Elasmotherium

Elasmotherium (from Ancient Greek ἔλασμα (élasma), meaning "metal plate" with the intended meaning "lamina" in reference to the tooth enamel, and θηρίον (theríon), meaning "beast") is an extinct genus of very large rhinoceros that lived in Eastern Europe, Central Asia and East Asia from the late Miocene through to the Late Pleistocene, until at least 39,000 years ago. It was the last surviving member of the subfamily Elasmotheriinae, a formerly diverse group of rhinoceroses separate from the subfamily Rhinocerotinae, that contains all living rhinoceroses.

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. Species of Elasmotherium are the largest known true rhinoceroses, reaching body lengths of at least , shoulder heights of over , and estimated body masses of , comparable to an elephant.

The skull of Elasmotherium exhibits a large dome on the top of the skull roof, which is hollow and forms an enlargement of the nasal cavity. No remains of a horn of Elasmotherium have ever been found and the appearance of the horn, if any, has been subject to considerable speculation. Elasmotherium sibericum has often been conjectured and depicted as having borne an enormous nearly long straight horn projecting from the dome. However, a 2021 study found that such a horn was implausible due to the thinness of the outer wall of the dome and the lack of attachment points for a horn on the dome itself, and suggested that the dome (which they proposed was primarily for enhancing the sense of smell) was instead covered by a hard keratinous pad (comparable to the bosses of muskox and African buffalo) and that may have resembled a small horn in mature males.

Later Elasmotherium species are thought to have had a relatively narrow and specialised ecological niche as an inhabitant of the central Eurasian steppe. The high crowned (becoming evergrowing/hypselodont in E. sibiricum, uniquely among rhinoceroses) and complicated enamel pattern of Elasmotherium teeth indicate that they had an abrasive diet of low growing vegetation. Elasmotherium may have fed on grass and/or used its well muscled head to turn over the soil to feed on roots and other subterranean parts of plants.

Due to the widespread assumption that it bore a single elongate horn, Elasmotherium sibiricum has been claimed by some to be the basis of the mythical unicorn (and sometimes consequently nicknamed the "Siberian unicorn"), though the evidence in support of this hypothesis is highly speculative.

Taxonomy

left|thumb|The "Moscow mandible", [[holotype of E. sibiricum|261x261px]]

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.

thumb|Cheek teeth of Elasmotherium sibiricum, showing thin enamel exhibiting laminated folding from which the genus takes its name

The genus name derives from Ancient Greek ἔλασμα (élasma), meaning "metal plate", with the intended meaning "lamina", in reference to the laminated folding of the tooth enamel; and θηρίον (theríon), meaning "beast" and the species name sibericum is probably a reference to the predominantly Siberian origin of Princess Dashkova's collection. However, the specimen's exact origins are unknown. The specimen narrowly escaped destruction by being evacuated to Nizhny Novgorod during the French invasion of Russia in 1812, when most of the rest of Dashkova's collection was destroyed. The specimen was transferred to the Palaeontological Institute of the Academy of Sciences of the USSR (now the Russian Academy of Sciences) in Moscow during the mid-20th century.^24-25 In 1916, a new genus, Enigmatherium and species E. stavropolitanum were named by Pavlova based on a single tooth found in the Northern Caucasus. This species was later recognised as a synonym of E. fischeri, named by Desmarest in 1820, which itself is now considered a synonym of the type species E. sibiricum. while younger estimates place the split during the Oligocene, around 35-23 million years ago. Unambiguous members of Elasmotheriinae first appeared during the Early Miocene, and the subfamily was speciose and widespread across Europe, Africa and Asia during the Miocene epoch.

Cladogram of Rhinocerotidae after Borrani et al. 2025 (note: only a limited selection of elasmothere genera were included in the tree).

