throwing its body into a series of S-shaped, zigzag curves, similar to the movement of eels; fish inherited the same swimming movement, but they generally have stiffer backbones. These adaptations may have allowed Pikaia to filter particles from the water as it swam along.[27] Pikaia was probably a slow swimmer, since it lacked the fast-twitch fibers that are associated with rapid swimming in modern chordates.[27]

Reinterpretations edit

 
Plate 20 of Walcott Cambrian Geology and Paleontology II (1911), by Charles Doolittle Walcott, showing fossils of Pikaia and Oesia

Walcott's original summary of the description of Pikaia reads:

Body elongate, slender, and tapering at each end. It is formed of many segments that are defined by strong annular shiny lines. Head small with two large eyes and two tentacles... Back of the head the first five segments carry short parapodia that appear to be divided into two parts. The enteric canal extends from end to end without change in character... This was one of the active, free-swimming annelids that suggest the Nephthydidae of the Polychaeta.[15]

Whittington and Conway Morris were the first to realise that Walcott's description and classification were not reliable and mostly inaccurate. They compared the body segments as described by Walcott with living animals and found that they were similar to the muscle bundles of chordates such as the living Amphioxus (Branchiostoma) as well as fishes, and not to superficial segments of annelids. They pictured that the muscles would be essential for swimming in water in wriggling motions. The enteric canal as observed by Walcott was not an ordinary digestive tract, it runs along with a stiff rod that resembles a notochord. They reported in 1979: "Although Pikaia differs from Amphioxus, in several important respects, the conclusion is that it is not a worm but a chordate appears inescapable."[18]

Conway Morris was convinced that the longitudinal rod was a notochord and the segments were muscle blocks that he concluded that Pikaia "is a primitive chordate rather than a polychaete. The earliest fish scales are Upper Cambrian, and Pikaia may not be far removed from the ancestral fish."[19] In 1982, he added further description in his Atlas of the Burgess Shale that Pikaia had one or more fins, but did not specify where they were present.[12]

Pikaia was not popularly known as a chordate fossil or as an ancient chordate until 1989.[34] That year, Harvard University palaeontologist Stephen Jay Gould wrote in his book Wonderful Life: The Burgess Shale and the Nature of History: "Pikaia is not an annelid worm. It is a chordate, a member of our own phylum—in fact, the first recorded member of our immediate ancestry."[35] From this remark Pikaia became generally recognised as a chordate and ancestor of vertebrates.[24][36][37]

In 1993, Conway Morris came up with another possible chordate feature. He identified structures that looked like gill slits but gave a cautious remark: "[They] may have been present, but are hard to identify with certainty in the compressed material available.[26] The tiny pores on the side of the pharynx are normally gill slits in living chordates.[38] He also noticed that Pikaia is similar to Amphioxus in most general aspects, with major difference in its notochord not reaching the anterior end.[26]

Not all palaeontologists were convinced of the chordate designation without better analysis.[21][22] In 2001, Nicholas D. Holland from the Scripps Institution of Oceanography and Junyuan Chen from the Chinese Academy of Sciences criticised the presentation in Wonderful Life, saying that the "reinterpretation [of Pikaia as a chordate] became almost universally accepted after its unqualified and forceful endorsement by Gould"; concluding that "the cephalochordate affinity of Pikaia is at best only weakly indicated by the characters visible in fossils discovered so far."[39] In 2010, an international team of palaeontologists argued that Pikaia has sufficiently invertebrate characters, and that it mostly look like a much younger extinct animal, the Tully monster (Tullimonstrum gregarium),[23] which is still debated as either an invertebrate or a chordate.[40][41]

Another component of Pikaia fossils that constrains the animal to be accepted as a chordate is its distinct invertebrate character; its preservational mode suggests that it had cuticle. The cuticle as a body covering is uncharacteristic of the vertebrates,[42] but is a dominant feature of invertebrates.[30] The presence of earlier chordates among the Chengjiang, including Haikouichthys and Myllokunmingia, appears to show that cuticle is not necessary for preservation, overruling the taphonomic argument,[25] but the presence of tentacles remains intriguing, and the organism cannot be assigned conclusively, even to the vertebrate stem group. Its anatomy closely resembles the modern creature Branchiostoma.[43]

A fossil species Myoscolex ateles, discovered in 1979 from Cambrian Emu Bay shale of Kangaroo Island in South Australia, had been debated as among the oldest annelids, or at least other invertebrate groups.[44][45] Polish palaeontologist Jerzy Dzik in his formal description in 2003 notes that it "closely resembles the slightly geologically younger Pikaia" in having smooth cuticle as well as muscular segmentation, and projections on its backside (ventral chaetae) that look like Pikaia's tentacles. He concluded:

