Elasmotherium

Elasmotherium ("Thin Plate Beast") is an extinct genus of giant rhinoceros endemic to Eurasia during the Late Pliocene through the Pleistocene, documented from 2.6 Ma to as late as 50,000 years ago, possibly later, in the Late Pleistocene, an approximate span of slightly less than 2.6 million years. Three species are recognised. The best known, E. sibiricum was the size of a mammoth and is thought to have borne a large, thick horn on its forehead which was used for defense, attracting mates, driving away competitors, sweeping snow from the grass in winter and digging for water and plant roots. Like all rhinoceroses, elasmotheres were herbivorous. Unlike any others, 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.

Discovery
The fossil received its name from Johann Fischer von Waldheim,[2] the Dirécteur Perpétuel of the Natural History Museum, Moscow University, at a presentation before the Societé Impériale des Naturalistes in 1808. The next year in the Mémoires of the society he reported what he had said in the Programme d'invitation:[3]

Elasmotherium is an animal with an elongated head without incisors or canines, with 5 molars on each side [made] from sinuous layers.[4]

Then he noted on his derivation of the name:

From the Greek word ἐλασμος [elasmos], layer, to designate the layered form of the molar teeth.[5]

All he had before him was one lower jaw donated to the museum by Yekaterina Romanovna Vorontsova-Dashkova, which he named Elasmotherium sibiricum, lamenting that it was the sole species of which he knew. The molars, the only teeth in the jaw, had formed in layers like tree rings, except the "rings", or lamellae, were highly corrugated. The edges in the grinding surface were elaborately sinuous to better break down the grasses on which the animal fed.

Description
Various theories of Elasmothere morphology, nutrition and habits have been the cause of wide variation in reconstruction. Some show the beast trotting like a horse with a horn; others hunched over with head to the ground, like a bison, and still others immersed in swamps like a hippopotamus. The use of the horn and whether or not there was one, and how large, have been popular topics. The statistical correlations of modern paleontology have taken much of the speculation out of the subject, although some details remain undetermined.

Morphology
The known specimens of E. sibiricum reach up to 4.5 m (15 ft) in body length with shoulder heights 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), based on isolated molars that significantly exceed those known from the Siberian species.[6] Both species were among the largest in the family Rhinocerotidae, comparable in size to the woolly mammoth and larger than the contemporary woolly rhinoceros.[7][8] The feet were unguligrade, the front larger than the rear, with 4 digits at the front and 3 at the rear.

Diet
Herbivores can be divided into two general groups on the basis of nutrition, which grade into each other morphologically: "foregut fermenters" and "hindgut fermenters". The border region is correlated to bulk: up to 600 to 1,200 kilograms (1,300 to 2,600 lb) are the former; over it, the latter. In foregut fermentation the animal must "browse" to select the most nutritious plants and then ruminate to make up for the shorter digestive tract. The hind-gut fermenters are "bulk-feeders": they ingest large quantities of low-nutrient food, which they process for a longer time in a much longer intestine. The main food in that category is grass, indicating that Elasmotherium, like the elephants, was probably a grassland "grazer" moving over long distances to take advantage of the growth phases of grass in different regions.[10] The standard is not without exception, as Indricotherium, the largest land mammal ever, with a weight of 15–20 tons, subsisted by browsing the treetops.

There are also paleontological indications of the grazing. In general, the normal position of the head can be determined by the angle between a vertical plane coinciding with the occiput of the cranium, which is always vertical, and a plane through the base of the cranium. A right or acute angle would indicate a head held high for browsing leaves at various heights. Elasmotherium had the most obtuse angle of the Rhinocerotids. It could only reach the lowest levels and therefore must have grazed habitually.[11] This morphological feature favors the identification of the one-horned beast depicted in Rouffignac Cave (shown in this article) as Elasmotherium and lends some validity to the bison-like restorations based on it.

Dentition
A third type of evidence that Elasmotherium was a grazer is that of tooth wear and morphology. Like all Rhinocerotidae, E. had cheek teeth evolved for herbivory: two premolars and three molars (originally taken for five molars), no incisors, no canines.[12] Where some of the browsers kept the incisors in the form of tusks, E, had instead a spoon-like symphysis, or tip, of the lower mandible and a rostrum, or beak, of the upper, which served as a bony basis for a soft-tissue labial grasping and tearing mechanism.[13] Grass, a very tough, fibrous material, contains phytoliths, microscopic granules mainly of silica, which act as sandpaper on the molars of grazers. Their response in geologic time is to evolve cheek teeth with large crowns (hypsodonty). There appears to be a correlation between grazing and hypsodonty:[14]

As a general rule, extant herbivores with low-crowned teeth are predominantly browsers and species with high-crowned teeth are predominantly grazers.

