Proboscidea

For the plant genus, see Proboscidea (plant).

Proboscidea (/ˌprɒbəˈsɪdiə/; from Latin proboscis, from Ancient Greek προβοσκίς (proboskís) 'elephant's trunk') is a taxonomic order of afrotherian mammals containing one living family (Elephantidae) and several extinct families. First described by J. Illiger in 1811, it encompasses the elephants and their close relatives.^[1] Proboscideans include some of the largest known land mammals. The largest land mammal of all time may have been a proboscidean; the elephant Palaeoloxodon namadicus has been estimated to be up to 5.2 m (17.1 ft) at the shoulder and may have weighed up to 22 t (24.3 short tons), surpassing the paraceratheres, the otherwise largest known land mammals, though this estimate was made based on a single fragmentary femur and is speculative.^[2] The largest extant proboscidean is the African bush elephant, with a record of size of 4 m (13.1 ft) at the shoulder and 10.4 t (11.5 short tons).^[2] In addition to their enormous size, later proboscideans are distinguished by tusks and long, muscular trunks, which were less developed or absent in early proboscideans.

Proboscidea Temporal range: Middle Paleocene-Holocene 60.0–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Proboscidean diversity: Indian elephant, Elephas maximus indicus, African bush elephant, Loxodonta africana and African forest elephant, Loxodonta cyclotis
Skeleton of Moeritherium
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Mirorder:	Tethytheria
Order:	Proboscidea Illiger, 1811
Subclades
†Eritherium †Moeritherium †Saloumia †Numidotheriidae †Barytheriidae †Deinotheriidae Elephantiformes

Three species of elephant are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. Elephantidae is the only surviving family of the order Proboscidea; extinct members include the mastodons, gomphotheres and stegodonts. The family Elephantidae also contains several extinct groups, including the mammoths and straight-tusked elephants. The distinctive features of proboscideans include a trunk, tusks, and massive legs. Large ear flaps are present in some proboscideans, including elephants. Some also have tough but sensitive skin; others, like the woolly mammoth, have a coat. The trunk is used for breathing, bringing food and water to the mouth, and grasping objects. Tusks, which are derived from the incisor teeth, serve both as weapons and as tools for moving objects and digging. The large ear flaps assist in maintaining a constant body temperature as well as in communication. The pillar-like legs carry their great weight.

Evolution

Over 180 extinct members of Proboscidea have been described.^[3] The earliest proboscideans, Eritherium and Phosphatherium are known from the late Paleocene of Africa.^[4] The Eocene included Numidotherium, Moeritherium and Barytherium from Africa. These animals were relatively small and some, like Moeritherium and Barytherium were probably amphibious.^[5][6] Later on, genera such as Phiomia and Palaeomastodon arose; the latter likely inhabited more forested areas. Proboscidean diversification changed little during the Oligocene.^[5]

A major event in proboscidean evolution was the collision of Afro-Arabia with Eurasia, during the Early Miocene, around 18-19 million years ago allowing proboscideans to disperse from their African homeland across Eurasia, and later, around 16-15 million years ago into North America across the Bering Land Bridge. Proboscidean groups prominent during the Miocene include the deinotheres, along with the more advanced elephantimorphs, including mammutids (mastodons), gomphotheres, amebelodontids (which includes the "shovel tuskers" like Platybelodon), choerolophodontids and stegodontids.^[7] Around 10 million years ago, the earliest members of the family Elephantidae emerged in Africa, having originated from gomphotheres.^[8] The Late Miocene saw major climatic changes, which resulted in the decline and extinction of many proboscidean groups such as amebelodontids and choerolophodontids.^[7] The earliest members of modern genera of Elephantidae appeared during the latest Miocene-early Pliocene around 5 million years ago. The elephantid genera Elephas (which includes the living Asian elephant) and Mammuthus (mammoths) migrated out of Africa during the late Pliocene, around 3.6 to 3.2 million years ago.^[9]

Over the course of the Early Pleistocene, all non-elephantid probobscideans outside of the Americas became extinct (including mammutids, gomphotheres and deinotheres), with the exception of Stegodon.^[7] Gomphotheres dispersed into South America during this era as part of the Great American interchange,^[10] and mammoths migrating into North America around 1.5 million years ago.^[11] At the end of the Early Pleistocene, around 800,000 years ago the elephantid genus Palaeoloxodon dispersed outside of Africa, becoming widely distributed in Eurasia.^[12] By the beginning of the Late Pleistocene, proboscideans were represented by around 23 species. Proboscideans underwent a dramatic decline during the Late Pleistocene as part of the Late Pleistocene extinctions, with all remaining non-elephantid proboscideans (including Stegodon, mastodons, and the American gomphotheres Cuvieronius and Notiomastodon) and Palaeoloxodon becoming extinct, with mammoths only surviving in relict populations on islands around the Bering Strait into the Holocene, with their latest survival being on Wrangel Island around 4,000 years ago.^[7][13]