In fact, there is little evidence for chordate affinities of Pikaia. Its relationship with Myoscolex [as annelid in his proposition] appears a much better solution. Both were initially identified as polychaetes and this line of inference perhaps deserves confrontation with more recent evidence than that available to the authors who proposed these genera.[46]

Comprehensive description edit

 
Anatomy of Pikaia gracilens based on Conway Morris & Caron (2012)

The first comprehensive description of Pikaia was published by Conway Morris and Jean-Bernard Caron in the May 2012 issue of Biological Reviews. The anatomical examination and interpretation based on 114 fossil specimens confirm the classification as a chordate.[3] According to the new assessment, Pikaia fossils indicate important features that define the animal as a primitive chordate.[12] All Pikaia fossils are in the range of 1.5 to 6 centimetres (0.59 to 2.36 in) in length, with an average of 4 centimetres (1.6 in). Having a laterally compressed (taller than wide) and fusiform (tapering at both ends) body, the exact width and height are variable, and normally its height is twice that of its width throughout it body.[3]

The head is bilaterally symmetrical with a distinct pair of tentacles. Due to its small size, only about 1 mm in diameter, the structural details are indistinguishable. Some specimens show a darker central line on the tentacles which may represent a nervous fibre; thus making the tentacles as sensory feelers.[47] A mouth is marked by a small opening at the anterior end of the gut towards the underside of the head. There are no jaws and teeth. Walcott had mentioned the presence of two large eyes,[15] but no specimens, including Walcott's original collection, show any evidence of eyes.[3]

One of the most unusual body parts is a series of appendages just posterior to the tentacles. Walcott had called the appendages parapodia, as a kind of body protrusions that aid locomotion in snails,[48] and mentioned five parapodia in each individual.[15] He was even puzzled by the absence on the major part of the body.[15] Other specimens have up to nine such appendages, and could not be parapodia. Fins are present as an expansion of the body on the dorsal and ventral sides. They are not present in many specimens indicating that they are delicate membranes and were lost during fossilisation.[3]

The backside of Pikaia fossils show a hollow tubular structure that extends throughout most of the body length, but not the anterior region. It is easily noticeable as a highly light-reflective portion and is known as the dorsal organ. Once described as the notochord,[18][19] its nature is not yet fully resolved and could be a storage organ. The true notochord, along with a nerve chord, is a fine lateral line that runs just beneath the thick dorsal organ.[3]

The main chordate character is a series of myomeres that extends from the anterior to the posterior region. On average, there are 100 such myomeres in each individual. The muscle segments are not simply "annular shiny lines" as Walcott described,[15] but are in concentric bends in the form of V-shaped chevron. The myomeres at the anterior end as simpler in appearance and show circular arrangement. Conway Morris and Caron concluded:

Whilst the possibility that Pikaia is simply convergent on the chordates cannot be dismissed, we prefer to build a scenario that regards Pikaia as the most stem-ward of the chordates with links to the phylogenetically controversial yunnanozoans. This hypothesis has implications for the evolution of the myomeres, notochord and gills.[3]

Evolutionary importance edit

Much debate on whether Pikaia is a vertebrate ancestor, its worm-like appearance notwithstanding, exists in scientific circles. It looks like a worm that has been flattened sideways (lateral compression). The fossils compressed within the Burgess Shale show chordate features such as traces of an elongate notochord, dorsal nerve cord, and blocks of muscles (myotomes) down either side of the body – all critical features for the evolution of the vertebrates.

The notochord, a flexible rod-like structure that runs along the back of the animal, lengthens and stiffens the body so that it can be flexed from side to side by the muscle blocks for swimming. In the fish and all subsequent vertebrates, the notochord forms the backbone (or vertebral column). The backbone strengthens the body, supports strut-like limbs, and protects the vital dorsal nerve cord, while at the same time allowing the body to bend.

