Extinct members of Proboscidea include thedeinotheres,mastodons,gomphotheres,amebelodonts andstegodonts. The family Elephantidae also contains several extinct groups, includingmammoths andPalaeoloxodon. Proboscideans include some of the largest known land mammals, with the elephantPalaeoloxodon namadicus and mastodon"Mammut" borsoni suggested to have body masses surpassing 16 tonnes (35,000 lb), rivalling or exceedingparaceratheres, the otherwise largest known land mammals in size.[2] The largest living proboscidean is the African bush elephant, with a recorded maximum size of 4 meters (13.1 feet) at the shoulder and a weight of 10.4 tonnes (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.
Over 180 extinct members of Proboscidea have been described.[3] The earliest members of Proboscidea likeEritherium are known from thePaleocene of Africa, around 60 million years ago, the earliest proboscideans were much smaller than living elephants, withEritherium having a body mass of around 3–8 kilograms (6.6–17.6 lb).[4] By the lateEocene, some members of Proboscidea likeBarytherium had reached considerable size, with an estimated mass of around 2 tonnes,[2] while others likeMoeritherium are suggested to have been semi-aquatic.[5]
A major event in proboscidean evolution was the collision of Afro-Arabia with Eurasia, during the EarlyMiocene, 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 thedeinotheres, along with the more advancedelephantimorphs, includingmammutids (mastodons),gomphotheres,amebelodontids (which includes the "shovel tuskers" likePlatybelodon),choerolophodontids andstegodontids.[6] Around 10 million years ago, the earliest members of the familyElephantidae emerged in Africa, having originated from gomphotheres.[7] The Late Miocene saw major climatic changes, which resulted in the decline and extinction of many proboscidean groups such as amebelodontids and choerolophodontids.[6] The earliest members of modern genera of Elephantidae appeared during the latest Miocene-early Pliocene around 6-5 million years ago. The elephantid generaElephas (which includes the living Asian elephant) andMammuthus (mammoths) migrated out of Africa during the late Pliocene, around 3.6 to 3.2 million years ago.[8]
Over the course of theEarly Pleistocene, all non-elephantid probobscideans outside of the Americas became extinct (including mammutids, gomphotheres and deinotheres), with the exception ofStegodon.[6] Gomphotheres dispersed into South America during this era as part of theGreat American interchange,[9] and mammoths migrating into North America around 1.5 million years ago.[10] At the end of the Early Pleistocene, around 800,000 years ago the elephantid genusPalaeoloxodon dispersed outside of Africa, becoming widely distributed in Eurasia.[11] By the beginning of theLate Pleistocene, proboscideans were represented by around 23 species. Proboscideans underwent a dramatic decline during the Late Pleistocene as part of theLate Pleistocene megafauna extinctions, with all remaining non-elephantid proboscideans (includingStegodon,mastodons, and the American gomphotheresCuvieronius andNotiomastodon) andPalaeoloxodon becoming extinct, with mammoths only surviving inrelict populations on islands around theBering Strait into the Holocene, with their latest survival being onWrangel Island around 4,000 years ago.[6][12]
The following cladogram is based on endocasts.[13]
Over the course of their evolution, proboscideans experienced a significant increase in body size. Some members of the familiesDeinotheriidae,Mammutidae,Stegodontidae andElephantidae are thought to have exceeded modern elephants in size, with shoulder heights over 4 metres (13 ft) and masses over 10 tonnes (22,000 lb), with average fully grown males of the mammutid"Mammut" borsoni having an estimated body mass of 16 tonnes (35,000 lb), making it one the largest and perhaps the largest land mammal ever, with a fragmentary specimen of the Indian elephant speciesPalaeoloxodon namadicus only known from a partial femur being speculatively estimated in the same study to have possibly reached a body mass of 22 tonnes (49,000 lb).[2] As with othermegaherbivores, including the extinctsauropod dinosaurs, the large size of proboscideans likely developed to allow them to survive on vegetation with low nutritional value.[14] Their limbs grew longer and the feet shorter and broader.