Vladimir Onufryevich Kovalevsky first proposed a connection between hypsodonty and grazing for horses in 1873.[15] Since then the concept has been expanded to all mammalian grazers at any time and has further been elaborated into hypsodonty or proto-hypsodonty and hypselodonty or euhypsodonty.[16] The euhypsodonts, of which, surprisingly for its bulk, E. was one,[17] have ever-growing high-crowned teeth. Most other examples are to be found among diminutive mammals such as Rodentia, which already casts doubt on the correlation, as they do not generally graze grass.

Teeth form from the top down through the deposition of enamel on a cement core by formative soft tissue in the jaw. The enamel of hypsodont Perissodactyla is highly rugose rather than sharp. In brachydont species, such as humans, when the crown is complete, the roots are deposited and finally the completed tooth erupts. Hypselodonty is a condition of tooth eruption and continued crown formation before a delayed root formation. In its most developed variety, the roots never form. Only rare fossils of E. show any sign of a root, and that on a premolar. No molars have roots, or, in the terminology of some, the roots are "open".

Habitat
If all grazers are hypsodont, not all hypsodonts are grazers. The supposed correlation between grass-eating and hypsodonty proved difficult to support in a number of instances. Koenigswald, for example, pointed out that hypsodonty had occurred among the Gondwanatheria of the Mesozoic, a group of mammals so primitive that he describes their cheek teeth as "molariform", as they are neither clearly molars nor premolars.[18] Molariform hypsodonty "cannot be correlated with a grass diet, since grasses were not present". Instead he suggests for his example, Sudamerica ameghinioi, that it lived a "semiaquatic and perhaps a burrowing way of life". Modern species provide many examples from beavers to hippopotamuses.

Attempts have been made to link the wear on Elasmotherium teeth to grazing. In 1938, H.E. Wood, a Rhinocerotid tooth specialist, pointed out that interproximal wear, or loss of tooth surface between teeth, due to abrasion during mastication, of Elasmotherium is similar to that of the white rhinoceros, the only remaining Rhinocerotid grazer, which has hypsodont teeth.[19] Data such as this led to an intuitive concept of some sort of correlation between grass-eating and hypsodonty, but it has been difficult to isolate mathematically.

A 2007 study by Mendoza and Palmqvist compared the habitats and diets of 134 species of living ungulates, for which this data is known, both Artiodactyla and Perissodactyla, against "thirty-two craniodental measurements" to discover what correlations exist for the group studied and to test hypotheses concerning hypsodonty and mode of life.[20] Habitats considered were open (savanna, deserts), mixed (wooded savanna, brush) and closed (riverine and forest). Diets considered were grazing, mixed grazing and browsing, browsing, omnivory and special niches, such as treetop browsing. Measurements included the Hypsodonty Index (HI) and Muzzle Width (MZW). The results showed that, except for the "high-level" browsers, hypsodonty is correlated to "open and mixed habitats". The HI was not precise enough to discriminate between open and mixed. However, high MZW is correlated to grazing in the open category, although some forest species also have wide muzzles. Grazers therefore are distinguished by a combination of high-crowned cheek teeth and wide muzzles, both of which are possessed by Elasmotherium. Life in the open is implied.

Cursorial distal limbs
The preponderance of evidence is that Elasmotherium, as far as is known now, was a grassland grazer. It is in this context that Deng and Zheng, experts in the few surviving leg bones, conclude, concerning the morphology of the legs:[21]

Combined with hypsodont cheek teeth with much cement and strong enamel plication, the slender distal limb bones of E. caucasicum indicate that it is cursorial and dwells in an open steppe as a typical grazer.

By distal they mean "furthest outward"; that is, the extremities. Cursorial animals are unequivocally "runners" although the authors did not examine what sort of gait scientifically should be proposed as "running". They selected caucasicum for study because of the availability of a few dozen limb bone fragments from Nihewan, China. These made possible a selective comparison with the fewer bones remaining of other fossil rhinocerotids. In comparison with them, the long legs of Elasmotherium. are the most derived; that is, the others did not have the same cursorial capabilities. The authors approach but do not solve the problem of how to reconcile the weight with the supposed mobility. They say elsewhere in the article that the legs of caucasicum are to be distinguished from those of other fossil Rhinocerotids at Nihewan by their "enormous size".[22]

The white rhinoceros at an estimated weight of 2.5 ton[ambiguous] has been photographed galloping at a speed of about 30 km/h (19 mph). In a gallop, all feet are off the ground ("ballistic phase") twice a cycle, a feat that elephants, at 2.5-11 ton, cannot perform. They can walk up to 20 km/h (12 mph); however, their straight, relatively inflexible legs are those of striders, not the bent and spring-like legs of gallopers, which utilize haunches, ankle mobility and knee flexion to spring off the ground on alternative legs of a pair. Elasmotherium legs are sufficiently like those of the White Rhino to hypothesize a similar gait even though Elasmotherium weighed 4.5-5 ton.