A Pikaia lookalike, the lancelet Branchiostoma, still exists today. With a notochord and paired muscle blocks, the lancelet and Pikaia belong to the chordate group of animals from which the vertebrates descended. Molecular studies have refuted earlier hypotheses that lancelets might be the closest living relative to the vertebrates, instead favoring tunicates in this position;[49] other extant and fossil groups, such as acorn worms and graptolites, are more primitive.[a]

The presence of cuticle, one of the principal characters of higher invertebrates,[30][31] in Pikaia can be understood from the evolutionary trends. A Cambrian invertebrate, Myoscolex ateles was described to be structurally similar to Pikaia particularly in having smooth cuticle as well as muscular segmentation, and projections on its backside (ventral chaetae) that look like Pikaia's tentacles.[46] Although chordates normally lack the cuticle, a type of cuticle is present in some cephalochordates, indicating that primitive characters are retained in lower chordates.[50]

Subsequently, Mallatt and Holland reconsidered Conway Morris and Caron's description, and concluded that many of the newly recognized characters are unique, already-divergent specializations that would not be helpful for establishing Pikaia as a basal chordate.[12]

Development of the head edit

The first sign of head development, cephalization, is seen in chordates such as Pikaia and Branchiostoma. It is thought that development of a head structure resulted from a long body shape, a swimming habit, and a mouth at the end that came into contact with the environment first, as the animal swam forward. The search for food required ways of continually testing what lay ahead so it is thought that anatomical structures for seeing, feeling, and smelling developed around the mouth. The information these structures gathered was processed by a swelling of the nerve cord (efflorescence) – the precursor of the brain. Altogether, these front-end structures formed the distinct part of the vertebrate body known as the head.[33]

Evolutionary interpretation edit

Once thought to be closely related to the ancestor of all vertebrates, Pikaia has received particular attention among the multitude of animal fossils found in the famous Burgess Shale and other Cambrian fauna. In 1979, Whittington and Conway Morris first explained the evolutionary importance of Pikaia. Realising the fossil to be that of a chordate in the Cambrian rocks, chordates could have originated much earlier than expected, as they commented: "The superb preservation of this Middle Cambrian organism [Pikaia] makes it a landmark history of the phylum [Chordata] to which all vertebrates, including man, belong."[18] It is for this knowledge Pikaia as an old chordate that it is often misleadingly and falsely[5] attributed to as an ancestor of all vertebrates,[6] or the oldest fish,[7] or the oldest ancestor of humans.[10][11]

Before Pikaia and other Cambrian chordates were fully appreciated, it was generally believed that the first chordates appeared much later, such as in Ordovician (484–443 mya). The establishment of Cambrian chordates, according to Stephen Jay Gould, prompted "revised views of evolution, ecology and development," and remarked: "So much for chordate uniqueness marked by slightly later evolution."[51] However, Gould did not believe that Pikaia itself was unique as an early chordate or that it was "the actual ancestor of vertebrates;" he presumed that there could be undiscovered fossils that are more closely linked to vertebrate ancestry.[52]

The presence of a creature as complex as Pikaia some 530 million years ago reinforces the controversial view that the diversification of life must have extended back well before Cambrian times – perhaps deep into the Precambrian.[33] The discovery of Metaspriggina, a primitive fish of similar age which seems to have already started to develop a jaw and the presence of conodonts, the teeth of an extinct type of fish belonging to the Agnatha some 20 Ma before Pikaia, does provide support for that view.

Gould's interpretation and evolutionary contingency edit

Gould, in his presidential address of the Paleontological Society on 27 October 1988, cited Pikaia to explain the trends of evolutionary changes:

Wind back life's tape to the Burgess (first erasing what actually came after), let it play again, and this time a quite different cast may emerge. If the cast lacked Pikaia, the first chordate, we might not be here—and the world would be no worse... Let us thank our lucky stars for the survival of Pikaia.[53]

He elaborated the same idea in "An epilogue on Pikaia" in his book Wonderful Life "to save the best for the last," in which he made a statement:

Pikaia is the missing and final link in our story of contingency—the direct connection between Burgess decimation and eventual human evolution... Wind the tape of life back to Burgess times, and let it play again. If Pikaia does not survive in the replay, we are wiped out of future history—all of us, from shark to robin to orangutan... And so, if you wish to ask the question of the age—why do humans exist?—a major part of the answer, touching those aspects of the issue that science can treat at all, must be: because Pikaia survived the Burgess decimation.[52]

This interpretation that the chances of evolutionary products are unpredictable is known as evolutionary contingency.[54][55] Gould, from this statement, is regarded as "the most famous proponent" of the concept.[56] His idea has inspired many research involving enolutionary contingency from palaeontology to molecular biology.[57][58][59] He used Pikaia among the Cambrian animals as an epitome of contingent event in the entire evolution of life; if Pikaia had not existed, the rest of chordate animals might not have evolved, thus completely changing the diversity of life as we know. According to him, contingency is a major factor that drives large-scale evolution (macroevolution) and dictates that evolution has no inevitable destiny or outcome.[60] However, as Gould explained, "The bad news is that we can't possibly perform the experiment."[59]

Ecology edit

Pikaia is suggested to have been an active swimming organism that swam close to the seafloor (nektobenthic) using side to side undulations of its flattened posterior for propulsion. The anterior appendages are unlikely to have been used in feeding, and may have had a respiratory function. Pikaia is suggested to have fed on small particles of organic matter.[3]

See also edit

Notes edit

  1. ^ Acorn worms are classified as hemichordates, which have a notochord-like structure only at the early stage of their lives.

