Paraceratherium

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Paraceratherium
Temporal range: Oligocene, 34–23Ma
Indricotherium.jpg
Skeleton cast of P. transouralicum, National Museum of Nature and Science, Tokyo
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Perissodactyla
Superfamily: Rhinocerotoidea
Family: Hyracodontidae
Subfamily: Indricotheriinae
Genus: Paraceratherium
Forster-Cooper, 1911
Type species
Aceratherium bugtiense
Pilgrim, 1908
Species
  • P. bugtiense Pilgrim, 1908
  • P. transouralicum Pavlova, 1922
  • ?P. orgosensis Chiu, 1973
Synonyms

Paraceratherium is an extinct genus of hornless rhinoceros, and the largest terrestrial mammal known to have existed. It lived during the Oligocene epoch, and remains have been found in Eurasia, stretching from China to the former Yugoslavia. Paraceratherium is classified as a member of the hyracodont subfamily Indricotheriinae.

The exact size of Paraceratherium is unknown, due to the incompleteness of the fossils, but the weight is estimated to have been 15-20 tonnes at most, the shoulder height 6 m (20 ft), and the length about 8.0 m (26.2 ft). It had long, pillar-like legs, held in a columnar posture, a long neck, and a skull that was about 1.3 metres (5 ft) long. It had large, tusk-like incisors, and a nasal incision that suggests it had a prehensile upper lip or proboscis. Its lifestyle may have been similar to that of large modern mammals, such as elephants and extant rhinos. Due to its size, it would have had few predators, but also a slow reproduction rate. Paraceratherium was a browser, mainly eating soft plants such as leaves and shrubs. It lived in habitats ranging from arid desert environment with few, scattered trees, to subtropical forest.

The classification of the genus and the species within has a long and complicated history. Other genera of Oligocene indricotheres have been named, such as Indricotherium, Baluchitherium, and Dzungariotherium, but no complete specimens exist, making comparison and classification difficult. Most modern scientists consider these genera as junior synonyms of Paraceratherium, and that it contains three discernible species, P. bugtiense (the type species), P. transouralicum, and P. orgosensis, though the latter may be a distinct genus. P. transouralicum is the most completely known species, so most restorations 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[edit]

The taxonomic history of Paraceratherium 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, while publishing mainly in their respective languages. Outdated species concepts were also being used in the East long after they had been abandoned by Western scientists.[1] The opposing taxonomic tendencies of "lumping and splitting" have also contributed to the problem.[2] Furthermore, inaccurate geological dating previously caused scientists to believe various geological formations that are now known to be contemporaneous were of different ages.[3] Many genera were named on the basis of subtle differences in molar characteristics - features that vary within other rhino taxa, and are therefore not accepted for distinguishing species by western scientists.[3]

A soldier named Vickaery collected the first known indricothere fossils from Balochistan in 1846, but these fragments were unidentifiable at the time.[4] 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 Chitarwata Formation of Dera Bugti, Balochistan in what is now Pakistan, 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.[1][5] Fossil incisors that Pilgrim had previously referred to the unrelated genus Bugtitherium were later shown to belong to the new species.[6]

Illustration of a lower jaw of P. bugtiense, from Forster-Cooper, 1911

In 1910, more partial fossils were discovered in Dera Bugti, during an expedition by the English palaeontologist and later Cambridge University Museum of Zoology director Sir Clive Forster-Cooper. Based on these remains, he moved A. bugtiense to the new genus Paraceratherium, meaning "near the hornless beast", in reference to Aceratherium.[1][7] His rationale for this reclassification was the distinctly down turned lower tusks.[6] In 1913, Forster-Cooper named Thaumastotherium ("wonderful beast") osborni based on larger fossils from the same excavations, but renamed the genus Baluchitherium later that year, as the former name was preoccupied by a hemipteran insect.[8][9] The fossils of Baluchitherium were so fragmentary that he was unsure what kind of perissodactyl they belonged to, but he noted the possibility of confusion with Paraceratherium.[10]

A Russian Academy of Sciences expedition later found fossils in the Aral Formation near the Aral Sea in Kazakhstan, the most complete indricothere skeleton known, yet lacking the skull. In 1916, Aleksei Alekseeivich Borissiak erected the genus Indricotherium based on these remains, named for the "Indrik beast", a mythological monster. He did not assign a species name until 1923, I. asiaticum, but Maria Pavlova had already named it I. transouralicum by 1922.[1][11] Borissiak also created the subfamily Indricotheriinae in 1923, to include the various related forms known by then.[12]

Hind foot of P. transouralicum, American Museum of Natural History

In 1922, 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 it had died while trapped in quicksand, as well as a very complete skull. These remains became the basis of Baluchitherium grangeri, named by Henry Fairfield Osborn in 1923.[13][14]

Dzungariotherium orgosensis was described in 1973 based on fossils, mainly teeth, from Dzungaria in Xinjiang, northwest China.[15] A multitude of other species and genus names have been coined for various indricothere remains, mostly based on differences in size, snout shape, and front tooth arrangement. Remains attributable to Paraceratherium continue to be discovered across Eurasia, but the political situation in Pakistan has become too unstable for further excavations.[4]

