Indricotherium

Indricotherium, also known as Paraceratherium, is a genus that included the extinct hornless rhinoceros, which was considered to be the largest land mammal to ever exist. The meaning of Paraceratherium is - near the hornless beast and this particular group of rhinos were found to have a similar lifestyle to that of modern large mammals, for example, elephants and extant rhinoceros. It was mainly dependent upon leaves, shrubs, soft plants and branches.  The species A. bugtiense was originally placed under the genus Paraceratherium with reference to Aceratherium.

The exact size of Indricotherium is not known due to incomplete and unidentifiable fossils. But it is estimated that the species belonging to this genus had an average height of 5.5 metres (18 feet) at the shoulder and were 8 metres (26 feet) long. The weight of an Indricotherium was estimated to be between 15 to 20 tonnes (33,000 to 44,000 lb), which was four times heavier than that of the weight of a modern elephant. The skull was comparatively smaller than its body, having a length of more than 1.2 metres (4 feet). It had relatively long front legs and neck, along with strongly constructed limbs. Their massive size was also a reason behind their slow rate of reproduction.

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Formerly named Baluchitherium, this group of species were mainly distributed in habitats ranging from the arid deserts (consisting of few scattered trees) to the subtropical forests. This particular page, thus, describes the evolution of the Paraceratherium genus and some of its unique characteristics. 

Scientific Classification of Paraceratherium

The taxonomy of the genus Paraceratherium and its species has a very long and complicated history. Formerly known as Balucitherium, this particular genus belonging to the family Hyracodontidae is thought to have three discernible species such as  P. bugtiense, P. transouralicum, and P. huangheense. Out of these three,  P. transouralicum is the most commonly known species and hence, most of the reconstructions of the genus were made depending on this type. 

The major differences between the species P. bugtiense and P. transouralicum were due to sexual dimorphism. Given below is the scientific classification of the genus Paraceratherium.

Kingdom: Animalia

Class: Mammalia

Phylum: Chordata

Order: Perissodactyla

Family: Hyracodontidae

Genus: Paraceratherium

Species: P. bugtiense, P. transouralicum, P. huangheense 

Description of Paraceratherium

Paraceratherium is known as the largest land mammals that ever existed. Although the precise size of the mammal is still unknown due to incomplete fossil records and lack of complete specimens, it is assumed that it had a total body length of 8.7 m (28.5 ft) from front to back ( as per the theory of Granger and Gregory in 1936). The Paraceratherium had a weight similar to that of some extinct proboscideans, which was known to have the largest complete skeleton belonging to the steppe mammoth (Mammuthus trogontherii). In spite of their roughly equivalent mass, the Paraceratherium was assumed to be taller than any proboscidean. They had an estimated shoulder height of 5.25 m (17.2 ft). The neck was estimated to be 2 to 2.5 m (6.6 to 8.2 ft) long as suggested by the palaeontologists Michael P. Taylor and Mathew J. Wedel in 2013.

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These calculations have been done based on the fossil remains of P. transouralicum as this particular species is known to have the most complete remains. The estimates have been based on skull, teeth, and limb bone measurements.

There have been no records on the indications of the skin texture and colour of the mammal due to lack of skin impressions or mummies. But as per the life restorations, it is assumed that the creature had a thick and folded skin texture, which was grey and hairless based on modern rhinoceroses. Because hair retains body heat, modern large mammals such as elephants and rhinoceroses are largely hairless. As proposed by Prothero, contrary to most depictions, the genus Paraceratherium had large elephant-like ears which were used for thermoregulation. 

It is because of the fragmentary nature of the Paraceratherium fossils that the skeletons of this particular animal have been reconstructed in several different ways since its discovery. It was in the year 1923, Matthew supervised an artist to draw a reconstructed image of the Paraceratherium skeleton based on the complete specimen of P. transouralicum and using the proportions of a modern rhinoceros as a guide. The result was too squat and compact, but later Osborn had a more slender version drawn in the same year for better understanding. These later life restorations eventually made the animal too slender, with little regard to the underlying skeleton. In the year 1959, Gromova published a more complete skeletal reconstruction based on the P. transouralicum skeleton from the Aral Formation, but this also lacked several neck vertebrae.

Indricotherium Skull

The Paraceterium or Indricotherium was found to have the largest skull, which was 1.3 metres (4.3 ft) long and measured around 33 to 38 centimetres (13 to 15 in) at the back of the skull. It was also 61 centimetres (24 in) wide across by the zygomatic arches. The bones present above the nasal region of Paraceratherium are found to be very long, with the nasal incision extending into the skull. This indicates that the genus Indicotherium had a prehensile upper lip which is similar to that of the Indian Rhinoceros, the Black Rhinoceros as well as a short proboscis (trunk) as in tapirs. 

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The skull also had a low and narrow back, without any large lambdoidal crests at the top and the sagittal crest. These crests, however,  are commonly found in horned and tusked animals that require strong muscles to push and fight. The deep pit present in the skull was used for the attachment of nuchal ligaments, which helped in holding up the skull automatically. The Paraceratherium (earlier known as Balucitherium) had a very wide occipital condyle and appeared to have had large and strong neck muscles, which helped in sweeping its head strongly downwards while foraging from branches. Out of the three species of Paraceratherium, P. transouralicum had a domed forehead, whereas the other species had a flat forehead, possibly due to sexual dimorphism. A brain endocast of P. transouralicum showed that the species had only 8 percent of the skull length, while in the Indian rhinoceros, the brain is 17.7 percent of its skull length.

