Elasmotherium

''Elasmotherium'' is an extinct genus of large rhinoceros that lived in Eastern Europe, Central Asia and East Asia during Late Miocene through to the Late Pleistocene, with the youngest reliable dates of at least 39,000 years ago. It was the last surviving member of Elasmotheriinae, a distinctive group of rhinoceroses separate from the group that contains living rhinoceros (Rhinocerotinae).

Five species are recognised. The genus first appeared in the Late Miocene in present-day China, likely having evolved from '' Sinotherium'', before spreading to the Pontic–Caspian steppe, the Caucasus and Central Asia. The best known ''Elasmotherium'' species, ''E. sibiricum'', sometimes called the Siberian unicorn, was among the largest known rhinoceroses, with an estimated body mass of around , comparable to an elephant, and is often conjectured to have borne a single very large horn. However, no horn has ever been found, and other authors have conjectured that the horn was likely much smaller. Like all rhinoceroses, elasmotheres were herbivorous. Unlike any other rhinos and any other ungulates aside from some notoungulates, its high-crowned molars were ever-growing, and it was likely adapted for a grazing diet. Its legs were longer than those of other rhinos and were adapted for galloping, giving it a horse-like gait.

Taxonomy

''Elasmotherium'' was first described in 1808-1809 by German/Russian palaeontologist Gotthelf Fischer von Waldheim based on a left lower jaw, four molars, and the tooth root of the third premolar, which was gifted to Moscow University by princess Ekaterina Dashkova in 1807. He first announced the genus name at an 1808 presentation before the Moscow Society of Naturalists, and named the type species ''E. sibiricum'' a year later in 1809. The genus name derives from Ancient Greek ''elasmos'' "laminated" and ''therion'' "beast" in reference to the laminated folding of the tooth enamel; and the species name ''sibericus'' is probably a reference to the predominantly Siberian origin of Princess Dashkova's collection. However, the specimen's exact origins are unknown. In 1877, German naturalist Johann Friedrich von Brandt placed it into the newly erected subfamily Elasmotheriinae, separate from modern rhinos.

The genus is known from hundreds of find sites, mainly of cranial fragments and teeth, but in some cases nearly complete skeletons of post-cranial bones, scattered over Eurasia from Eastern Europe to China. Dozens of crania have been reconstructed and given archaeological identifiers. The division into species is based mainly on the fine distinctions of the teeth and jaws and the shape of the skull.

Evolution

''Elasmotherium'' belongs to the subfamily Elasmotheriinae, distinct from the subfamily which includes all living rhinceroses, Rhinocerotinae. The depth of the split between Elasmotheriinae and Rhinocerotinae is disputed. Older estimates place the age of divergence around 47 million years ago, during the Eocene, while younger estimates place the split around 35 million years ago, during the Oligocene. Unambiguous members of Elasmotheriinae first appeared during the Early Miocene, and were widespread across Europe, Africa and Asia during the Miocene epoch.

''Elasmotherium'' is the only known member of Elasmotheriinae from after the Miocene, with elasmotheriines declining as part of a broader decline of rhinocerotids and many other species of mammals during the late Miocene period. The oldest known species of ''Elasmotherium'' is ''Elasmotherium primigenium'' from the Late Miocene of Dingbian County in Shaanxi, China. ''Elasmotherium'' likely evolved from '' Sinotherium'', a genus of elasmothere also found in China. ''Elasmotherium'' arrived in Eastern Europe around 2.5 million years ago, during the earliest part of the Pleistocene epoch.

Hypsodonty, a dentition pattern where the molars have high crowns and the enamel extends below the gum line, is thought to be a characteristic of Elasmotheriinae, perhaps as an adaptation to the heavier grains featured in riparian zones on riversides.

Species

There are four chronospecies of ''Elasmotherium'' aside from the aforementioned ''E. primigenium'', which are—from oldest to youngest—''E. chaprovicum'', ''E. peii, E. caucasicum'' and ''E. sibiricum,'' and which together span from the Late Pliocene to the Late Pleistocene.


An elasmotherian species turned up in the preceding Khaprovian or Khaprov Faunal Complex, which was at first taken to be ''E. caucasicum'', and then on the basis of the dentition was redefined as a new species, ''E. chaprovicum'' (Shvyreva, 2004), named after the Khaprov Faunal Complex. The Khaprov is in the Middle Villafranchian, MN17, which spans the Piacenzian of the Late Pliocene and the Gelasian of the Early Pleistocene of Northern Caucasus, Moldova and Asia and has been dated to 2.6–2.2 Ma.

''E. peii'' was first described by (Chow, 1958) for remains found in Shaanxi, China. The species is also known from numerous remains from the classical range of ''Elasmotherium,'' and some sources have considered this species to be a synonym of ''E. caucasicum'', but it is currently considered distinct. It is mainly found in the Psekups faunal complex between 2.2 and 1.6 Ma, and additional remains from Shaanxi were described in 2018.

