Paraceratherium

1923 skeletal reconstructions of B. grangeri (now P. transouralicum), in rhinoceros-like and slender versions

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.[18][19] 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.[20] 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.[3][21]

In 1989, the American palaeontologists Spencer G. Lucas and Jay C. Sobus published a revision of indricothere taxa, which is followed by most western scientists today. 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.[3][22][23]

Skull (left) and vertebrae (right) of P. linxiaense

According to Lucas and Sobus, the type species P. bugtiense from the late Oligocene of Pakistan includes junior synonyms such as B. osborni and P. zhajremensis. P. transouralicum from the late Oligocene of Kazakhstan, Mongolia, and northern China includes B. grangeri and I. minus.[22] 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 genus Benaratherium.[3][22] 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.[24]

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.[25] A 2004 paper by Deng and colleagues also recognised three distinct genera.[26] Some western writers have similarly used names otherwise considered invalid since the 1989 revision, but without providing detailed analysis and justification.[3] Deng and colleagues recognised six Paraceratherium species in 2021, including some that had previously been declared synonyms, P. grangeri, P. asiaticum, and P. lepidum, while keeping Indricotherium and Baluchitherium as synonyms of the genus.[17]

Phylogenetic relationships of Paraceratherium with other rhinos, according to Tao Deng and colleagues, 2021

The superfamily Rhinocerotoidea, which includes modern rhinoceroses, can be traced back to the early Eocene—about 50 million years ago—with early precursors such as Hyrachyus. Rhinocerotoidea contains three families; Amynodontidae, Rhinocerotidae ("true rhinoceroses"), and Hyracodontidae. The diversity within the rhinoceros group was much larger in prehistoric times; they ranged from dog-sized to the size of Paraceratherium. There were long-legged, cursorial forms adapted for running and squat, semi aquatic forms. Most species did not have horns. Rhinoceros 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 rhinoceroses have a pi-shaped (π) pattern on the crown, and each lower molar has paired L-shapes. Various skull features are also used for identification of fossil rhinoceroses.[27]

The subfamily Indricotheriinae, to which Paraceratherium belongs, was first classified as part of the family Hyracodontidae by the American palaeontologist Leonard B. Radinsky in 1966. Previously, they had been regarded as a subfamily within Rhinocerotidea, or even a full family, Indricotheriidae.[28] 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.[29] 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.[22][27] Paraceratherium itself lived in Eurasia during the Oligocene period, 23 to 34 million years ago.[30] The genus is distinguished from other indricotheres by its large size, nasal incision that would have supported a muscular snout, and its down-turned premaxillae.[22] It had also lost the second and third lower incisors, lower canines, and lower first premolars.[27]

The cladogram below follows the 1989 analysis of Indricotheriinae by Lucas and Sobus, and shows the closest relatives of Paraceratherium:[22]

Lucas and colleagues had reached similar conclusions in a previous 1981 analysis of Forstercooperia, wherein they still retained Paraceratherium and Indricotherium as separate genera.[31] 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.[32] Deng and colleagues found in that their 2021 analysis confirmed previous studies, 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.[17]

P. transouralicum skull, AMNH

The largest skulls of Paraceratherium are around 1.3 metres (4.3 ft) long, 33 to 38 centimetres (13 to 15 in) at the back of the skull, and 61 centimetres (24 in) 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.[4] A distinguishing feature was that the nasal incision was retracted to the P2-P3 premolars.[17]

The back of the skull was low and narrow, without the large lambdoid crests at the top and along the sagittal crest, which are otherwise found in horned and tusked animals that need strong muscles to push and fight. It also had a deep pit for the attachment of nuchal ligaments, which hold up the skull automatically. The occipital condyle was very wide and Paraceratherium appears to have had large, strong neck muscles, which allowed it to sweep its head strongly downwards while foraging from branches.[4] The upper profile of the skull was arched, a distinguishing feature of the genus.[17] One skull of P. transouralicum has a domed forehead, whereas others have flat foreheads, possibly because of sexual dimorphism.[22] A brain endocast of P. transouralicum shows it was only 8 percent of the skull length, while the brain of the Indian rhinoceros is 17.7 percent of its skull length.[20]

