Paraceratherium

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. 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 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. Fossil incisors that Pilgrim had previously assigned to the unrelated genus Bugtitherium were later shown to belong to the new species. 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 American palaeontologist Henry Fairfield Osborn, after which B. osborni was named, suggested it may have been a titanothere. 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. 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. These fossils are now thought to have belonged to an aberrant Paraceratherium bugtiense specimen that lacked the M3 molar. 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. The American palaeontologist William Diller Matthew and Forster-Cooper himself had expressed similar doubts few years earlier. Although it had already been declared a junior synonym, the genus name Baluchitherium remained popular in various media because of the publicity surrounding Osborn's B. grangeri. 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. In contrast to the revision by Lucas and Sobus, a 2003 paper by Chinese palaeontologist Jie Ye and colleagues suggested that Indricotherium and Dzungariotherium were valid genera, and that P. prohorovi did not belong in Paraceratherium. They also recognised the validity of species such as P. lipidus, P. tienshanensis, and P. sui. A 2004 paper by Deng and colleagues also recognised three distinct genera. Some western writers have similarly used names otherwise considered invalid since the 1989 revision, but without providing detailed analysis and justification. In a 1999 cladistic study of tapiromorphs, the American palaeontologist 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 period, 23 to 34 million years ago. In 2016, the Chinese researchers Haibing Wang and colleagues used the name Paraceratheriidae for the family and Paraceratheriine for the subfamily, and placed them outside of Hyracodontidae. Deng and colleagues confirmed previous studies with their 2021 analysis, suggesting that Juxia evolved from a clade consisting of Forstercooperia and Pappaceras 40 million years ago, with the resulting stock evolving into Urtinotherium in the late Eocene and Paraceratherium in the Oligocene. These researchers did not find Hyracodontidae to form a natural group, and found Paraceratheriidae to be closer to Rhinocerotidae, unlike previous studies. ==Description==

, the extinct elephant Palaeoloxodon recki, and the African bush elephant compared to humans Paraceratherium is one of the largest known land mammals that have ever existed, but its precise size is unclear because of the lack of complete specimens. Despite its roughly equivalent mass, Paraceratherium might have been taller than any proboscidean. The neck was estimated at long by the palaeontologists Michael P. Taylor and Mathew J. Wedel in 2013. of P. transouralicum Early estimates of are now considered exaggerated; it may have been in the range of at maximum, and as low as on average. Calculations have mainly been based on fossils of P. transouralicum because this species is known from the most complete remains. Asier Larramendi in 2015 estimated that the largest known remains of Paraceratherium transouralicum, represented by two neck (cervical) vertebrae and a metacarpal bone from the hand, belonged to individuals with a shoulder height of and a body mass of . The palaeontologists Pierre-Olivier Antoine and Darren Naish have expressed scepticism towards this idea. Due to the fragmentary nature of known Paraceratherium fossils, the skeleton of the animal has been reconstructed in several different ways since its discovery. In 1923, Matthew supervised an artist to draw a reconstruction of the skeleton based on the even less complete P. transouralicum specimens known by then, using the proportions of a modern rhinoceros as a guide. The result was too squat and compact, and Osborn had a more slender version drawn later the same year. Some later life restorations have made the animal too slender, with little regard to the underlying skeleton. Skull The largest skulls of Paraceratherium are around long, at the back of the skull, and wide across by the zygomatic arches. Paraceratherium had a long forehead, which was smooth and lacked the roughened area that serves as attachment point for the horns of other rhinoceroses. 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 that of the black rhinoceros and the Indian rhinoceros, or a short proboscis (trunk) as in tapirs. The limbs were large and robust to support the animal's large weight, and were in some ways similar to and convergent with those of elephants and sauropod dinosaurs with their likewise graviportal (heavy and slow moving) builds. Unlike such animals, which tend to lengthen the upper limb bones while shortening, fusing and compressing the lower limb, hand, and foot bones, Paraceratherium had short upper limb bones and long hand and foot bonesexcept for the disc-shaped phalangessimilar to the running rhinoceroses from which they descended. Some foot bones were almost long. The thigh bones typically measured , a size only exceeded by those of some elephants and dinosaurs. The thigh bones were pillar-like and much thicker and more robust than those of other rhinoceroses, and the three trochanters on the sides were much reduced, as this robustness diminished their importance. The limbs were held in a column-like posture instead of bent, as in smaller animals, which reduced the need for large limb muscles. ==Palaeobiology==

