Paraceratherium

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.[2] It was first discovered in 1910 in Balochistan of what is now Pakistan, hence the name, during an expedition by the English paleontologist and Cambridge University Museum of Zoology director Sir Clive Forster Cooper.

Description
Paraceratherium is regarded as the largest land mammal known, with the largest species having an estimated mean adult mass of 11 t (12 tons)[4] 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.[5]

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. It had a long, low, hornless skull and vaulted frontal and nasal bones. Its front teeth were reduced to a single pair of incisors in either jaw, but they were conical and so large that they looked like small tusks. The upper incisors pointed straight downwards, while the lower ones jutted outwards. The upper lip was evidently extremely mobile. The neck was very long, the trunk robust, and the limbs long and thick, column-like.

It lived in and browsed the forests of Central Asia between 23 and 34 million years ago.

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.

Taxonomy
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. One more (P. zhajremensis) has been tentatively added. 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).

Paraceratherium 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.

Other remains referable to Paraceratherium or related taxa were found in Southeastern Europe and Asia Minor, notably in Turkey (Antoine et al., 2008).