Temporal range: Oligocene, 34–23Ma
|Skeleton cast of P. transouralicum, National Museum of Nature and Science, Tokyo|
Forster Cooper, 1911
Paraceratherium, also commonly known as Indricotherium or Baluchitherium (see taxonomic discussion below), is an extinct genus of gigantic hornless rhinoceros-like mammals of the family Hyracodontidae, endemic to Eurasia and Asia during the Oligocene epoch. It is the largest terrestrial mammal known to have existed.
The first fossils of Paraceratherium were discovered by Guy Ellcock Pilgrim during his time in British India in 1907–1908. His material consisted of an upper jaw, lower teeth, and the back of a jaw. They were collected in the Dera Bugti area of Balochistan in what is now Pakistan, south of the Siwalik Hills, where Pilgrim had previously been exploring. In 1908, he referred the species to the extinct rhino genus Aceratherium, as the new species A. bugtiense. Aceratherium was by then a wastebasket taxon which included several unrelated species of hornless rhinos, many of which have since been moved to other genera. A soldier named Vickaery brought back the first known indricothere fossils from Baluchistan in 1846, but these fragments were unidentifiable at the time.
The subsequent taxonomic history of Paraceratherium and the species within the genus is complex, due to the fragmentary nature of the known fossils, as well as the fact that western, Soviet and Chinese scientists worked in isolation from each other for much of the 20th century.
While more distinct at the species level, there is uncertainty and disagreement with regard to the genus level of taxonomy. Paraceratherium was first described by Clive Forster Cooper in 1911. The genus Baluchitherium was first described by Forster Cooper in 1913. The genus Indricotherium was first described by Borissiak in 1915.
Baluchitherium is now widely regarded as a synonym of (i.e. the same as) either Paraceratherium or Indricotherium.
However, there has been disagreement over whether Indricotherium is a distinct genus from Paraceratherium. Lucas and Sobus in their 1989 review of the subfamily Indricotheriinae (see reference below), argue for synonymy, and consider that the differences between the two are of species level at most, and may even be the result of sexual dimorphism in a single species, with the larger more robust Indricotherium with larger incisors being probably the male, and the more gracile Paraceratherium the female. Others,[who?] however, have expressed doubts about this (concerning the interpretation of the shape of the skull). Even if these two do turn out to be distinct genera, they would still be similar in size and appearance.
If they are considered the same genus, then Indricotherium would become a junior synonym of Paraceratherium, because, according to the priority principle of scientific classification, the first publication, and hence the oldest valid name, takes priority and the name Paraceratherium predates the other.
Here Lucas and Sobus are followed. They consider Indricotherium, Baluchitherium, Thaumastotherium Forster Cooper, 1913a, Aralotherium Borissiak, 1939, and Dzungariotherium Xu and Wang, 1973 all as junior synonyms of Paraceratherium.
Lucas and Sobus recognise four valid species of Paraceratherium. The Paraceratherium species are:
Paraceratherium bugtiense (Forster Cooper, 1911) from the Oligocene of Pakistan is the type species of Paraceratherium. Baluchitherium osborni Forster Cooper, 1913a is a junior synonym. It was first found in the Chitarwata Formation of the Bugti Hills, Balochistan, after which it was originally named. New specimens of P. bugtiense were unearthed in the last decade by a French-Pakistani team (Antoine et al., 2004; Métais et al., 2009).
P. transouralicum (Pavlova, 1922). Also known as Indricotherium transouralicum, this is the best known and most widespread species, known from the middle and late Oligocene of Kazakhstan, Mongolia, and Nei Monggol in northern China. Lucas and Sobus list the following species as synonyms: Baluchitherium grangeri Osborn, 1923, Indricotherium asiaticum Borissiak, 1923, Indricotherium minus Borissiak, 1923.
Paraceratherium orgosensis (Chiu, 1973) is the largest species, the teeth being at least a quarter again as big as P. transouralicum (see Lucas and Sobus p. 363/fig.19.2). It is known from the middle and late Oligocene of Xinjiang, northwest China. The three synonyms are Dzungariotherium orgosensis Chiu, 1973 and (each of the following named after a separate skull) Dzungariotherium turfanensis Xu & Wang, 1978 and Paraceratherium lipidus Xu & Wang, 1978. While there is some variation in details of the proportions of the skull (perhaps due to sexual dimorphism), all occur in a close geographical region and have distinct first and second upper molar crochets.
Paraceratherium prohorovi (Borissiak, 1939) from the late Oligocene or early Miocene of eastern Kazakhstan. Paraceratherium zhajremensis (Osborn, 1923) from the Middle and late Oligocene of India.
The cladogram below follows the 1989 analysis by Lucas and Sobus:
Paraceratherium is regarded as the largest land mammal known, with the largest species having an estimated mean adult mass of 11 t (12 tons) and the largest individual known estimated at 4.8 m (16 ft) tall at the shoulders, 8.0 m (26.2 ft) in length from nose to rump, and 16 t (18 tons) in weight.
The neck was very long, the trunk robust, and the limbs long and thick, column-like.
Paraceratherium had a long, domed forehead, which was smooth and lacked the roughened area that serves as attachment point for the horns of other rhinos. The bones above the nasal region are long, and the nasal incision goes far into the skull. This indicates that Paraceratherium had a prehensile upper lip, similar to the black rhino and the Indian rhino, or a short proboscis or trunk, as in tapirs and elephants.
There are no indications of the colour and skin texture of the animal, as no skin impressions or mummies are known, but most restorations show it as thick, folded, grey and hairless, based on modern rhinos. Hair holds in body heat, so large modern animals such as elephants and rhinos are largely hairless.
The American palaeontologist Donald Prothero has proposed that, contrary to most depictions, Paraceratherium had large, elephant-like ears, used for thermoregulation. The ears of elephants enlarge the body surface and are full of blood vessels, which makes it easier for them to release excess heat. Prothero believes this would have been true for Paraceratherium as well, and also points to it having robust bones around the ear openings. Palaeontologist Darren Naish expressed scepticism towards this idea.
Unlike most primitive rhinos, the front teeth of Paraceratherium were reduced to a single pair of incisors in either jaw, which were large and conical, and have been described as tusks. The upper incisors pointed downwards, while the lower ones were shorter and pointed forwards. Among known rhinos, this arrangement is unique to Paraceratherium and the related Urtinotherium. The canines otherwise found behind the incisors were lost.
The incisors were separated from the row of cheek teeth by a large diastema (gap). The upper molars were characteristic in having a pi (π) shaped pattern, except for the third upper molar, which was V-shaped, and had a reduced metastyle.
It lived in and browsed the forests of Central Asia between 23 and 34 million years ago.
Paraceratherium was a browsing herbivorous perissodactyl that stripped leaves from trees with its downward-pointing, tusk-like upper teeth that occluded forward-pointing lower teeth.
Its type of dentition, its mobile upper lip and its long legs and neck indicate that it was a browser that lived on the leaves and twigs of trees and large shrubs.
It has been argued that the large incisors were used for defence, or for jerking loose shrubs by moving the neck downwards, thereby acting as picks and levers. Tapirs use their proboscis to wrap around branches while stripping off bark with the front teeth, and such a feature would have been helpful to Paraceratherium as well.