References edit

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Further reading edit

  • Bishop, A., Woolley, A. and Hamilton, W. (1999) Minerals, Rocks and Fossils. London: Phillip's
  • Conway Morris, Simon. 1998. The Crucible of Creation: The Burgess Shale and the Rise of Animals. Oxford University Press, New York, New York.
  • Norman, D. (1994) Prehistoric Life: the Rise of the Vertebrates, London: Boxtree
  • Sheldon, P., Palmer D., Spicer, B. (2001). Fossils and the History of Life. Aberystwyth: Cambrian Printers/The Open University. p. 41-42.

External links edit

  • "Pikaia gracilens". Burgess Shale Fossil Gallery. Virtual Museum of Canada. 2011. Archived from the original on 2020-11-12.
  • La evolución de las especies: ¿por qué sobrevivió Pikaia? (Spanish)
  • Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia

pikaia, this, article, about, extinct, chordate, anime, series, gracilens, extinct, primitive, chordate, animal, known, from, middle, cambrian, burgess, shale, british, columbia, described, 1911, charles, doolittle, walcott, annelid, 1979, harry, whittington, . This article is about the extinct chordate For the anime series see Pikaia Pikaia gracilens is an extinct primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia Described in 1911 by Charles Doolittle Walcott as an annelid and in 1979 by Harry B Whittington and Simon Conway Morris as a chordate it became the most famous early chordate fossil 1 or famously known as the earliest described Cambrian chordate 2 It is estimated to have lived during the latter period of the Cambrian explosion Since its initial discovery more than a hundred specimens have been recovered 3 PikaiaTemporal range Early Cambrian to Middle Cambrian Stage 3 to Wuliuan 513 505 Ma PreꞒ Ꞓ O S D C P T J K Pg N Life reconstruction of Pikaia gracilens Scientific classification Domain Eukaryota Kingdom Animalia Phylum Chordata Family Pikaiidae Genus PikaiaWalcott 1911 Species P gracilens Binomial name Pikaia gracilensWalcott 1911 The body structure resembles that of the lancelet and it swam perhaps much like an eel A notochord and myomeres segmented blocks of skeletal muscles span the entire length of the body and are considered the defining signatures of chordate characters Its primitive nature is indicated by the body covering a cuticle which is characteristic of invertebrates and some protochordates The exact phylogenetic position is unclear Proposed affinities include those of cephalochordata craniata or a stem chordate not closely related to any extant lineage 4 Popularly but falsely 5 attributed as an ancestor of all vertebrates 6 or the oldest fish 7 or the oldest ancestor of humans 8 9 it is generally viewed as a basal chordate alongside other Cambrian chordates it is a close relative of vertebrate ancestors 10 11 but it is not an ancestor itself 2 12 Contents 1 Discovery 2 Description 2 1 Reinterpretations 2 2 Comprehensive description 3 Evolutionary importance 3 1 Development of the head 3 2 Evolutionary interpretation 3 3 Gould s interpretation and evolutionary contingency 4 Ecology 5 See also 6 Notes 7 References 8 Further reading 9 External linksDiscovery edit nbsp Fossil of Pikaia gracilens Syntype USNM PAL 57628 The fossils of Pikaia gracilens was discovered by Charles Walcott from the Burgess shale member of the Stephen formation in British Columbia and described it in 1911 He named it after Pika Peak a mountain in Alberta Canada Based on the obvious and regular segmentation of the body as is the feature of annelids 13 Walcott classified it as a polychaete worm and created a new family Pikaidae for it Princeton palaeontologist Benjamin Franklin Howell changed the name of the family to Pikaiidae in 1962 14 Walcott was aware of the limitation of his classification as he noted I am unable to place it within any of the families of the Polychaeta owing to the absence of parapodia paired protrusions on the sides of polychaete worms on the body segments back of the fifth 15 University of Cambridge palaeontologist Harry B Whittington and his student Simon Conway Morris re examined the Burgess Shale fauna and noted the anatomical details of Pikaia for the first time The fossil specimens bears features of notochord and muscle blocks that are fundamental structures of chordates and not of annelids In 1977 Conway Morris presented a paper that indicated the possible chordate position without further explanation 16 17 He and Whittington were convinced that the animal was obviously a chordate as they wrote in Scientific American in 1979 Finally we find among the Burgess Shale fauna one of the earliest known invertebrate representatives of our own conspicuous corner of the animal kingdom the chordate phylum The chordates are represented in the Burgess Shale by the genus Pikaia and the single species P gracilens 18 Conway Morris formally placed P gracilens among the chordates in a paper in the Annual Review of Ecology and Systematics that same year 19 However he provided no structural analyses such as using microscopes to confirm the chordate features The comparative description only earned a putative chordate status 20 The fossil s chordate nature was received sceptically for several decades 21 22 23 