Paraceratherium is an extinct genus of hornless rhinocerotoids belonging to the family Paraceratheriidae. It is one of the largest terrestrial mammals that has ever existed and lived from the Early to Late Oligocene epoch (34–23 million years ago). The first fossils were discovered in what is now Pakistan, and remains have been found across Eurasia between China and the Balkans. Paraceratherium means "near the hornless beast", in reference to Aceratherium, the genus in which the type species P. bugtiense was originally placed.
The exact size of Paraceratherium is unknown because of the incompleteness of the fossils. The shoulder height was about , and the length about . Its weight is estimated to have been about . The long neck supported a skull that was about long. It had large, tusk-like incisors and a nasal incision that suggests it had a prehensile upper lip or proboscis (trunk). The legs were long and pillar-like. 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 long gestation period. 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.
The taxonomy of the genus and the species within has a long and complicated history. Other genera of Oligocene indricotheres, such as Baluchitherium, Indricotherium, and Pristinotherium, 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 the following species; P. bugtiense, P. transouralicum, P. huangheense, and P. linxiaense. 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.
Taxonomy
The taxonomic history of Paraceratherium is complex due to the fragmentary nature of the known fossils and because Western, Soviet, and Chinese scientists worked in isolation from each other for much of the 20th century and published research mainly in their respective languages. Scientists from different parts of the world tried to compare their finds to get a more complete picture of these animals, but were hindered by politics and wars. The opposing taxonomic tendencies of "lumping and splitting" have also contributed to the problem. Inaccurate geological dating previously led scientists to believe various geological formations that are now known to be contemporaneous were of different ages. Many genera were named on the basis of subtle differences in molar tooth characteristicsfeatures that vary within populations of other rhinoceros taxaand are therefore not accepted by most scientists for distinguishing species.
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.Prothero, 2013. pp. 35–52 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 Aceratherium; A. bugtiense. Aceratherium was by then a wastebasket taxon; it included several unrelated species of hornless rhinoceros, many of which have since been moved to other genera.Prothero, 2013. pp. 17–34 Fossil incisors that Pilgrim had previously assigned to the unrelated genus Bugtitherium were later shown to belong to the new species.
In 1910, more partial fossils were discovered in Dera Bugti during an expedition by the British palaeontologist Clive Forster-Cooper. Based on these remains, Forster-Cooper moved A. bugtiense to the new genus Paraceratherium, meaning "near the hornless beast", in reference to Aceratherium. His rationale for this reclassification was the species' distinctly down-turned lower tusks. In 1913, Forster-Cooper named a new genus and species, Thaumastotherium ("wonderful beast") osborni, based on larger fossils from the same excavations (some of which he had earlier suggested to belong to male P. bugtiense), but he renamed the genus Baluchitherium later that year because the former name was preoccupied, as it had already been used for a hemipteran insect. The fossils of Baluchitherium were so fragmentary that Forster-Cooper was only able to identify it as a kind of odd-toed ungulate, but he mentioned the possibility of confusion with Paraceratherium. The American palaeontologist Henry Fairfield Osborn, after which B. osborni was named, suggested it may have been a titanothere.
A Russian Academy of Sciences expedition later found fossils in the Aral Formation near the Aral Sea in Kazakhstan; it was the most complete indricothere skeleton known, but it lacked the skull. It is mounted in the Moscow Paleontological Museum. In 1916, based on these remains, Aleksei Alekseeivich Borissiak erected the genus Indricotherium named for a mythological monster, the "Indrik beast". He did not assign a species name, I. asiaticum, until 1923, but the Russian palaeontologist Maria Pavlova had already named it I. transouralicum in 1922. Also in 1923, Borissiak created the subfamily Indricotheriinae to include the various related forms known by then.