In 1936, Walter Granger and William K. Gregory pointed out that Forster-Cooper's Baluchitherium osborni was likely a junior synonym of Paraceratherium bugtiense, since these specimens were collected at the same locality, and therefore part of the same morphologically variable species.[16] Forster-Cooper had himself had similar doubts.[17] This had also been pointed out by William Diller Matthew in 1931. In spite of already being declared a junior synonym, the genus name Baluchitherium remained popular in various media, due to publicity surrounding Osborn's B. grangeri.[2]

P. transouralicum skull cast mounted in a life-sized silhouette, AMNH

In 1989, palaeontologists Spencer G. Lucas and Jay C. Sorbus 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, which contained four valid species, P. bugtiense, P. transouralicum, P. orgosensis, and P. prohorovi. They considered most other names as junior synonyms of those taxa, or as nomina dubiae based on remains too fragmentary to properly identify. 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 also been pointed out in the 1930s. The fact that the single skull referred to P. transouralicum or Indricotherium was domed while others are flat at the top, was attributed to sexual dimorphism. Therefore it is possible that P. bugtiense fossils represent the female, while P. transouralicum represents the male of the same species.[18][2]

The type species P. bugtiense, from the late Oligocene of Pakistan, includes junior synonyms such as B. osborni and P. zhajremensis. P. transouralicum, formerly Indricotherium, from the late Oligocene of Kazakhstan, Mongolia and northern China includes B. grangeri and I. minus. P. orgosensis, formerly Dzungariotherium, from the middle and late Oligocene of northwest China, includes D. turfanensis and P. lipidus.[18] P. orgosensis may be distinct enough to warrant its original genus name, but its exact position requires evaluation. P. prohorovi, from the late Oligocene of Kazakhstan, is now considered too incomplete for its position to be resolved in relation to the other species, and the same applies to species such as I. intermedium and P. tienshanensis.[2][18] Though the genus name Indricotherium is now a junior synonym of Paraceratherium, the subfamily name Indricotheriinae is still in use, as 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.[19]

Evolution[edit]

Comparison of the incisors of Amynodon (Amynodontidae), Paraceratherium (Hyracodontidae), and Trigonias (Rhinocerotidae)
Skull and neck of Juxia, a cow sized relative, Paleozoological Museum of China

The superfamily Rhinocerotoidea 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. Various skull features are also used for identification of fossil rhinos.[20]

The Indricotherinae subfamily, of which Paraceratherium belongs, are considered part of the Hyracodontidae, a group containing long legged members adapted to running, such as Hyracodon. Indricotheres are distinguished by their larger size, and derived structure of their snout, 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, and by the late Eocene, the genus Urtinotherium of Asia had almost reached the size of Paraceratherium.[20][18] Paraceratherium itself lived during the Oligocene period of Eurasia, 23 to 34 million years ago.[21] The genus is distinguished from other indricotheres by its large size, nasal incision which would have supported a muscular snout, and down turned premaxillae.[18] It had also lost the second and third lower incisors, lower canines, and lower first premolars.[20]

The cladogram below follows the 1989 analysis by Lucas and Sobus, which found Triplopodinae to be the sister taxon of Indricotheriinae:



 Triplopodinae


 Indricotheriinae

 Forstercooperia




 Juxia




 Urtinotherium



 Paraceratherium






In a 1999 study, Luke Holbrook instead found the indricotheres to be outside the hyracodontid group, and also suggested that the indricotheres may not be a monophyletic grouping. It has also been suggested that they are most closely related to rhinocerotids.[22]

Description[edit]

Size (dark grey) compared to that of a human and other rhinos

Paraceratherium is considered the largest known land mammal that has ever existed, but its 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 at maximum, and as low as 11 tonnes on average. Calculations have mainly been based on fossils of P. transouralicum, since this species is known from the most complete remains.[3] Estimates have been based on skull, teeth, and limb bone measurements, but the known bone elements are represented by individuals of different sizes, so all skeletal restorations are composite extrapolations, resulting in several different weight ranges.[23][24] Height has been estimated as 6 m (20 ft) tall at the shoulders.[21] The length was around 8.0 m (26.2 ft) from front to back. The weight of Paraceratherium was approached by some extinct proboscideans, with the largest complete skeleton known belonging to the steppe mammoth, Mammuthus trogontherii.[23] In spite of the roughly equivalent mass, Paraceratherium was still taller than any proboscidean.[3]

Restoration of P. transouralicum

No complete set of vertebrae and ribs of Paraceratherium have yet been found, and the tail is completely unknown. 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.[3] Like sauropod dinosaurs, Paraceratherium had pleurocoel-like openings in their presacral vertebrae, which may have helped to lighten the skeleton.[25] 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.[3]

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, 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.[3]

1923 skeletal restorations of P. transouralicum (then B. grangeri), in rhino-like and slender versions