Only in 2012 when detailed analysis was reported by Conway Morris and Jean Bernard Caron that the chordate position became generally accepted 12 24 The fossils are found only in a restricted series of horizons in the strata exposed on Fossil Ridge close to the Yoho National Park From the same location other fish like animal fossils named Metaspriggina were discovered in 1993 Conway Morris identified the animals as another Cambrian chordate 25 The fossil specimens are preserved in the Smithsonian Institution and the Royal Ontario Museum 3 Description editPikaia has a lancelet like body tapering at both ends laterally flat and lacked a well defined head It measures an average of about 1 1 2 inches 3 8 cm in length Walcott recorded the longest individuals as 5 cm 2 0 in in length 15 Pikaia has a pair of large antenna like tentacles on its head that resembles those of invertebrates such as snails 23 The attachment of the tentacles makes a two lobed structure of the head 26 The tentacles may be comparable to those in the present day hagfish a jawless chordate 27 It has a small circular mouth that could be used to eat small food particles in a single bite There are a series of short appendages on either side of the underside of the head just after the mouth and their exact nature or function is unknown The pharynx is associated with six pairs of slits with tiny filaments that could be used for respiratory apparatus 28 In these ways it differs from the modern lancelets which have distinct pharyngeal gill slits on either sides of the pharynx and are used for filter feeding 29 A major primitive structure of Pikaia is a cuticle as its body covering Cuticle is a hard protein layer predominantly found in invertebrates such as arthropods molluscs echinoderms and nematodes 30 31 Unlike a typical cuticle the cuticle of Pikaia does not have hard extracellular exoskeleton protection and the entirely body is essentially soft bodied 32 Although primitive Pikaia shows the essential prerequisites for vertebrates When alive Pikaia was a compressed leaf shaped animal with an expanded tail fin the flattened body is divided into pairs of segmented muscle blocks seen as faint vertical lines The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail 33 Pikaia was an active and free swimmer 15 It likely swam by throwing its body into a series of S shaped zigzag curves similar to the movement of eels fish inherited the same swimming movement but they generally have stiffer backbones These adaptations may have allowed Pikaia to filter particles from the water as it swam along 27 Pikaia was probably a slow swimmer since it lacked the fast twitch fibers that are associated with rapid swimming in modern chordates 27 Reinterpretations edit nbsp Plate 20 of Walcott Cambrian Geology and Paleontology II 1911 by Charles Doolittle Walcott showing fossils of Pikaia and OesiaWalcott s original summary of the description of Pikaia reads Body elongate slender and tapering at each end It is formed of many segments that are defined by strong annular shiny lines Head small with two large eyes and two tentacles Back of the head the first five segments carry short parapodia that appear to be divided into two parts The enteric canal extends from end to end without change in character This was one of the active free swimming annelids that suggest the Nephthydidae of the Polychaeta 15 Whittington and Conway Morris were the first to realise that Walcott s description and classification were not reliable and mostly inaccurate They compared the body segments as described by Walcott with living animals and found that they were similar to the muscle bundles of chordates such as the living Amphioxus Branchiostoma as well as fishes and not to superficial segments of annelids They pictured that the muscles would be essential for swimming in water in wriggling motions The enteric canal as observed by Walcott was not an ordinary digestive tract it runs along with a stiff rod that resembles a notochord They reported in 1979 Although Pikaia differs from Amphioxus in several important respects the conclusion is that it is not a worm but a chordate appears inescapable 18 Conway Morris was convinced that the longitudinal rod was a notochord and the segments were muscle blocks that he concluded that Pikaia is a primitive chordate rather than a polychaete The earliest fish scales are Upper Cambrian and Pikaia may not be far removed from the ancestral fish 19 In 1982 he added further description in his Atlas of the Burgess Shale that Pikaia had one or more fins but did not specify where they were present 12 Pikaia was not popularly known as a chordate fossil or as an ancient chordate until 1989 34 That year Harvard University palaeontologist Stephen Jay Gould wrote in his book Wonderful Life The Burgess Shale and the Nature of History Pikaia is not an annelid worm It is a chordate a member of our own phylum in fact the first recorded member of our immediate ancestry 35 From this remark Pikaia became generally recognised as a chordate and ancestor of vertebrates 24 36 37 In 1993 Conway Morris came up with another possible chordate feature He identified structures that looked like gill slits but gave a cautious remark