The zoologist Robert M. Alexander suggested in 1988 that overheating may have been a serious problem in Paraceratherium due to its size. According to Prothero, the best living analogues for Paraceratherium may be large mammals such as elephants, rhinoceroses and hippopotamuses. To aid in thermoregulation, these animals cool down during the day by resting in the shade or by wallowing in water and mud. They also forage and move mainly at night. Because of its large size, Paraceratherium would not have been able to run and move quickly, but they would have been able to cross large distances, which would be necessary in an environment with a scarcity of food. They may therefore have had large home ranges and have been migratory. The reasons mammals cannot reach the much larger size of sauropod dinosaurs are unknown. The reason may be ecological instead of biomechanical, and perhaps related to reproduction strategies. Like its perissodactyl relatives, the horses, tapirs, and other rhinoceroses, Paraceratherium would have been a hindgut fermenter; it would extract relatively little nutrition from its food and would have to eat large volumes to survive. Like other large herbivores, Paraceratherium would have had a large digestive tract. Granger and Gregory argued that the large incisors were used for defence or for loosening 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; this ability would have been helpful to Paraceratherium. Some Russian authors suggested that the tusks were probably used for breaking twigs, stripping bark and bending high branches and that, because species from the early Oligocene had larger tusks than later ones, they probably had a more bark than leaf based diet. Since the species involved are now known to have been contemporaneous, and the differences in tusks are now thought to be sexually dimorphic, the latter idea is not accepted today. Herds of Paraceratherium may have migrated while continuously foraging from tall trees, which smaller mammals could not reach. Osborn suggested that its mode of foraging would have been similar to that of the high-browsing giraffe and okapi, rather than to modern rhinoceroses, whose heads are carried close to the ground. ==Distribution and habitat==

'' below Remains assignable to Paraceratherium have been found in early to late Oligocene (34–23 million years ago) formations across Eurasia, in modern-day China, Mongolia, India, Pakistan, Kazakhstan, Georgia, Turkey, Romania, Bulgaria, and the Balkans. The fauna which coexisted with Paraceratherium included other rhinocerotoids, artiodactyls, rodents, amphicyonids, mustelids, hyaenodonts, nimravids and felids. A study of fossil pollen showed that much of China was woody shrubland, with plants such as saltbush, mormon tea (Ephedra), and nitre bush (Nitraria), all adapted to arid environments. Trees were rare, and concentrated near groundwater. The parts of China where Paraceratherium lived had dry lakes and abundant sand dunes, and the most common plant fossils are leaves of the desert-adapted Palibinia. Trees in Mongolia and China included birch, elm, oaks, and other deciduous trees, while Siberia and Kazakhstan also had walnut trees. Dera Bugti in Pakistan had dry, temperate to subtropical forest. , according to Deng and colleagues, 2021 Deng and colleagues speculated about the palaeobiogeography of Paraceratherium based on their phylogenetic analys in 2021. They found that P. bugtiense was the only species of the genus represented in the Oligocene of western Pakistan, while the genus was highly diversified across the Mongolian Plateau, northwestern China, and Kazakhstan north to the Tibetan Plateau. They hypothesised that P. asiaticum dispersed westward to Kazakhstan during the early Oligocene from the ancestral area of Mongolia, where the most primitive member of the genus, P. grangeri, lived, and descendants may have continued to South Asia as P. bugtiense, dispersing through the Tibetan region. P. lepidum existed in Xinjiang and Kazakhstan and P. linxiaense in Linxia during the late Oligocene, and it is possible that these sister species of P. bugtiense had been able to migrate back north to Central Asia during this time when that area had become tropical (it was arid during the early Oligocene). This implies the Tibetan region was not yet a high-elevation plateau that could act as a barrier, and large animals may therefore have been able to move freely along the eastern coast of the Tethys sea, and through lowlands in the area, some of which were possibly under in elevation at the time. ==Extinction==

, 1923 The reasons Paraceratherium and its relatives became extinct after surviving for about 11 million years are unknown, but it is unlikely that there was a single cause. Theories include that their large size was related to the now outdated concept of inadaptive evolution, climate change, vegetational change, and low reproduction rate. Prothero and the Russian zoologist Pavel V. Putshkov have considered these causes unlikely since these animals managed to survive regardless of these issues for millions of years under the harsh conditions of their environment, and were not much larger than the biggest proboscideans, extinct as well as extant, which faced similar challenges. Prothero has pointed out that gomphotheres are not known to have generally coexisted with paraceratheres, and there are no known co-occurrences between paraceratheres and the large deinotheres, which would have been their most likely competitors. While cautioning that the true cause of their extinction will never be known for certain, Prothero found it to be more than a coincidence that paraceratheres disappeared just as large predators and other large herbivores entered Asia during the early Miocene (between 23 and 16 million years ago). ==References==