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 Late Miocene through to the Late Pleistocene, until 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).^[2]

Elasmotherium Temporal range: Late Miocene to Late Pleistocene, 7–0.039 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Reconstructed E. caucasicum skeleton, Azov Museum of History, Archaeology and Palaeontology
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	Rhinocerotidae
Subfamily:	†Elasmotheriinae
Genus:	†Elasmotherium Fischer, 1808[1]
Type species
†Elasmotherium sibiricum Fischer, 1809
Other Species
†E. caucasicum Borissiak, 1914 †E. chaprovicum Shvyreva, 2004 †E. peii Chow, 1958 †E. primigenium Sun et al., 2021
Approximate range map for Elasmotherium
Synonyms
Stereoceros Enigmatherium E. fischeri = E. sibiricum E. inexpectatum = E. caucasicum

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.^[3] Species of Elasmotherium are the largest known true rhinoceroses, with the youngest and best known species, Elasmotherium sibiricum, having an estimated body mass of around 4.5 tonnes (9,900 lb), comparable to an elephant. The skull exhibits a large, hollow dome on the top of the skull roof. 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 conjecured and depicted as having borne an enormous nearly 2 metres (6 ft 7 in) long straight horn projecting from the dome. However, a 2021 study found that such a horn was implausible due to the thinness of the bone 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 keratinous pad that may have resembled a small horn in mature males.

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 diet on low-growing vegetation. It may have used its well muscled head to turn over the soil to feed on roots and other subterranean parts of plants.

Taxonomy

The "Moscow mandible", holotype of E. sibiricum

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.^[4][5]

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 sibericus 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 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.^[5]

Due limited known remains and the unique morphology of Elasmotherium, for much of the 19th century its affinities to other mammals were the subject of considerable speculation. Fischer suggested in his original description that Elasmotherium was a "pachyderm which finds a place between elephants and rhinoceroses", while others such as Georges Cuvier in 1852, Anselme Gaëtan Desmarest in 1820, and Richard Owen in 1845 saw it as a "missing link" between rhinoceroses and horses, as a edentate by Henri Marie Ducrotay de Blainville in publications from 1839-1864, and as an aquatic, possibly even marine mammal by Henri Milne-Edwards in 1868. Johann Jakob Kaup in 1840-41 was the first author to recognise that Elasmotherium was a rhinoceros, a view followed by Charles Lucien Bonaparte in 1845, who created the group Elasmotheriina to house it. Other authors supporting Elasmotherium as a rhinoceros include George Louis Duvernoy in 1852, who in 1855 erronously assigned a partial skull correctly attributed to Elasmotherium by Kaup to the new genus and species Stereoceros gall. In 1877, Brandt described the first complete skull of Elasmotherium sibiricum, which put to rest the debate about its affinities and demonstrated unequivocally that Elasmotherium was a rhinoceros.^[6]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 synonym of E. fischeri, named by Desmarest in 1820, which itself is now considered a synonym of the type species E. sibiricum.^[5]

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.^[7] Dozens of crania have been reconstructed and given identifiers. The division into species is based mainly on the fine distinctions of the teeth and jaws and the shape of the skull.^[8]

Evolution

Elasmotherium belongs to the subfamily Elasmotheriinae, distinct from the subfamily which includes all living rhinoceroses, 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,^[2] while younger estimates place the split during the Oligocene, around 35-23 million years ago.^[9][10] Unambiguous members of Elasmotheriinae first appeared during the Early Miocene, and the subfamiliy was speciose and widespread across Europe, Africa and Asia during the Miocene epoch.^[11]

Cladogram after Lu, Deng and Pandolfi (2023):^[12]

Rhinocerotidae	†Trigonias †Ronzotherium †Epiaceratherium †Molassitherium †Skinneroceras †Aceratheriinae (including Teleoceratini) †Protaceratherium †Diceratherium †Menoceras Rhinocerotinae (modern rhinoceroses) †Elasmotheriinae †Turkanatherium †Hispanotherium beonense †Hispanotherium matritense †Iranotherium †Sinotherium †Paraelasmotherium †Ningxiatherium †Elasmotherium

Elasmotherium is the only known member of Elasmotheriinae to haved survived after the Early Pliocene.^[13] with elasmotheriines declining as part of a broader decline of rhinocerotids and many other species of mammals during the late Miocene period.^[14] 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.^[15] Elasmotherium arrived in Eastern Europe around 2.5 million years ago, during the earliest part of the Pleistocene epoch.^[16]

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.^[17]

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.^[18]

First published restoration (1878) of E. sibiricum, by Rashevsky, under supervision of A.F. Brant

An elasmotherian species turned up in the preceding Khaprovian or Khaprov Faunal Complex, which was at first taken to be E. caucasicum,^[19] and then on the basis of the dentition was redefined as a new species, E. chaprovicum (Shvyreva, 2004), named after the Khaprov Faunal Complex.^[8] 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.^[20]

E. peii was first described by (Chow, 1958) for remains found in Shaanxi, China.^[21] 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.^[18] It is mainly found in the Psekups faunal complex between 2.2 and 1.6 Ma,^[18] and additional remains from Shaanxi were described in 2018.^[22]

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.^[23][24] 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.^[25]

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,^[26] and into Central Asia in western Kazakhstan.^[27]

Description

Skeletal reconstruction of E. sibiricum with a dome instead of large horn.