They may have been present but are hard to identify with certainty in the compressed material available 26 The tiny pores on the side of the pharynx are normally gill slits in living chordates 38 He also noticed that Pikaia is similar to Amphioxus in most general aspects with major difference in its notochord not reaching the anterior end 26 Not all palaeontologists were convinced of the chordate designation without better analysis 21 22 In 2001 Nicholas D Holland from the Scripps Institution of Oceanography and Junyuan Chen from the Chinese Academy of Sciences criticised the presentation in Wonderful Life saying that the reinterpretation of Pikaia as a chordate became almost universally accepted after its unqualified and forceful endorsement by Gould concluding that the cephalochordate affinity of Pikaia is at best only weakly indicated by the characters visible in fossils discovered so far 39 In 2010 an international team of palaeontologists argued that Pikaia has sufficiently invertebrate characters and that it mostly look like a much younger extinct animal the Tully monster Tullimonstrum gregarium 23 which is still debated as either an invertebrate or a chordate 40 41 Another component of Pikaia fossils that constrains the animal to be accepted as a chordate is its distinct invertebrate character its preservational mode suggests that it had cuticle The cuticle as a body covering is uncharacteristic of the vertebrates 42 but is a dominant feature of invertebrates 30 The presence of earlier chordates among the Chengjiang including Haikouichthys and Myllokunmingia appears to show that cuticle is not necessary for preservation overruling the taphonomic argument 25 but the presence of tentacles remains intriguing and the organism cannot be assigned conclusively even to the vertebrate stem group Its anatomy closely resembles the modern creature Branchiostoma 43 A fossil species Myoscolex ateles discovered in 1979 from Cambrian Emu Bay shale of Kangaroo Island in South Australia had been debated as among the oldest annelids or at least other invertebrate groups 44 45 Polish palaeontologist Jerzy Dzik in his formal description in 2003 notes that it closely resembles the slightly geologically younger Pikaia in having smooth cuticle as well as muscular segmentation and projections on its backside ventral chaetae that look like Pikaia s tentacles He concluded In fact there is little evidence for chordate affinities of Pikaia Its relationship with Myoscolex as annelid in his proposition appears a much better solution Both were initially identified as polychaetes and this line of inference perhaps deserves confrontation with more recent evidence than that available to the authors who proposed these genera 46 Comprehensive description edit nbsp Anatomy of Pikaia gracilens based on Conway Morris amp Caron 2012 The first comprehensive description of Pikaia was published by Conway Morris and Jean Bernard Caron in the May 2012 issue of Biological Reviews The anatomical examination and interpretation based on 114 fossil specimens confirm the classification as a chordate 3 According to the new assessment Pikaia fossils indicate important features that define the animal as a primitive chordate 12 All Pikaia fossils are in the range of 1 5 to 6 centimetres 0 59 to 2 36 in in length with an average of 4 centimetres 1 6 in Having a laterally compressed taller than wide and fusiform tapering at both ends body the exact width and height are variable and normally its height is twice that of its width throughout it body 3 The head is bilaterally symmetrical with a distinct pair of tentacles Due to its small size only about 1 mm in diameter the structural details are indistinguishable Some specimens show a darker central line on the tentacles which may represent a nervous fibre thus making the tentacles as sensory feelers 47 A mouth is marked by a small opening at the anterior end of the gut towards the underside of the head There are no jaws and teeth Walcott had mentioned the presence of two large eyes 15 but no specimens including Walcott s original collection show any evidence of eyes 3 One of the most unusual body parts is a series of appendages just posterior to the tentacles Walcott had called the appendages parapodia as a kind of body protrusions that aid locomotion in snails 48 and mentioned five parapodia in each individual 15 He was even puzzled by the absence on the major part of the body 15 Other specimens have up to nine such appendages and could not be parapodia Fins are present as an expansion of the body on the dorsal and ventral sides They are not present in many specimens indicating that they are delicate membranes and were lost during fossilisation 3 The backside of Pikaia fossils show a hollow tubular structure that extends throughout most of the body length but not the anterior region It is easily noticeable as a highly light reflective portion and is known as the dorsal organ Once described as the notochord 18 19 its nature is not yet fully resolved and could be a storage organ The true notochord along with a nerve chord is a fine lateral line that runs just beneath the thick dorsal organ 3 The main chordate character is a series of myomeres that extends