All the three species under the genus Paraceratherium are mainly differentiated based on their skull characteristics. The species P. bugtiense consisted of relatively slender maxillae and premaxillae, along with shallow skull roofs and mastoid-paroccipital processes that were relatively thin and placed back on the skull. It also had a lambdoid crest and an occipital condyle with a horizontal orientation, which was also found in the genus Dzungariotherium. 

P. transouralicum, on the other hand, had a skull that consisted of robust maxillae and premaxillae, upturned zygomata and domed frontal bones. It also had thick mastoid-paroccipital processes, a lambdoid crest and an occipital condyle with a vertical orientation.

The species P. huangheense and P. bugtiense differed from each other on the basis of the anatomy and size of the rear portion of the jaw.

Unlike most of the primitive rhinoceroses, the Paraceratherium consisted of a single pair of large and conical incisors in either jaw, which were described as tusks. The upper incisors used to point downwards, whereas the lower ones were comparatively shorter and pointed forwards. This particular arrangement is assumed to be unique to the genus Paraceratherium and the related Urtinotherium. The incisors were found to be larger in the case of males and were separated from the row of cheek teeth by a large diastema (gap). This particular feature can also be found in mammals having different specialisations for incisors and cheek teeth. The upper molars, found in this group of animals were V-shaped and had a pi-shaped (π) pattern along with a reduced metastyle. The premolars, on the other hand, partially formed the pi pattern. Each molar had the size of a human fist but was relatively smaller than the size of the skull. The lower cheek teeth were L-shaped, which can be commonly seen in the family of rhinoceroses.

Indicotherium - Diet and Feeding

The genus Indicotherium consisted of simple, low-crowned teeth which indicate that the major portion of their diet mainly consisted of relatively soft leaves and shrubs. Several studies have also confirmed that the genus Paraceratherium was dependent on a diet of soft leaves and fed chiefly on C3 plants, which are mainly leaves. Like some of its perissodactyl relatives, which includes the horses, tapirs, and other rhinoceroses, this particular group was predicted to be a hindgut fermenter as it extracted relatively low nutrition from its food and depended on large volumes to survive. The Paraceratherium also had a large digestive tract similar to that of other large herbivores.

As per several theories, it is assumed that these animals had large incisors that helped in defending or loosening shrubs by moving the neck downwards, thereby acting as picks and levers. Some Russian authors also predicted that the tusks present in the species were mostly used to break twigs, stripping bark and bending high branches. These assumptions were done based on the early Oligocene species that had larger tusks than Paraceratherium and probably had a bark-based diet. 

Distribution of Paraceratherium

The remains of the genus Paraceratherium were discovered in early to late Oligocene (34 – 23 million years ago) formations across the countries of China, Mongolia, India, Pakistan, Kazakhstan, Georgia, Turkey, Romania, Bulgaria, and the Balkans. The distribution and habitat of this animal group can be correlated with the palaeogeographic development of the Alpine-Himalayan mountain belt. The range of Paraceratherium finds also indicates that the species were mostly found to inhabit a continuous landmass with a similar environment across it. But as per various palaeogeographic maps, it is seen that this area had various marine barriers, hence, the genus was successful in being widely distributed despite this. Several other faunas also coexisted with Paraceratherium which included the rhinoceroses, artiodactyls, rodents, bear dogs, weasels, hyaenodonts, nimravids and cats.

The Paraceratherium appeared to have a varied range of habitats, depending on the types of geological formations. The Paraceratherium mainly lived in parts of China that had dry lakes and abundant sand dunes. The most common plant fossils found in those areas are leaves of the desert-adapted Palibinia. As per a fossil-based study of the pollens, it is found that much of China was woody shrubland, with plants such as saltbush, Mormon tea (Ephedra), and nitre bush (Nitraria), which were all adapted to the arid environments. The Hsanda Gol Formation of Mongolia that represented an arid desert basin is thought to have had a few tall trees and limited brush cover, as the fauna present there mainly fed on treetops or close to the ground. Thus, the trees in Mongolia and China majorly included birch, elm, oaks, and other deciduous trees, while Siberia and Kazakhstan had walnut trees.  Dera Bugti in Pakistan had dry, temperate to subtropical forest.

During the Early Oligocene, the climate around Central Asia was arid, while South Asia was more humid and filled with a patchwork of open landscape and green forests. The giant rhinos likely migrated to South Asia in search of food to fill their bellies. Then, during the late Oligocene, the rhinos ventured back northward when the climate changed again and Central Asia was no longer arid. The trail of various giant rhino fossils suggest they migrated from the Mongolian Plateau into northwest China and Kazakhstan and then down through Tibet into Pakistan

Extinction of Paraceratherium

The reasons behind the extinction of Paraceratherium are still unknown. They survived for about 11 million years, and thus, it is very unlikely that their extinction was caused due to a single reason. Theories include climate change, low reproduction rate, and invasion by gomphothere proboscideans from Africa in the late Oligocene (between 28 and 23 million years ago). Gomphotheres may have been able to considerably change the habitats they entered, in the same way that African elephants do today, by destroying trees and turning woodland into grassland. Once their food source became scarce and their numbers dwindled, Paraceratherium populations would have become more vulnerable to other threats.[51] Large predators like Hyainailurus and Amphicyon also entered Asia from Africa during the early Miocene (between 23 and 16 million years ago); these may have preyed on Paraceratherium calves. Other herbivores also invaded Asia during this time.