Size of Elasmotherium sibiricum compared to a human

Depiction of Elasmotherium sibiricum with a keratinous covering of the dome, as has been conjectured by some modern authors

Elasmotherium is the largest known rhinocerotid,^[28] and one of the largest members of Rhinocerotoidea, excluding the largest paraceratheres like Paraceratherium.^{[29]supplementary material} The known specimens of E. sibiricum reach up to 4.5 m (15 ft) in length, with shoulder heights up to 2.5 m (8 ft 2 in), while E. caucasicum reaches at least 5 m (16 ft) in body length with an estimated mass of 3.5–5 tonnes (3.9–5.5 short tons),^[5][30] These make Elasmotherium comparable in size to the woolly mammoth and larger than the contemporary woolly rhinoceros.^[31][32] The feet were unguligrade, the front larger than the rear, with three digits at the front and rear, with a vestigial fifth metacarpal.^[33] The cervical vertebrae of the neck were very robust. The thoracic vertebrae of the back have elongate neural spines, reaching up to 50 centimetres (20 in) in length.^[5] The neck of Elasmotherium is thought to have been heavily muscled.^[27]

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.^[5]

Skull

The skulls of E. sibiricum were large and relatively elongate, reaching a length of 86–89 centimetres (34–35 in) in mature male individuals. The skull is less than half as wide at maximum width as it is long. The nasal septum is heavily ossified (bony) rather than cartilaginous.^[27]

The most conspicuous feature of the skull of Elasmotherium is the large dome/protuberance on the top of the frontal bone of the skull roof, generally around 25–35 centimetres (9.8–13.8 in) in diameter. The development of the dome depended on age and sex, being more developed in adult and especially in male individuals. The dome is covered in a rough hummocky texture of uniformly spaced tubercules, though the degree to which this roughness was developed varies considerably between individuals, with some individuals, assume to be female, exhibiting relatively little roughness on the dome. Unlike living rhinoceroses, these tubercules were not arranged into ring-like structures. Along the side of the domes ran sulci grooves, which in life would have supported arterial blood vessels. The interior of the dome was largely hollow and formed an extension of the nasal cavity, with the outer wall being relatively thin, varying from 0.5–1.35 centimetres (0.20–0.53 in) thick across the dome.^[27]

E. caucasicum reconstructed without hair and with the traditional large horn

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

Elasmotherium had two premolars and three molars in each half of both the upper and lower jaws, and lacked any incisors or canines.^[5] These teeth were very high crowned (hypsodont), the highest crowned among ungulates, and evergrowing (hypselodont) similar to those of many rodents but uniquely among rhinoceroses (ever growing teeth also evolved in South American notoungulates).^[34][27] In E. sibiricum, the teeth completely lacked roots in adult teeth, though some earlier Elasmotherium species had roots in their adult teeth.^[27] The cheek teeth exhibit highly complex laminated enamel folding.^[5] The roughness of the nasal bone and the interior of the nasal septum suggests that Elasmotherium had a relatively mobile prehensile upper lip.^[27]

Cranial ornamentation

Closeup of the head of Elasmotherium sibiricum, following the keratinous pad intepretation of Titov and colleagues, 2021

Beginning with the first reconstructions of Elasmotherium by Rashevsky under Brant's guidance in 1878, many authors have contented that the dome in Elasmotherium served as the base for a large keratinous horn,^[5] often conjectured to be around 2 metres (6 ft 7 in) long.^{[29]supplementary material} The inferred large musclature of the neck has been used to support the hypothesis, with supporters arguing the neck muscles were needed to support a heavy horn.^[35] This interpretation, while widely popularised in encyclopedias and other works,^[5] has not been without its critics, such as Valentin Teryaev in 1948, who argued that a much smaller covering of the dome was present instead.^[27]

A 2021 study by Vadim Titov and colleagues affirmed Terayev's view, and found that the cranial dome was quite fragile because it was largely hollow, and only had a thin outer wall, and was therefore ill suited for a large horn, and the pattern of the rugosity of the skull did not match the structure found at the base of the horns of living rhinoceroses, and was more indicative of a smaller keratinous covering that may have resembled a small horn in mature male individuals, and that the dome primarily functioned to enhance the sense of smell, and possibly secondarily as a resonating chamber for vocalisation. The also proposed the front tip of the snout had a keratinous pad that was used for scraping the ground.^[27]