In 1922, the American explorer Roy Chapman Andrews led a well-documented expedition to China and Mongolia sponsored by the American Museum of Natural History. Various indricothere remains were found in formations of the Mongolian Gobi Desert, including the legs of a specimen standing in an upright position, indicating that it had died while trapped in quicksand, as well as a very complete skull. These remains became the basis of Baluchitherium grangeri, named by Osborn in 1923.Prothero, 2013. pp. 1–16
In 2017, a new species, P. huangheense, was named by the Chinese palaeontologist Yong-Xiang Li and colleagues based on jaw elements from the Hanjiajing Formation in the Gansu Province of China; the name refers to the nearby Huanghe River. In 2021, the Chinese palaeontologist Tao Deng and colleague described the new species P. linxiaense, based on a complete skull with an associated mandible and an atlas-axis complex followed by two thoracic vertebrae of another individual, all the fossils coming from the Jiaozigou Formation of the Linxia Basin (to which the name refers) of northwestern China. A multitude of other species and genus namesmostly based on differences in size, snout shape, and front tooth arrangementhave been coined for various indricothere remains. Fossils attributable to Paraceratherium continue to be discovered across Eurasia, but the political situation in Pakistan had become too unstable for further excavations to occur there.
Species and synonyms
In 1922 Forster-Cooper named the new species Metamynodon bugtiensis based on a palate and other fragments from Dera Bugti, thought to belong to a giant member of that genus. These fossils are now thought to have belonged to an aberrant Paraceratherium bugtiense specimen that lacked the M3 molar. Forster-Cooper stated in 1932 that the generic rank of Indricotherium was fairly well-established and while there were similarities in features of the jaws and teeth of Baluchitherium and Paraceratherium, there were also considerable differences, leading him to conclude keeping them separate was permissible as a working hypothesis.
In 1936, the American palaeontologists Walter Granger and William K. Gregory proposed that Forster-Cooper's Baluchitherium osborni was likely a junior synonym (an invalid name for the same taxon) of Paraceratherium bugtiense, because these specimens were collected at the same locality and were possibly part of the same morphologically variable species. They conceded there were minor differences that could be specific, and nonetheless opted to continue using the name Baluchitherium grangeri for the Mongolian material. Prothero speculated that Granger and Gregory hesitated to sink Baluchitherium entirely due to the politics at the American Museum; their boss, Osborn, had heavily publicised the genus and one of its species was named after him. The genus name Baluchitherium remained popular in various media for decades because of the publicity surrounding B. grangeri.Prothero, 2013. pp. 67–86
In 1989, the American palaeontologists Spencer G. Lucas and Jay C. Sobus published a revision of indricothere taxa, which was subsequently followed by western scientists. They concluded that Paraceratherium, as the oldest name, was the only valid indricothere genus from the Oligocene, and contained four valid species, P. bugtiense, P. transouralicum (originally in Indricotherium), P. prohorovi (originally in Aralotherium), and P. orgosensis (originally in Dzungariotherium). They considered most other names to be junior synonyms of those taxa, or as dubious names, based on remains too fragmentary to identify properly. By analysing alleged differences between named genera and species, Lucas and Sobus found that these most likely represented variation within populations, and that most features were indistinguishable between specimens, as had been pointed out in the 1930s. The fact that the single skull assigned to P. transouralicum or Indricotherium was domed, while others were flat at the top was attributed to sexual dimorphism; it is possible that P. bugtiense fossils represent the female, while P. transouralicum represents the male of the same species.
According to Lucas and Sobus, the type species P. bugtiense from the late Oligocene of Pakistan included junior synonyms such as B. osborni and P. zhajremensis. P. transouralicum from the late Oligocene of Kazakhstan, Mongolia, and northern China included B. grangeri and I. minus. By this scheme, P. orgosensis from the middle and late Oligocene of northwest China included D. turfanensis and P. lipidus. In 2013, the American palaeontologist Donald Prothero suggested that P. orgosensis may be distinct enough to warrant its original genus name Dzungariotherium, though its exact position requires evaluation. P. prohorovi from the late Oligocene of Kazakhstan may be too incomplete for its position to be resolved in relation to the other species; the same applies to proposed species such as I. intermedium and P. tienshanensis, as well as the Georgian genus Benaratherium. Though the genus name Indricotherium is now a junior synonym of Paraceratherium, the subfamily name Indricotheriinae is still in use because genus name synonymy does not affect the names of higher level taxa that are derived from these. Members of the subfamily are therefore still commonly referred to as indricotheres.