Due to the fragmentary nature of known Paraceratherium fossils, the animal has been restored in several different ways since its discovery.[26] In 1923, W. D. Matthew supervised an artist to draw a restoration of the skeleton based on the even less complete specimens known by then, by using the proportions of a modern rhino as guide.[27] The result was too squat and compact, and Osborn had a more slender version drawn later the same year. Some later restorations have made the animal too slender, with little regard to the underlying skeleton.[3]

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.[3] The palaeontologists Pierre-Olivier Antoine and Darren Naish have expressed scepticism towards this idea.[28][29]

Skull[edit]

P. transouralicum skull, AMNH

The largest skulls of 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. Paraceratherium had a long 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 and along the sagittal crest, which are otherwise found in horned and tusked animals which 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 and strong neck muscles, which allowed it to sweep the head strongly downwards, while foraging from branches.[3] One skull of P. transouralicum has a domed forehead, whereas others have flat foreheads, possibly due to sexual dimorphism.[18] A brain endocast of P. transouralicum shows that it was only 8 percent of the skull length, while the brain of the Indian rhino is 17.7 percent of its skull length.[16]

The three species of Paraceratherium are mainly discernible through skull characteristics. P. bugtiense and P. orgosensis share features such as relatively slender maxillae and premaxillae, shallow skull roofs, mastoid-paroccipital processes that are relatively thin and placed back on the skull, a lambdoid crest which extends less back, and an occipital condyle with a horizontal orientation. P. transouralicum has robust maxillae and premaxillae, upturned zygomata, domed frontal bones, thick mastoid-paroccipital processes, a lambdoid crest that extends back, and occipital condyles with a vertical orientation.[2]

Upper teeth of P. transouralicum, Musee d'Histoire Naturelle, Paris

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 incisors may have been larger in males. The canines otherwise found behind the incisors were lost. The incisors were separated from the row of cheek teeth by a large diastema (gap).[3] This feature is found in mammals where the incisors and cheek teeth have different specialisations.[20] 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 proboscideans, though they were small relative to the size of the skull. The lower cheek teeth were L-shaped, which is typical of rhinos.[3] The teeth of P. orgosensis are 25 percent bigger than those of P. transouralicum, making it the largest known indricothere.[2] The dental formula was \frac{1.1.4.3}{1.0.3.3}.[30]

Palaeobiology[edit]

Photo collage restoration of P. transouralicum with a calf

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 distances, which they would need in an environment with scarce food. They may therefore have had large "home ranges", and performed migrations.[3] Donald Prothero has proposed that animals as large as indricotheres would need very large "home ranges" or territories of at least 1000 square km, and that there would have been little room in Asia for many populations, let alone a multitude of nearly identical species and genera, due to scarcity of resources. This principle is called competitive exclusion, and is used to explain how the black rhino (a browser) and white rhino (a grazer) exploit different niches while living in the same areas of Africa.[2]

Most predators in their habitat were relatively small, about wolf-sized, and therefore not a threat.[21] 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.[3] Paraceratherium may have lived in small herds, perhaps consisting of females and their calves, which they protected from predators.[21] It has been proposed that 20 tonnes may be the maximum weight possible for land mammals, and Paraceratherium was close to this limit.[31] It is unknown why mammals cannot reach the much larger size of sauropod dinosaurs. The reason may be ecological instead of biomechanical, and perhaps related to reproduction strategies.[23] Movement, sound and other behaviour seen in CGI documentaries such as "Walking With Beasts" are entirely conjectural.[3]

Diet[edit]

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 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.[3] Isotope analysis show that Paraceratherium fed chiefly on C3 plants, which is mainly leaves.[32][33] Like its perissodactyl relatives, the horses, tapirs and other rhinos, Paraceratherium would have 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.[3]

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.[16] 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.[3] Herds of Paraceratherium may have moved from area to area, while continuously foraging from tall trees, which smaller mammals could not reach.[21] Osborn suggested that its mode of foraging would have been similar to that of the high browsing giraffe and okapi, rather than to modern rhinos, whose heads are carried close to the ground.[27]

Distribution and habitat[edit]

Locations of fossil finds

Remains referable to Paraceratherium have been found in Oligocene formations stretching across Eurasia, including what is now China, Mongolia, India, Pakistan, Kazakhstan, Georgia, Turkey, Romania, Bulgaria, and the former Yugoslavia.[4][34][35] The habitat of Paraceratherium appears to have varied from arid desert basin with few tall trees and brush, to woody shrubland, and dry, temperate to subtropical forest.[21] The Oligocene fauna which coexisted with Paraceratherium included various other rhinos, artiodactyls, rodents, beardogs, weasels, hyaenodonts, nimravids and cats.[21]

It is unknown why Paraceratherium went extinct after surviving for 11 million years, but it is unlikely that one factor was the single cause.[21] Theories include climate change, low reproduction rate, and invasion by gomphothere proboscideans from Africa in the late Oligocene. Gomphotheres 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 in an area. Once their food source became scarcer and their numbers dwindled, Paraceratherium populations would have become more vulnerable to various other threats.[36] 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.[21]

References[edit]

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Bibliography[edit]

  • Prothero, D. (2013). Rhinoceros Giants: The Palaeobiology of Indricotheres. Indiana: Indiana University Press. ISBN 978-0-253-00819-0.