''E. caucasicum'' was first described by Russian palaeontologist Aleksei Borissiak in 1914, who said it apparently flourished in the Black Sea region as a member of the Early Pleistocene Tamanian Faunal Unit (1.1–0.8 Ma, Taman Peninsula). It is the most commonly found mammal of the assemblage. ''E. caucasicum'' is thought to be more primitive than ''E. sibiricum'' and perhaps represents an ancestral stock. It is also known in northern China from the Early Pleistocene Nihewan Faunal assemblage and were extinct at approximately 1.6 Ma. This suggests ''Elasmotherium'' developed separately in Russia and China.

''E. sibiricum'', described by Johann Fischer von Waldheim in 1808 and chronologically the latest species of the sequence appeared in the Middle Pleistocene, ranging from southwestern Russia to western Siberia and southward into Ukraine and Moldova.

Description

''Elasmotherium'' is typically reconstructed as a woolly animal, generally based on the woolliness exemplified in contemporary megafauna such as mammoths and the woolly rhino. However, it is sometimes depicted as bare-skinned like modern rhinos. In 1948, Russian palaeontologist Valentin Teryaev suggested it was semi-aquatic with a dome-like horn, and resembled a hippo because the animal had four toes like a wetland tapir rather than the three toes in other rhinos, but ''Elasmotherium'' has since been shown to have had only three functional toes, and Teryaev's reconstruction has not garnered much scientific attention.

The known specimens of ''E. sibiricum'' reach up to in length, with shoulder heights up to , while ''E. caucasicum'' reaches at least in body length with an estimated mass of , making ''Elasmotherium'' the largest rhinos of the Quaternary. Both species were among the largest rhinos, comparable in size to the woolly mammoth and larger than the contemporary woolly rhinoceros. The feet were unguligrade, the front larger than the rear, with three digits at the front and rear, with a vestigial fifth metacarpal.

Dentition

Like other rhinos, ''Elasmotherium'' had two premolars and three molars for chewing, and lacked incisors and canines, relying instead on a prehensile lip to strip food. ''Elasmotherium'' were euhypsodonts, with large tooth crowns and enamel extending below the gum line, and continuously growing teeth.

''Elasmotherium'' fossils rarely show evidence of tooth roots.

Horn

''Elasmotherium'' is traditionally thought to have had a keratinous horn, indicated by a circular dome on the forehead, with a deep, furrowed surface, and a circumference of . The furrows are interpreted as the seats of blood vessels for horn-generating tissue.

In rhinos, the horn is not attached to bone, but grows from the surface of a dense skin tissue, anchoring itself by creating bone irregularities and rugosities. The outermost layer cornifies. As the layers age, the horn loses diameter by degradation of the keratin due to ultraviolet light, drying out, and continual wearing. However, melanin and calcium deposits in the centre harden the keratin there, which gives the horn its distinctive shape.

There was likely a large hump of muscle on the back, which is generally thought to have supported a heavy horn.

A 2021 study found that the cranial dome was quite fragile and ill suited for a large horn and was more indicative of a smaller horn, and that the dome could function as a resonating chamber of some sort, akin to that of '' Rusingoryx'' and hadrosaur crests.

Palaeobiology

Diet

Modern hypsodont hoofed mammals are generally grazers of open environments, with hypsodonty possibly an adaptation to chewing tough, fibrous grass. ''Elasmotherium'' dental wearing is similar to that of the grazing white rhino, and both of their heads have a downward orientation, indicating a similar lifestyle and an ability to only reach low-lying plants. In fact, the head of ''Elasmotherium'' had the most obtuse angle of any rhinoceros, and could only reach the lowest levels and therefore must have grazed habitually. ''Elasmotherium'' also displays euhypsodonty (evergrowing teeth), which is typically seen in rodents, and dental physiology could have been influenced by pulling up food from moist, grainy soil. Therefore, they may have inhabited both mammoth steppeland and riparian riversides, similar to contemporary mammoths.

Movement

''Elasmotherium'' had similar running limbs to the white rhinoceros–which run at with a top speed of . However, ''Elasmotherium'' had double the weight–about –and consequently had a more restricted gait and mobility, likely achieving much slower speeds. Elephants, weighing , cannot exceed a walking speed of .

Extinction

''Elasmotherium'' was previously thought to have gone extinct around 200,000 years ago as part of normal extinction, but ''E. sibiricum'' skull fragments from the Pavlodar Region, Kazakhstan, shows its persistence in the Western Siberian Plain about 39,000–35,000 years ago. Isolated remains dating to 50,000 years ago are known from the Siberian Smelovskaya and Batpak Caves, likely dragged there by a predator.

This timing is roughly coincident with the Pleistocene extinction, during which many mammal species with body weights greater than died out. This coincided with a shift to a cooler climate–which resulted in replacement of grasses and herbs by lichens and mosses–and the migration of modern humans into the area.

See also

* '' Aegyrcitherium''
* '' Bugtirhinus''
* '' Hispanotherium''
* '' Sinotherium''
* ''Coelodonta''

Notes

References

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External links

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{{Taxonbar, from=Q131678

Pleistocene mammals of Asia
Piacenzian first appearances
Pleistocene genus extinctions
Pliocene rhinoceroses
Unicorns
Pleistocene rhinoceroses
Fossil taxa described in 1808
Taxa named by Gotthelf Fischer von Waldheim