[15] The feet were originallyplantigrade and developed into adigitigrade stance with cushion pads and thesesamoid bone providing support, with this change developing around the common ancestor ofDeinotheriidae andElephantiformes.[16] Members ofElephantiformes and Deinotheriidae have retracted nasal regions of the skull indicating the development of a trunk.[17][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 ofpremolars, incisors andcanines decreased. The cheek teeth (molars and premolars) became larger and more specialised.[15] In Elephantiformes, the second upper incisor and lower incisor were transformed into ever growingtusks on the upper and lower jaws,[19][20] while in Deinotheriidae there are only tusks on the lower jaw.[18] The tusks are proportionally heavy for their size, being primarily composed ofdentine. 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 the mammutid"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 inDeinotherium (which lacks upper tusks), where they could grow over 1.5 metres (4.9 ft) long, theamebelodontidKonobelodon has lower tusks 1.61 metres (5.3 ft) long, with the longest lower tusks ever recorded being from the primitive elephantidStegotetrabelodon 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 cladeElephantimorpha.[22] While early Elephantimorpha generally had lower jaws with an elongatedmandibular 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 other 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]
Size comparison of the dwarf elephantPalaeoloxodon falconeri from the Pleistocene of Sicily and Malta to a human
Several species of proboscideans lived on islands and experiencedinsular 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 developgigantism in these conditions. Dwarf proboscideans are known to have lived inIndonesia, theChannel Islands of California, and several islands of theMediterranean.[26]
It has been suggested that members of Elephantimorpha, including mammutids,[29] gomphotheres,[30] and stegodontids,[31] lived in herds like modern elephants. Analysis of remains of the American mastodon (Mammut americanum) suggest that like modern elephants, that herds consisted of females and juveniles and that adult males lived solitarily or in small groups, and that adult males periodically engaged in fights with other males during periods similar tomusth found in living elephants. These traits are suggested to be inherited from the last common ancestor of elephantimorphs,[29] with musth-like behaviour also suggested to have occurred in gomphotheres.[32] All elephantimorphs are suggested to have been capable of communication viainfrasound, as found in living elephants.[33] Deinotheres may have also lived in herds, based on tracks found in the Late Miocene of Romania.[34] Over the course of the Neogene and Pleistocene, various members ofElephantida shifted from a browse-dominated diet towards mixed feeding or grazing.[35]
^H. Saegusa, H. Nakaya, Y. Kunimatsu, M. Nakatsukasa, H. Tsujikawa, Y. Sawada, M. Saneyoshi, T. SakaiEarliest elephantid remains from the late Miocene locality, Nakali, Kenya Scientific Annals, School of Geology, Aristotle University of Thessaloniki, Greece VIth International Conference on Mammoths and Their Relatives, vol. 102, Grevena -Siatista, special volume (2014), p. 175
^Carpenter, K. (2006). "Biggest of the big: a critical re-evaluation of the mega-sauropodAmphicoelias fragillimus Cope, 1878". In Foster, J.R.; Lucas, S.G. (eds.).Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin. Vol. 36. New Mexico Museum of Natural History and Science. pp. 131–138.
^Tikhonov, A.; Agenbroad, L.; Vartanyan, S. (2003). "Comparative analysis of the mammoth populations on Wrangel Island and the Channel Islands".Deinsea.9:415–20.ISSN0923-9308.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Shoshani, Jeheskel; Pascal Tassy (2005). "Advances in proboscidean taxonomy & classification, anatomy & physiology, and ecology & behavior".Quaternary International.126–128:5–20.Bibcode:2005QuInt.126....5S.doi:10.1016/j.quaint.2004.04.011.
^Wang, Shi-Qi; Deng, Tao; Ye, Jie; He, Wen; Chen, Shan-Qin (2017). "Morphological and ecological diversity of Amebelodontidae (Proboscidea, Mammalia) revealed by a Miocene fossil accumulation of an upper-tuskless proboscidean".Journal of Systematic Palaeontology.15 (8):601–615.Bibcode:2017JSPal..15..601W.doi:10.1080/14772019.2016.1208687.S2CID89063787.
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