Upper molars of P. transouralicum, Musée d'Histoire Naturelle, Paris

The species of Paraceratherium are mainly discernible through skull characteristics. P. bugtiense had features such as relatively slender maxillae and premaxillae, shallow skull roofs, mastoid-paroccipital processes that were relatively thin and placed back on the skull, a lambdoid crest, which extended less back, and an occipital condyle with a horizontal orientation, which it shared with Dzungariotherium. P. transouralicum had robust maxillae and premaxillae, upturned zygomata, domed frontal bones, thick mastoid-paroccipital processes, a lambdoid crest that extended back, and occipital condyles with a vertical orientation.[3] P. huangheense differed from P. bugtiense only in the anatomy of the rear portion of the jaw, as well as its larger size.[16] P. linxiaense differed from the other species in that the nasal notch was deeper, with the bottom placed above the middle of molar M2, a proportionally higher occipital condyle compared to the occipital surface's height, short muzzle bones and diastema in front of the cheek teeth, and a high zygomatic arch with a prominent hind end, and a smaller upper incisor I1.[17]

Unlike those of most primitive rhinoceroses, 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; the lower ones were shorter and pointed forwards. Among known rhinoceroses, this arrangement is unique to Paraceratherium and the related Urtinotherium. The incisors may have been larger in males. The canine teeth otherwise found behind the incisors were lost. The incisors were separated from the row of cheek teeth by a large diastema (gap).[4] This feature is found in mammals where the incisors and cheek teeth have different specialisations.[27] The upper molars, except for the third upper molar that was V-shaped, had a pi-shaped (π) pattern and a reduced metastyle. The premolars only partially formed the pi pattern. Each molar was the size of a human fist; among mammals they were only exceeded in size by proboscideans, though they were small relative to the size of the skull. The lower cheek teeth were L-shaped, which is typical of rhinoceroses.[4]

Hind foot of P. transouralicum, AMNH

No complete set of vertebrae and ribs of Paraceratherium has yet been found and the tail is completely unknown. The atlas and axis vertebrae of the neck were wider than in most modern rhinoceroses, with space for strong ligaments and muscles that would be needed to hold up the large head. The rest of the vertebrae were also very wide, and had large zygapophyses with much room for muscles, tendons, ligaments, and nerves, to support the head, neck, and spine. 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 rhinoceroses, but the ribcage would have looked smaller in proportion to the long legs and large bodies, because modern rhinoceroses are comparatively short-limbed. The last vertebra of the lower back was fused to the sacrum, a feature found in advanced rhinoceroses.[4] Like sauropod dinosaurs, Paraceratherium had pleurocoel-like openings (hollow parts of the bone) in their pre-sacral vertebrae, which probably helped to lighten the skeleton.[43]

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 bones—except for the disc-shaped phalanges—similar to the running rhinoceroses from which they descended. Some foot bones were almost 50 centimetres (20 in) long. The thigh bones typically measured 1.5 m (4.9 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 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.[4] The front limbs had three toes.[44]

Foraging herd of P. transouralicum, by Elizabeth Rungius Fulda, 1923

The simple, low-crowned teeth indicate that Paraceratherium was a browser with a diet consisting of relatively soft leaves and shrubs. Later rhinoceroses were grazers, with high-crowned teeth because their diets contained grit that quickly wore down their teeth. Studies of mesowear on Paraceratherium teeth confirm the creatures had a diet of soft leaves; microwear studies have yet to be conducted.[4] Isotope analysis shows that Paraceratherium fed chiefly on C3 plants, which are mainly leaves.[47][48] 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.[4]

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.[20] 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.[4] Herds of Paraceratherium may have migrated while continuously foraging from tall trees, which smaller mammals could not reach.[30] 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.[41]