Palaeobiology

The angle of the plane of the occipital bone with the skull base is around 105–115º , indicating that the head was habitually held low, which has resulted in it being widely agreed among researchers that Elasmotherium fed on low-growing vegetation.^[27][36]

A number of researchers, including Russian palaeontologist Valentin Teryaev in 1948, as well as publications by Zhegallo and colleagues in the 2000s, have argued that Elasmotherium was a semi-aquatic animal that lived close proximity to water, such as riverbanks and ponds, feeding on aquatic and nearshore plants with an ecology similar to a hippopotamus.^[27] However, other researchers have disagreed with this assessment, and argued that Elasmotherium primarily inhabited open terrestrial environments, including steppe, forest steppe and meadows.^[27][2]

Restoration of E. sibiricum in a steppe environment

Modern high crowned (hypsodont) hoofed mammals are generally grazers of open environments,^[37] with hypsodonty possibly an adaptation to chewing tough, fibrous grass.^[38] Dental wear analysis indicates that Elasmotherium had a highly abraisive diet, likely as a result of a high amount of sandy soil covering the food it ingested.^[27]

In the 21st century, a number of authors have argued multiple lines of evidence, including skull morphology and carbon and nitrogen isotope ratios (which are highly distinct from those of other rhinoceroses), that Elasmotherium heavily fed on roots and other underground parts of plants such as bulbs and tubers, perhaps even to a greater degree than above-ground parts of plants, likely using their snouts and prehensile lips, in combination with strong side to side head movement enabled by their large neck muscles, to excavate soil surrounding its food. Elasmotherium may have moved between different habitats, such as forest, steppe and meadows, seasonally to feed on different plant resources.^[27]

At the latest Middle Pleistocene-early Late Pleistocene Irgiz 1 site in European Russia, 7 Elasmotherium sibiricum individuals, ranging from adults from juveniles, appear to have died together in a castrophic event. Dental microwear analysis, which records an animals diet in their last days/weeks of life, suggests they switched from a low-growing vegetation/grazing diet typical for Elasmotherium (as indicated by their tooth mesowear, which records their long-term diet), towards an atypical browsing diet shortly before their death. This may suggest that the Elasmotherium individuals died due to starvation as a result of the ground being covered in an abnormally thick layer of snow, forcing them to attempt to feed on shrubs and other vegetation above the snow layer.^[39]

Relationship with humans and extinction

Upper Paleolithic cave art from Rouffignac cave in France, which has been controversially argued to depict Elasmotherium

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 from specimens obtained from southern European Russia shows its persistence in the region until at least 39,000 years ago, indicating that Elasmotherium survived well into the Last Glacial Period. This places Elasmotherium's extinction within the timeframe of the Late Pleistocene megafauna extinctions, when most large terrestrial animals became extinct, generally thought to be the result of climate change, hunting by the globally expanding population of modern humans, or the combination of both. However, there is very little evidence of human interaction with Elasmotherium, and their remains have never been found in any confirmed archaeological context. The last records of Elasmotherium, which date to Marine Isotope Stage 3, correspond to a period of pronounced climatic and environmental change induced by the oscillation between relatively warm interstadial and cool stadial periods, with the last records dating to around the onset of Heinrich stadial 4, caused by a Heinrich event (a large group of icebergs simultaneously breaking off the Laurentide Ice Sheet and travelling into the North Atlantic), corresponding with pronunced cooling, the disruption of grass and herb dominated ecosystems and the spread of the mammoth steppe-tundra across northern Eurasia.^[2]

A claimed Late Pleistocene presence in Western Europe, based a cave painting in Rouffignac Cave in France, which depicts a rhinoceros with a large single horn, has been doubted, because of the complete lack of any Western European remains of Elasmotherium,^[6]46-47and other authors have considered this to be a depiction of the woolly rhinoceros.^[5] Other claimed depictions of Elasmotherium have been reported Shulgan-Tash Cave in the southern Russian Urals, though other authors have considered the claims to be unproven, due to the ambiguous nature of the ochre drawings.^[5]

Since the 19th century, Elasmotherium has been claimed to be the origin of folkloric creatures, particularly the unicorn, though the evidence in support of this hypothesis is circumstantial, and Elasmotherium does not appear to have inhabited the areas in which the mythological unicorn is thought to have originated, and the depiction of the unicorn may have been influenced by living rhinoceroses rather than Elasmotherium.^[5] As a consequence, E. sibiricum has sometimes been nicknamed the "Siberian Unicorn".^[2]

See also

• Paleontology portal
• Prehistoric mammals portal

• Aegyrcitherium
• Bugtirhinus
• Hispanotherium
• Sinotherium
• Coelodonta