The following cladogram is based on endocasts^[14]

Proboscidea	Phosphatherium esculliei   Numidotherium koholense   Moeritherium lyonsi   Deinotheriidae   Elephantiformes Palaeomastodon beadnelli   Elephantimorpha Mammutida  Mammut americanum   Zygolophodon borsoni   Choerolophodon pentelici Gomphotherium augustidens   Cuvieronius andium   Stegomastodon humboldti   Elephantoidea Stegodon insignis   Elephantidae Mammuthus meridionalis   Mammuthus primigenius   Mammuthus columbi   Elephas maximus   Loxodonta africana   Palaeoloxodon Palaeoloxodon antiquus   Palaeoloxodon falconeri	"plesielephantiforms" "mastodonts"

Morphology

Proboscideans experienced several evolutionary trends, such as an increase in size, which led to many giant species that stood over 4 m (13 ft 1 in) tall, and body masses of over 10 tonnes.^[15] As with other megaherbivores, including the extinct sauropod dinosaurs, the large size of proboscideans likely developed to allow them to survive on vegetation with low nutritional value.^[16] Their limbs grew longer and the feet shorter and broader.^[17] The feet were originally plantigrade and developed into a digitigrade stance with cushion pads and the sesamoid bone providing support, with this change developing around the common ancestor of Deinotheriidae and Elephantiformes.^[18]

The skull grew larger, especially the cranium, while the neck shortened to provide better support for the skull. The increase in size led to the development and elongation of the mobile trunk to provide reach. The number of premolars, incisors and canines decreased. The cheek teeth (molars and premolars) became larger and more specialised.^[17] In Elephantiformes, the second upper incisor and lower incisor were transformed into ever growing tusks.^[19][20] The tusks are proportionally heavy for their size, being primarily composed of dentine. In primitive proboscideans, a band of enamel covers part of the tusk surface, though in many later groups including modern elephants the band is lost, with elephants only having enamel on the tusk tips of juveniles. The upper tusks were initially modest in size, but from the Late Miocene onwards proboscideans developed increasingly large tusks, with the longest ever recorded tusk being 5.02 metres (16.5 ft) long belonging to "Mammut" borsoni found in Greece, with some mammoth tusks likely weighing over 200 kilograms (440 lb). The lower tusks are generally smaller than the upper tusks, but could grow to large sizes in some species, like in Deinotherium (which lacks upper tusks), where they could grow over 1.5 metres (4.9 ft) long, the amebelodontid Konobelodon has lower tusks 1.61 metres (5.3 ft) long, with the longest lower tusks ever recorded being from the primitive elephantid Stegotetrabelodon which are around 2.2 metres (7.2 ft) long.^[21]

The molar teeth changed from being replaced vertically as in other mammals to being replaced horizontally in the clade Elephantimorpha.^[22] While early Elephantimorpha generally had lower jaws with an elongated mandibular symphysis at the front of the jaw with well developed lower tusks/incisors, from the Late Miocene onwards, many groups convergently developed brevirostrine (shortened) lower jaws with vestigial or no lower tusks.^[23][24] Elephantids are distinguished from earlier proboscideans by a major shift in the molar morphology to parallel lophs rather than the cusps of earlier proboscideans, allowing them to become higher crowned (hypsodont) and more efficient in consuming grass.^[25]

Dwarfism

Main article: Dwarf elephant

 

Skeleton of Palaeoloxodon falconeri, one of the smallest known dwarf elephants, with an adult shoulder height less than 1 metre (3.3 ft)

Several species of proboscideans lived on islands and experienced insular dwarfism. This occurred primarily during the Pleistocene, when some elephant populations became isolated by fluctuating sea levels, although dwarf elephants did exist earlier in the Pliocene. These elephants likely grew smaller on islands due to a lack of large or viable predator populations and limited resources. By contrast, small mammals such as rodents develop gigantism in these conditions. Dwarf proboscideans are known to have lived in Indonesia, the Channel Islands of California, and several islands of the Mediterranean.^[26]