Cladogram of Elasmotheriina (the subgroup including core members of Elasmotheriinae) after Sun et al. 2023:

Elasmotherium is the only known member of Elasmotheriinae to haved survived after the Early Pliocene. with elasmotheriines declining as part of a broader decline of rhinocerotids and many other species of mammals during the late Miocene period. After originating and initially evolving in China (likely from Sinotherium) during the latest Miocene and Pliocene, Elasmotherium migrated westwards into Central Asia and Eastern Europe around 2.6 million years ago, during the earliest part of the Pleistocene epoch. Elasmotherium species are largely distinguished by differences in their tooth anatomy. 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. This species is characterised by the relative thickness and roughness of its irregularly folded tooth enamel, its massive metapodial bones, and the morphology of the talus bone of the foot, which are relatively low, have a narrow trochlea and a wide part closest to the (distal) tip of the foot. 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. This species is distinguished from other Elasmotherium species by several aspects of its dental anatomy, with the teeth being characterised by early closure of the roots, though the roots remain distinct from the crown of the tooth, and several morphological characters of the teeth, including the persistent presence of a postfossette (a depression of the tooth surface), as well as the shape of the protoloph and metaloph (which are raised areas of teeth).

E. sibiricum, described by Johann Fischer von Waldheim in 1808 and chronologically the latest species of the sequence appeared in the Middle Pleistocene, ranging northwards to southern-central Russia, westwards into Ukraine and Moldova in Eastern Europe, eastwards into eastern Kazakhstan and southwards to Uzbekistan in Central Asia, and into Azerbaijan in the Caucasus. and one of the largest members of Rhinocerotoidea, excluding the largest paraceratheres like Paraceratherium.^{supplementary material} The largest known specimens of E. sibiricum reach up to in length, with shoulder heights of over , These make Elasmotherium comparable in size to the woolly mammoth (Mammuthus primigenius) and larger than the contemporary woolly rhinoceros (Coelodonta antiquitatis).

The cervical vertebrae of the neck were very robust,thumb|Size of Elasmotherium sibiricum compared to a humanElasmotherium is typically reconstructed covered in a considerable layer of fur, generally based on the woolliness exemplified in contemporary megafauna such as woolly mammoths and the woolly rhinoceros. However, it is sometimes depicted as bare-skinned like modern rhinoceroses.

Palaeobiology

The angle made by the intersection of the plane of the occipital bone with the skull base is around 105–115°, indicating that the head was habitually held low,

Valentin Teryaev in 1948 argued that Elasmotherium was a semi-aquatic animal that lived within and in close proximity to water including riverbanks and ponds, feeding on aquatic and nearshore plants with an ecology and physical appearance similar to a hippopotamus. Part of Teryaev's argument was based on the supposed presence of a fourth functional digit in the forefeet of Elasmotherium, similar to tapirs which predominantly inhabit wet environments, but this idea was later shown to be incorrect. Some other authors, while not necessarily agreeing with Teryaev's idea of a semi-aquatic Elasmotherium, have suggested that it predominantly lived near water, such as along river valleys, and foraged on aquatic and semi-aquatic plants. thumb|Restoration of E. sibiricum in a steppe environment with traditional large horn

thumb|Depiction of Elasmotherium sibiricum with a small horn-like keratinous covering of the dome, as has been conjectured for mature males by Titov et al. 2021

Modern high crowned (hypsodont) hoofed mammals are generally grazers of open environments, with the wrinkled enamel of Elasmotherium teeth suggested to possibly be an adaptation to feeding on tough, fibrous grass. The high crowned teeth of Elasmotherium have either been suggested to be an adaptation for feeding on grass or to compensate from the wear caused by the intake of grit on the food it was eating.

Relationship with humans and extinction

Elasmotherium sibiricum was previously thought to have gone extinct during the late Middle Pleistocene, around 200,000 years ago as a background extinction, but radiocarbon dates obtained from specimens found in southern European Russia shows its persistence in the region until at least 39,000 and possibly as late as 35,000 years ago, indicating that Elasmotherium survived well into the Last Glacial Period.