Elasmotherium

***Elasmotherium*** is an extinct genus of large cenozoic rhinoceros endemic to Eurasia during the Late Pliocene through the Pleistocene, existing from 2.6 Ma to at least as late as 39,000 years ago in the Late Pleistocene.[2] A more recent date of 26,000 BP[3] is considered less reliable.[2]

Three species are recognised. The best known, *E. sibiricum*, known as the **Siberian unicorn**,[4] was the size of a mammoth and is thought to have borne a large, thick horn on its forehead. Theories about the function of this horn include defense against predators, attracting mates, driving away competitors, sweeping snow from the grass in winter, and digging for water and plant roots.[5][*unreliable source?*][6][*unreliable source?*] Like all rhinoceroses, elasmotheres were herbivorous. Unlike any other rhinos, its high-crowned molars were ever-growing. Its legs were longer than those of other rhinos and were adapted for galloping, giving it a horse-like gait.

*Elasmotherium* was first described in 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 it at an 1808 presentation before the Moscow Society of Naturalists.[7] The genus name derives from Ancient Greek *elasmos* "laminated" and *therion* "mammal" 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.[8]

In 1997, the McKenna/Bell classification considered *Elasmotherium* to be closely related to the wooly and modern rhinos, and placed it into the subfamily Rhinocerotinae.

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.[9] 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.

*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 4 toes like a wetland tapir rather than the 3 toes in other rhinos, but *Elasmotherium* has since been shown to have had only 3 functional toes,[8] and Teryaev's reconstruction has not garnered much scientific attention.[8][15]

The known specimens of *E. sibiricum* reach up to 4.5 m (15 ft) in length, with shoulder heights of over 2 m (6 ft 7 in), while *E. caucasicum* reaches at least 5 m (16 ft) in body length with an estimated mass of 3.6–4.5 tonnes (4–5 short tons),[8] making *Elasmotherium* the largest rhinos of the Quaternary.[2] Both species were among the largest rhinos, comparable in size to the woolly mammoth and larger than the contemporary woolly rhinoceros.[19][17] The feet were unguligrade, the front larger than the rear, with 3 digits at the front and rear, with a vestigial fifth metacarpal.[20]

### Dentition[edit]

*E. sibiricum* skull cast at the Museum für Naturkunde, Berlin

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.[8] *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.[*citation needed*]

### Horn[edit]

*Elasmotherium* is thought to have had a keratinous horn, indicated by a circular dome on the forehead, with a 5-inch (13 cm) deep, furrowed surface, and a circumference of 3 feet (0.91 m). The furrows are interpreted as the seats of blood vessels for horn-generating tissue.[21][22]

Restoration of *E. sibiricum*

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.[23] The outermost layer cornifies.[24] As the layers age, the horn loses diameter by degradation of the keratin due to ultraviolet light, drying out, and continual wearing.[25] However, melanin and calcium deposits in the centre harden the keratin there, which gives the horn its distinctive shape.[26]

There was likely a large hump of muscle on the back, which is generally thought to have supported a heavy horn.[27]