Elephas celebensis of Sulawesi is believed to have descended from Elephas planifrons. Elephas falconeri of Malta and Sicily was only 1 m (3 ft), and had probably evolved from the straight-tusked elephant. Other descendants of the straight-tusked elephant existed in Cyprus. Dwarf elephants of uncertain descent lived in Crete, Cyclades and Dodecanese, while dwarf mammoths are known to have lived in Sardinia.^[26] The Columbian mammoth colonised the Channel Islands and evolved into the pygmy mammoth. This species reached a height of 1.2–1.8 m (4–6 ft) and weighed 200–2,000 kg (440–4,410 lb). A population of small woolly mammoths survived on Wrangel Island as recently as 4,000 years ago.^[26] After their discovery in 1993, they were considered dwarf mammoths.^[27] This classification has been re-evaluated and since the Second International Mammoth Conference in 1999, these animals are no longer considered to be true "dwarf mammoths".^[28]

Classification

Below is an unranked taxonomy of proboscidean genera as of 2019.^{[29][30][31][32]}

• Order Proboscidea Illiger, 1811
  • †Eritherium Gheerbrant, 2009
  • †Moeritherium Andrews, 1901
  • †Saloumia Tabuce et al., 2019
  • †Suborder Plesielephantiformes Shoshani et al, 2001
    • †Family Numidotheriidae Shoshani & Tassy, 1992
      • †Phosphatherium Gheerbrant et al., 1996
      • †Arcanotherium Delmer, 2009
      • †Daouitherium Gheerbrant & Sudre, 2002
      • †Numidotherium Mahboubi et al., 1986
    • †Family Barytheriidae Andrews, 1906
      • †Omanitherium Seiffert et al., 2012
      • †Barytherium Andrews, 1901
    • †Family Deinotheriidae Bonaparte, 1845
      • †Chilgatherium Sanders et al., 2004
      • †Prodeinotherium Ehik, 1930
      • †Deinotherium Kaup, 1829
  • Suborder Elephantiformes Tassy, 1988
    • †Eritreum Shoshani et al., 2006
    • †Hemimastodon Pilgrim, 1912
    • †Palaeomastodon Andrews, 1901
    • †Phiomia Andrews & Beadnell, 1902
    • Infraorder Elephantimorpha Tassy & Shoshani, 1997
      • †Family Mammutidae Hay, 1922
        • †Losodokodon Rasmussen & Gutierrez, 2009
        • †Eozygodon Tassy & Pickford, 1983
        • †Zygolophodon Vacek, 1877
        • †Sinomammut Mothé et al., 2016
        • †Mammut Blumenbach, 1799
      • Parvorder Elephantida Tassy & Shoshani, 1997
        • †Family Choerolophodontidae Gaziry, 1976
          • †Afrochoerodon Pickford, 2001
          • †Choerolophodon Schlesinger, 1917
        • †Family Amebelodontidae Barbour, 1927
          • †Afromastodon Pickford, 2003
          • †Progomphotherium Pickford, 2003
          • †Eurybelodon Lambert, 2016
          • †Serbelodon Frick, 1933
          • †Archaeobelodon Tassy, 1984
          • †Protanancus Arambourg, 1945
          • †Amebelodon Barbour, 1927
          • †Konobelodon Lambert, 1990
          • †Torynobelodon Barbour, 1929
          • †Aphanobelodon Wang et al., 2016
          • †Platybelodon Borissiak, 1928
        • †Family Gomphotheriidae Hay, 1922 (paraphyletic)
          • "trilophodont gomphotheres"
            • †Gomphotherium Burmeister, 1837
            • †Blancotherium May, 2019
            • †Gnathabelodon Barbour & Sternberg, 1935
            • †Eubelodon Barbour, 1914
            • †Stegomastodon Pohlig, 1912
            • †Sinomastodon Tobien et al., 1986
            • †Notiomastodon Cabrera, 1929
            • †Rhynchotherium Falconer, 1868
            • †Cuvieronius Osborn, 1923
          • "tetralophodont gomphotheres"
            • †Anancus Aymard, 1855
            • †Paratetralophodon Tassy, 1983
            • †Pediolophodon Lambert, 2007
            • †Tetralophodon Falconer, 1857
        • Superfamily Elephantoidea Gray, 1821
          • †Family Stegodontidae Osborn, 1918
            • †Stegolophodon Schlesinger, 1917
            • †Stegodon Falconer, 1857
          • Family Elephantidae Gray, 1821
            • †Subfamily Stegotetrabelodontinae Aguirre, 1969
              • †Stegodibelodon Coppens, 1972
              • †Stegotetrabelodon Petrocchi, 1941
              • †Selenotherium Mackaye, Brunet & Tassy, 2005
            • Subfamily Elephantinae Gray, 1821
              • †Primelephas Maglio, 1970
              • Loxodonta Anonymous, 1827
              • †Palaeoloxodon Matsumoto, 1924
              • †Mammuthus Brookes, 1828
              • Elephas Linnaeus, 1758

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Bibliography

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