Paraceratherium: Reconstructing the Largest Ever Land Mammal

Tuesday, May 1, 2018

Paraceratherium is often described as the largest land mammal ever to have lived. ABELOV2014/CC BY-SA 4.0

Piecing together a giant prehistoric rhinoceros is as hard as it looks.

Paraceratherium is a genus of hornless rhinoceros, and one of the largest terrestrial mammals that has ever existed. It lived from the early to late Oligocene epoch (34–23 million years ago); its remains have been found across Eurasia between China and the Balkans. It is classified as a member of the hyracodont subfamily Indricotheriinae. Paraceratherium means "near the hornless beast", in reference to Aceratherium, the genus in which the type species P. bugtiense was originally placed in.

The exact size of Paraceratherium is unknown because of the incompleteness of the fossils. Its weight is estimated to have been 15 to 20 tonnes (33,000 to 44,000 lb) at most; the shoulder height was about 4.8 metres (15.7 feet), and the length about 7.4 metres (24.3 feet). The legs were long and pillar-like. The long neck supported a skull that was about 1.3 metres (4.3 ft) long. It had large, tusk-like incisors and a nasal incision that suggests it had a prehensile upper lip or proboscis. The lifestyle of Paraceratherium may have been similar to that of modern large mammals such as the elephants and extant rhinoceroses. Because of its size, it would have had few predators and a slow rate of reproduction. It was a browser, eating mainly leaves, soft plants, and shrubs. It lived in habitats ranging from arid deserts with a few scattered trees to subtropical forests. The reasons for the animal's extinction are unknown, but various factors have been proposed.

Estimated size of P. transouralicum (olive green) compared with that of humans, other large mammals, and the dinosaur Patagotitan

Paraceratherium is one of the largest known land mammals that have ever existed, but its exact size is unclear because of the lack of complete specimens. Early estimates of 30 tonnes (66,000 lb) are now considered exaggerated; it may have been in the range of 15 to 20 tonnes (33,000 to 44,000 lb) at maximum, and as low as 11 tonnes (24,000 lb) on average. Calculations have mainly been based on fossils of P. transouralicumbecause this species is known from the most complete remains. Estimates have been based on skull, teeth, and limb bone measurements, but the known bone elements are represented by individuals of different sizes, so all skeletal reconstructions are composite extrapolations, resulting in several weight ranges. Its total body length was estimated as 8.7 m (28.5 ft) from front to back by Granger and Gregory in 1936, and 7.4 m (24.3 ft) by Vera Gromova in 1959, but the former estimate is now considered exaggerated. The weight of Paraceratherium was similar to that of some extinct proboscideans, with the largest complete skeleton known belonging to the steppe mammoth (Mammuthus trogontherii). In spite of the roughly equivalent mass, Paraceratherium may have been taller than any proboscidean. Its shoulder height was estimated as 5.25 m (17.2 ft) at the shoulders by Granger and Gregory, but 4.8 m (15.7 ft) by Gregory S. Paul in 1997. The neck was estimated at 2 to 2.5 m (6.6 to 8.2 ft) long by Michael P. Taylor and Mathew J. Wedel in 2013. P. huangheense differs from P. bugtiense only in the anatomy of the rear portion of the jaw, as well as its larger size.

The taxonomy of the genus and the species within has a long and complicated history. Other genera of Oligocene indricotheres, such as Baluchitherium and Indricotherium, have been named, but no complete specimens exist, making comparison and classification difficult. Most modern scientists consider these genera to be junior synonyms of Paraceratherium, and it is thought to contain at least four discernible species; P. bugtienseP. transouralicumP. lepidum, and P. huangheense. The most completely-known species is P. transouralicum, so most reconstructions of the genus are based on it. Differences between P. bugtiense and P. transouralicum may be due to sexual dimorphism, which would make them the same species.

Preparator Otto Falkenbach with P. transouralicum skull (specimen AMNH 18650), formerly assigned to Baluchitherium grangeri, American Museum of Natural History

Early discoveries of indricotheres were made through various colonial links to Asia. The first known indricothere fossils were collected from Balochistan (in modern-day Pakistan) in 1846 by a soldier named Vickary, but these fragments were unidentifiable at the time. The first fossils now recognised as Paraceratherium were discovered by the British geologist Guy Ellcock Pilgrim in Balochistan in 1907–1908. His material consisted of an upper jaw, lower teeth, and the back of a jaw. The fossils were collected in the Chitarwata Formation of Dera Bugti, where Pilgrim had previously been exploring. In 1908, he used the fossils as basis for a new species of the extinct rhinoceros genus AceratheriumA. bugtienseAceratherium was by then a wastebasket taxon; it included several unrelated species of hornless rhinoceros, many of which have since been moved to other genera. Fossil incisors that Pilgrim had previously assigned to the unrelated genus Bugtitherium were later shown to belong to the new species.

The subfamily Indricotheriinae, to which Paraceratherium belongs, was first classified as part of the family Hyracodontidae by Leonard B. Radinsky in 1966. Previously, they had been regarded as a subfamily within Rhinocerotidea, or even a full family, Indricotheriidae. In a 1999 cladistic study of tapiromorphs, Luke Holbrook found indricotheres to be outside the hyracodontid clade, and wrote that they may not be a monophyletic(natural) grouping. Radinsky's scheme is the prevalent hypothesis today. The hyracodont family contains long-legged members adapted to running, such as Hyracodon, and were distinguished by incisor characteristics. Indricotheres are distinguished from other hyracodonts by their larger size and the derived structure of their snouts, incisors and canines. The earliest known indricothere is the dog-sized Forstercooperia from the middle and late Eocene of western North America and Asia. The cow-sized Juxia is known from the middle Eocene; by the late Eocene the genus Urtinotherium of Asia had almost reached the size of Paraceratherium. Paraceratherium itself lived in Eurasia during the Oligocene, 23 to 34 million years ago. The genus is distinguished from other indricotheres by its large size, nasal incision that would have supported a muscular snout, and its down-turned premaxillae. It had also lost the second and third lower incisors, lower canines, and lower first premolars.

Locations of fossil finds

The reasons Paraceratherium became extinct after surviving for about 11 million years are unknown, but it is unlikely that there was a single cause. Theorised reasons include climate change, low reproduction rate, and invasion by gomphothere proboscideans from Africa in the late Oligocene (between 28 and 23 million years ago). Gomphotheres may have been able to considerably change the habitats they entered, in the same way that African elephants do today, by destroying trees and turning woodland into grassland. Once their food source became scarce and their numbers dwindled, Paraceratherium populations would have become more vulnerable to other threats. Large predators like Hyaenaelurus and Amphicyon also entered Asia from Africa during the early Miocene (between 23 and 16 million years ago ); these may have predated Paraceratherium calves. Other herbivores also invaded Asia during this time.

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