Proboscidea (/ˌprɒbəˈsɪdiə/; from
Latinproboscis, from
Ancient Greekπροβοσκίς (proboskís) 'elephant's trunk') is a
taxonomicorder 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.
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]
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]
Over the course of their evolution, proboscideans experienced a significant increase in body size. Some members of the families
Deinotheriidae,
Mammutidae,
Stegodontidae and
Elephantidae 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).[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] Members of
Elephantiformes which have retracted nasal regions of the skull indicating the development of a trunk, as well as well-developed tusks on the upper and lower jaws.[19]
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.[20][21] 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
amebelodontidKonobelodon 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.[22] Elephantiformes primitively had an elongated
mandibular symphysis (the region at the front of the lower jaw), but this region was shortened in many later species (including modern elephants), along with the reduction/loss of the lower tusks which happened
convergently multiple times across different groups.[23][24]
The molar teeth changed from being replaced vertically as in other mammals to being replaced horizontally in the clade
Elephantimorpha.[25] 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.[26][27] 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.[28]
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.[29]
^H. Saegusa, H. Nakaya, Y. Kunimatsu, M. Nakatsukasa, H. Tsujikawa, Y. Sawada, M. Saneyoshi, T. Sakai
Earliest 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-sauropod Amphicoelias 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.
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