from the anterior to the posterior region On average there are 100 such myomeres in each individual The muscle segments are not simply annular shiny lines as Walcott described 15 but are in concentric bends in the form of V shaped chevron The myomeres at the anterior end as simpler in appearance and show circular arrangement Conway Morris and Caron concluded Whilst the possibility that Pikaia is simply convergent on the chordates cannot be dismissed we prefer to build a scenario that regards Pikaia as the most stem ward of the chordates with links to the phylogenetically controversial yunnanozoans This hypothesis has implications for the evolution of the myomeres notochord and gills 3 Evolutionary importance editMuch debate on whether Pikaia is a vertebrate ancestor its worm like appearance notwithstanding exists in scientific circles It looks like a worm that has been flattened sideways lateral compression The fossils compressed within the Burgess Shale show chordate features such as traces of an elongate notochord dorsal nerve cord and blocks of muscles myotomes down either side of the body all critical features for the evolution of the vertebrates The notochord a flexible rod like structure that runs along the back of the animal lengthens and stiffens the body so that it can be flexed from side to side by the muscle blocks for swimming In the fish and all subsequent vertebrates the notochord forms the backbone or vertebral column The backbone strengthens the body supports strut like limbs and protects the vital dorsal nerve cord while at the same time allowing the body to bend A Pikaia lookalike the lancelet Branchiostoma still exists today With a notochord and paired muscle blocks the lancelet and Pikaia belong to the chordate group of animals from which the vertebrates descended Molecular studies have refuted earlier hypotheses that lancelets might be the closest living relative to the vertebrates instead favoring tunicates in this position 49 other extant and fossil groups such as acorn worms and graptolites are more primitive a The presence of cuticle one of the principal characters of higher invertebrates 30 31 in Pikaia can be understood from the evolutionary trends A Cambrian invertebrate Myoscolex ateles was described to be structurally similar to Pikaia particularly in having smooth cuticle as well as muscular segmentation and projections on its backside ventral chaetae that look like Pikaia s tentacles 46 Although chordates normally lack the cuticle a type of cuticle is present in some cephalochordates indicating that primitive characters are retained in lower chordates 50 Subsequently Mallatt and Holland reconsidered Conway Morris and Caron s description and concluded that many of the newly recognized characters are unique already divergent specializations that would not be helpful for establishing Pikaia as a basal chordate 12 Development of the head edit The first sign of head development cephalization is seen in chordates such as Pikaia and Branchiostoma It is thought that development of a head structure resulted from a long body shape a swimming habit and a mouth at the end that came into contact with the environment first as the animal swam forward The search for food required ways of continually testing what lay ahead so it is thought that anatomical structures for seeing feeling and smelling developed around the mouth The information these structures gathered was processed by a swelling of the nerve cord efflorescence the precursor of the brain Altogether these front end structures formed the distinct part of the vertebrate body known as the head 33 Evolutionary interpretation edit Once thought to be closely related to the ancestor of all vertebrates Pikaia has received particular attention among the multitude of animal fossils found in the famous Burgess Shale and other Cambrian fauna In 1979 Whittington and Conway Morris first explained the evolutionary importance of Pikaia Realising the fossil to be that of a chordate in the Cambrian rocks chordates could have originated much earlier than expected as they commented The superb preservation of this Middle Cambrian organism Pikaia makes it a landmark history of the phylum Chordata to which all vertebrates including man belong 18 It is for this knowledge Pikaia as an old chordate that it is often misleadingly and falsely 5 attributed to as an ancestor of all vertebrates 6 or the oldest fish 7 or the oldest ancestor of humans 10 11 Before Pikaia and other Cambrian chordates were fully appreciated it was generally believed that the first chordates appeared much later such as in Ordovician 484 443 mya The establishment of Cambrian chordates according to Stephen Jay Gould prompted revised views of evolution ecology and development and remarked So much for chordate uniqueness marked by slightly later evolution 51 However Gould did not believe that Pikaia itself was unique as an early chordate or that it was the actual ancestor of vertebrates he presumed that there could be undiscovered fossils that are more closely linked to vertebrate ancestry 52 The presence of a creature as complex as Pikaia some 530 million years ago reinforces the controversial view that the diversification of life