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 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. Many genera were named by Russian and Chinese scientists on the basis of subtle differences in molar characteristics, features that otherwise vary within populations of other rhinos, and are therefore not accepted for distinguishing species by western scientists.
A soldier named Vickaery brought back to London the first known indricothere fossils from Baluchistan in 1846, but these fragments were unidentifiable at the time. 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.
In 1910, more material was discovered in Balochistan, during an expedition by the English paleontologist and Cambridge University Museum of Zoology director Sir Clive Forster Cooper. Paraceratherium was first described by 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 superfamily Rhinocerotoidae can be traced back to about 50 million years ago, in the Early Eocene period, with early precursors such as Hyrachyus. Rhinocerotoidea contains three families, the Amynodontidae, the Rhinocerotidae ("true rhinos"), and the Hyracodontidae. The diversity within the rhino group was much larger in prehistoric times, with sizes ranging from dog-sized to the size of Paraceratherium, with long-legged, cursorial forms, and squat, semi aquatic forms. Most species did not possess horns.
Rhino fossils are identified as such mainly by characteristics of their teeth, which is the part of the animals most likely to be preserved. The upper molars of most rhinos are characteristic in having a pi (π) shaped pattern on the crown, whereas each lower molar has paired L-shapes. Skull features are also used for identification of fossil rhinos. The Indricotherinae subfamily, of which Paraceratherium belongs, are considered part of the Hyracodontidae, a group containing long legged members, adapted to running. All indricothere genera are known from Eurasia. The earliest known indricothere is the dog-sized Forstercooperia from the Middle and late Eocene. The dog-sized Juxia is known from the Middle Eocene, and by the Late Eocene, the genus Urtinotherium (U. incisivum was initially described as a species of Indricotherium) had almost reached the size of Paraceratherium. Paraceratherium itself lived during the Oligocene period, 23 and 34 million years ago, a span of 11 million years.
The cladogram below follows the 1989 analysis by Lucas and Sobus:
Indricotheres have traditionally been regarded as part of the hyracodontidae family, but Luke Holbrook found them to be outside this group in a 1999 study.
Paraceratherium is considered the largest known land mammal that has ever existed, but it's exact size is unclear, due to the lack of complete specimens. Early estimates of 30 tonnes are now considered exaggerated, and it may rather have been in the 15-20 ton range. Estimates have been made based on skull, teeth, and upper limb bone measurements, each resulting in different weights. Height has been estimated as 6 m (20 ft) tall at the shoulders. 8.0 m (26.2 ft) in length from nose to rump.
No complete set of vertebrae and ribs of Paraceratherium have yet been found, and the tail is completely unknown, which has proved hard to make composite reconstructions, as those elements that have been found belonged to individuals of different sizes. The atlas and axis vertebrae of the neck are wider than in most modern rhinos, with space for strong ligaments and muscles, which would be needed to hold up the large head. The rest of the vertebrae were also very wide, and large zygapophyses, with much room for muscles, tendons, ligaments, and nerves, to support the head, neck, and spine. Like sauropod dinosaurs, Paraceratherium had pleurocoel-like openings in their presacral vertebrae, which may have helped to lighten the skeleton. The neural spines were long, and formed a long "hump" along the back, where neck muscles and nuchal ligaments for holding up the skull were attached. The ribs were similar to those of modern rhinos, but the ribcage would have looked smaller in proportion to the long legs and large bodies, as modern rhinos are comparatively short limbed. The last vertebrae of the lower back was fused to the sacrum, a feature found in advanced rhinos.
The limbs were large and robust to support the large weight of the animal, and were in some ways similar to and convergent with those of elephants and sauropod dinosaurs with their likewise graviportal builds. Unlike such animals, which tend to lengthen the upper limb bones, while shortening, fusing and compressing the lower limb bones, hand and foot bones, Paraceratherium had short upper limb bones, and long hand and foot bones (except for the phalanges, which were disc-shaped), similar to the running rhinos which they descended from. Some foot bones were almost 50 cm (20 in) long. The thigh bones typically measured 1.5 m (5 ft), 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 rhinos, and the three trocanthers on the sides were much reduced, as this robustness diminished their importance. The limbs were held in a columnar posture instead of bent, as in smaller animals, which reduced the need for large limb muscles.