Horn morphology
The Ceratomorpha are so-called because their families, such as the Rhinocerotidae, of which Elasmotherium is undisputably one, are characterized by the presence of hooves, or horns and hooves, made of keratin, the same substance of which hair is made. These keratin structures appear to have formed in the Mesozoic, a remnant in humans being the nails. A keratin horn is to be distinguished from a bone horn and a tusk. Bone forms the base of most horns but in some cases the horn is entirely of bone. A tusk is a modified canine or incisor tooth. Rhinocerotidae have had tusks, but not Elasmotherium. Two open questions are whether they were horned or hornless, hairy or hairless. Most Rhinocerotidae have and have had horns, but there are some instances of hornlessness, and most are or were hairy, such as the Wooly Rhinoceros, but no instances of hair or horn have yet been found for Elasmotherium. Only circumstantial evidence of them exists.

The main evidence suggestive of a horn on Elasmotherium is a frontal protuberance, which struck the attention of the late 19th century paleontologists and was immediately interpreted as the bony basis for a horn by most investigators from that time forward. A skull of E. sibiricum from the Volga region (cast shown in this article's lead picture) described by Alexander Brandt in the Russian journal, Niwa, and reported in Nature in 1878 offers the following description of the protuberance: hemispherical, 5 inches (13 cm) deep, furrowed surface, circumference of 3 feet (0.91 m). The furrows are interpreted as the seats of blood vessels for the tissues that generated the horn:[24]

The whole analogy with the rhinoceros points with the greatest certainty to the previous existence of a horn, which, to judge from the size of the blood vessels once encircling the base, must have possessed enormous dimensions.

Brandt was already familiar with the legend of a unicorn among the Tatars of Siberia with a horn so large it required a sledge for transport, and made the connection in interpreting the bump as the base of a horn. He also interpreted the rostrum of the upper mandible as the basis of a nasal horn, a hypothesis now rejected in favor of the cropping labia. In any case the non-circular base indicates a section through the horn would not have been circular. This possibility is supported by another fossil with a non-circular partially healed puncture wound in the base, chiefly interpreted as the result of dueling other males with the horn.[13]

The ungulates typically combine keratin and bone in various structures. If horns are keratinous, they have a bone core. Rhinocerotids horns, however, are uniquely derived. Hieronymus, an expert in Rhinoceros dermatology, says:[25]

... extant rhinocerotids are unique in possessing a massive entirely keratinous horn that approximates the functions of keratin-and-bone horns such as those of bovid artiodactyls ...."

He defines rhinocerotid horns as:

... cylindrical blocks of constantly growing cornified papillary epidermis.

This tissue is "strikingly convergent" with other "cornified epidermis" in horses, cetaceans, artiodactyls and birds. The horn is not attached to the bone of the boss but grows from the surface of a dense dermal tissue. The top layer keratinizes itself to form tubules about 1-2 millimeter high, the cells of which then die. The next layer forms below it.[26] As the layers age the horn loses diameter by degradation of the keratin due to ultraviolet light, desiccation and mechanical wear from contact with objects and agonistic behavior.[27] However, melanin and calcium deposits in the center harden the keratin there, causing differential wear and shaping of the horn.[28]

The dermis generating the horn is anchored to the boss by interpenetration between rugosities – various irregularities of bone, which it creates by deposition.[29] This tissue is a specialization of dermal armor, which, whenever it attaches to bone, deposits the rugosities to strengthen the attachment. The author states that rugosity is "a bony signature of dermal armor".[30] Cranial rugosity is an indication, but not a sure sign, of a horn. If, on the other hand, an annular (ring-shaped) pattern is visible in the rugosity, it is due to "stress concentration at the edges of horns" and is "the signature for epidermal horns".

Hieronymus found annular rugosities in all living and some fossil Rhinocerotidae. The rings had previously been noted on additional fossils. To date Elasmotherium has not been examined for rings under lighting designed to show them up; however, based on the observations of other paleontologists, the author says "squamosal rugosity" is the "most pronounced cranial rugosity in the elasmotherine lineage".[31] This fact suggests an especially firm attachment was required, which, combined with the extraordinarily large hump of muscle for managing the head, suggest a large and heavy horn. In the early 19th century the state of the fossils had not yet revealed the presence of a horn. By 1910 and for all the time since then the paleontologists have not ventured a mathematical estimate but have preferred to refer to the horn as immense, enormous, great or huge. As the size and shape of the horn depended on the concentration of melanin and calcium, and no known indicator of those remains, any further estimate of horn morphology is purely speculative.

In 1968 Björn Kurtén's The Age of Dinosaurs pronounced, without a clue of the reasoning followed, that Elasmotherium "carried a single frontal horn two meters in length".[32] This figure is often repeated in non-technical books and articles, as it is the only one available. The number is speculative, but coincidentally it happens to be the shoulder height of E. sibiricum. If in fact Elasmotherium swept snow with the horn, or excavated holes in the terrain with it, the length can hardly have been less to reach the ground.