must have extended back well before Cambrian times perhaps deep into the Precambrian 33 The discovery of Metaspriggina a primitive fish of similar age which seems to have already started to develop a jaw and the presence of conodonts the teeth of an extinct type of fish belonging to the Agnatha some 20 Ma before Pikaia does provide support for that view Gould s interpretation and evolutionary contingency editGould in his presidential address of the Paleontological Society on 27 October 1988 cited Pikaia to explain the trends of evolutionary changes Wind back life s tape to the Burgess first erasing what actually came after let it play again and this time a quite different cast may emerge If the cast lacked Pikaia the first chordate we might not be here and the world would be no worse Let us thank our lucky stars for the survival of Pikaia 53 He elaborated the same idea in An epilogue on Pikaia in his book Wonderful Life to save the best for the last in which he made a statement Pikaia is the missing and final link in our story of contingency the direct connection between Burgess decimation and eventual human evolution Wind the tape of life back to Burgess times and let it play again If Pikaia does not survive in the replay we are wiped out of future history all of us from shark to robin to orangutan And so if you wish to ask the question of the age why do humans exist a major part of the answer touching those aspects of the issue that science can treat at all must be because Pikaia survived the Burgess decimation 52 This interpretation that the chances of evolutionary products are unpredictable is known as evolutionary contingency 54 55 Gould from this statement is regarded as the most famous proponent of the concept 56 His idea has inspired many research involving enolutionary contingency from palaeontology to molecular biology 57 58 59 He used Pikaia among the Cambrian animals as an epitome of contingent event in the entire evolution of life if Pikaia had not existed the rest of chordate animals might not have evolved thus completely changing the diversity of life as we know According to him contingency is a major factor that drives large scale evolution macroevolution and dictates that evolution has no inevitable destiny or outcome 60 However as Gould explained The bad news is that we can t possibly perform the experiment 59 Ecology editPikaia is suggested to have been an active swimming organism that swam close to the seafloor nektobenthic using side to side undulations of its flattened posterior for propulsion The anterior appendages are unlikely to have been used in feeding and may have had a respiratory function Pikaia is suggested to have fed on small particles of organic matter 3 See also editFossils of the Burgess Shale Paleobiota of the Burgess ShaleNotes edit Acorn worms are classified as hemichordates which have a notochord like structure only at the early stage of their lives References edit Gee Henry 2018 Across the Bridge Understanding the Origin of the Vertebrates University of Chicago Press p 83 ISBN 978 0 226 40319 9 a b McMenamin Mark A S 2019 Cambrian Chordates and Vetulicolians Geosciences 9 8 354 Bibcode 2019Geosc 9 354M doi 10 3390 geosciences9080354 ISSN 2076 3263 a b c d e f g h i Conway Morris Simon Caron Jean Bernard 2012 Pikaia gracilens Walcott a stem group chordate from the Middle Cambrian of British Columbia Biological Reviews 87 2 480 512 doi 10 1111 j 1469 185X 2012 00220 x PMID 22385518 S2CID 27671780 Pikaia gracilens Burgess Shale Fossil Gallery Virtual Museum of Canada 2011 a b Danchin Etienne G J Pontarotti Pierre 2004 Towards the reconstruction of the bilaterian ancestral pre MHC region Trends in Genetics 20 12 587 591 doi 10 1016 j tig 2004 09 009 PMID 15522451 a b SciTechDaily 2012 03 06 Pikaia Gracilens Is a Relative of but not the Genetic Ancestor to All Vertebrates SciTechDaily Retrieved 2022 09 23 a b An Early 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Science 362 6415 eaam5979 Bibcode 2018Sci 362M5979B doi 10 1126 science aam5979 ISSN 1095 9203 PMID 30409860 S2CID 53239705 Briggs Derek E G 2002 Stephen Jay Gould 1941 2002 Nature 417 6890 706 doi 10 1038 417706a ISSN 1476 4687 S2CID 4356596 Further reading editBishop A Woolley A and Hamilton W 1999 Minerals Rocks and Fossils London Phillip s Conway Morris Simon 1998 The Crucible of Creation The Burgess Shale and the Rise of Animals Oxford University Press New York New York Norman D 1994 Prehistoric Life the Rise of the Vertebrates London Boxtree Sheldon P Palmer D Spicer B 2001 Fossils and the History of Life Aberystwyth Cambrian Printers The Open University p 41 42 External links edit nbsp Wikimedia Commons has media related to Pikaia nbsp Paleontology portal Pikaia gracilens Burgess Shale Fossil Gallery Virtual Museum of Canada 2011 Archived from the original on 2020 11 12 La evolucion de las especies por que sobrevivio Pikaia Spanish Fossils of the Burgess Shale Middle Cambrian Pikaia gracilens Walcott a stem group chordate from the Middle Cambrian of British Columbia Retrieved from https en wikipedia org w index php title Pikaia amp oldid 1220973741, wikipedia, wiki, book, books, library,

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