Due to the fragmentary nature of known Paraceratherium fossils, the animal has been restored in several different ways since its discovery. In 1923, Osborn supervised an artist to draw a restoration of the skeleton based on the then even less complete specimens known by then, by using the proportions of a modern rhino as guide. The result was too squat and compact, and he had a more slender version drawn later the same year. 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. The palaeontologists Pierre-Olivier Antoine and Darren Naish have expressed scepticism towards this idea.
The largest skulls of supposed male Paraceratherium measure around 1.3 metres (5 ft) long, 33-38 cm at the back of the skull, and 61 cm (2 ft) wide across by the zygomatic arches. Skulls of supposed females were almost as large. 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. The back of the skull was low and narrow, without the large lambdoid crests at the top, which are found in horned and tusked animals which need strong muscles to push and fight. The occipital condyle was very wide, and Paraceratherium appears to have had large and strong neck muscles, which allowed it to sweep the head strongly downwards, while foraging from branches.
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 had a pi (π) shaped pattern, except for the third upper molar, which was V-shaped, and had a reduced metastyle. The premolars only partially formed the pi pattern Each molar was the size of a human fist, and among mammals they were only exceeded in size by elephants, though they were small relative to the size of the skull. The lower cheek teeth were L-shaped, which is typical of rhinos.
The best living analogues for Paraceratherium may be elephants, rhinos and hippos, due to their large size. 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. Due to its large size, Paraceratherium would not have been able to run and move fast, but they would have been able to move across large distance, which they would need in an environment with scarce food. They may therefore have had large home ranges, and performed migrations. Adult individuals would be too large for most predators, but the young would have been vulnerable. Bite marks on bones from the Bugti beds indicate that even adults may have been preyed upon by 10-11 metre (33-36 ft) large crocodiles, Crocodylus bugtiensis. Like in elephants, the gestation period of Paraceratherium may have been long, and individuals may have had long lifespans. Paraceratherium may have lived in small herds, perhaps consisting of females and their calves, which they protected from predators. Movement, sound and other behaviour seen in CGI documentaries such as "Walking With Beasts" are entirely conjectural.
The simple, low crowned teeth indicate that Paraceratherium was a browser with a diet consisting of relatively soft leaves and shrubs. Later rhinos are grazers and instead have high crowned teeth due to their diet containing grit, which quickly wears down their teeth. Studies of mesowear on Paraceratherium teeth confirm a soft diet of leaves, but microwear studies have yet to be conducted. Isotope analysis show that Paraceratherium fed chiefly on C3 plants, which is mainly leaves. Like its perissodactyl relatives, the horses, tapirs and other rhinos, Paraceratherium would had been a hindgut fermenter, which means 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 and gut.
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. Herds of Paraceratherium may have moved from area to area, while continuously foraging from tall trees, which smaller mammals could not reach.
The Oligocene fauna which coexisted with Paraceratherium included various other rhinos, artiodactyls, rodents, beardogs, weasels, hyaenodonts, nimravids and cats. Most predators in their habitat were relatively small, and therefore not a threat. It is unknown why Paraceratherium went extinct after surviving for 11 million years, but it is unlikely that one factor was the single cause. Theories include climate change, low reproduction rate, and invasion by gomphothere proboscideans from Africa in the late Oligocene. These may have been able to change the habitats they entered considerably, like African elephants do today, by turning forests into grasslands by destroying the trees of an area. Once their food source became scarcer and their populations dwindled, Paraceratherium populations would have become more vulnerable to various other threats. Large predators like Hyaenaelurus and Amphicyon also entered Asia from Africa during the early Miocene, and these may have preyed upon Paraceratherium calves. Other herbivores invaded Asia during this time as well.