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Mammals of Southern Africa

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The Evolution of the Rhino or Rhinoceros species of the world

Evolution of the Rhino

Indricotherium is the name given by palaeontologists to the huge hornless browsing rhinoceros that became extinct approximately ten million years ago. The species represented the peak of rhinoceros evolution at a time when the group as a whole was very successful both in the Old World and the New.

In evolutionary terms, rhinoceros are the surviving members of a branch of very ancient animals, the first mammals to develop hooves, and their ancestors, including the paenungulates or 'nearly hooved', and the protungulates, or 'first hooved' animals, which are species long extinct. Rhinoceros belong to the group known as the perissodactyls, or 'odd toed ungulates' which includes horses and tapirs.

The group's origins are in the Eocene Period approximately 50 million years ago. During the following 40 million years, leading up to the Pliocene and Pleistocene Periods, the perissodactyls were themselves replaced by the modern artiodactyls, the 'even-toed ungulates', including deer, antelopes and gazelles, pigs, camels and hippopotamus. These species became fast movers, with keen senses and well developed brains but most importantly many of them became ruminants with stomachs that enabled them to digest cellulose by fermenting it with the aid of bacteria.

Although all surviving species of rhinoceros look rather similar, and are descended from a remote single common ancestor, they are the modern ends of two evolutionary lines which diverged about 30 million years ago, in the middle of the Oligocene. One line led to the advanced grazing, Indian one-horned rhinoceros, and the other gave rise to the two-horned species, a branch of which was the woolly rhinoceros, that survived the Stone Age. About ten million years ago, a branch of the two horned group found its way to Africa and thereby began the evolution of today's black and white rhinoceroses. They evolved to feed without cutting teeth, and therefore lost the incisors which form the tusks of the other species.

The surviving species are a remnant of the vast and varied range of animals that lived during the heyday of the rhinoceros. The fossil record is surprisingly full. In North America as well as in Eurasia, there were tubby, hippopotamus-like creatures, the Amynodonts, or 'defensive toothed' rhinoceroses, which were partly aquatic, and like the modern animal in appearance. The oldest rhinoceroses, the Caenopenes and the Aceratheres, appeared in the Oligocene in North America and Europe about 30 million years ago. They were distinctly rhinoceros-shaped, but their fossil skeletons show no sign of horns until just before they died out in the Pliocene when it seems they had begun to develop small horns, but their principal form of defence was still the lower incisors, which had developed into a rather formidable set of tusks.

Close relatives to Caenopenes and Aceratheres, the Paraceratheres, produced some of the biggest rhinoceroses which were also the largest terrestrial mammals ever to have lived. The largest, Indricotherium asiaticum 's fossilised remains were discovered in Kazakhstan in central Russia and were dated at 35 million years old. A similar find was made in the Gobi Desert in the early 1920's. Paraceratherium had no horns but formidable tusks and low crowned molars indicated it would have been a browser with a reach not much less than a modern day giraffe.

Asiatic two-horned rhinoceros appeared in the Miocene, 15 million years ago. One of their descendants was the woolly rhinoceros, Coelodonta antiquitatis, which was first discovered in 1799 in the permafrost of Siberia, complete with skin and fur. Drawings of this species occur in the cave paintings of the early Stone Age, but it was extinct by the end of the last Ice Age 15 000 years ago. Although frozen specimens have been found with willow leaves and fragments of coniferous twigs between their teeth, it seems to have been a principally grass eating species.

The two African species, the black rhinoceros, Diceros bicornis and the white rhinoceros, or square lipped rhinoceros, Ceratotherium simum, are a separate branch of the family which split from the Asiatic two-horned rhinoceroses, approximately 10 million years ago, at the end of the Miocene. The black rhino is considered to be more primitive and separated from the white, between four and five million years ago. Taxonomists doubt whether they should be separated into two genera because they are so closely related.

The Last of Their Line

Rhinoceroses are considered an example of a major animal group which has long past the peak of its evolutionary development. The giant browsers like Indricotherium and Paraceratherium, roamed the plains of Mongolia and opinions differ as to the reason for the decline of these huge animals. With their weight poised over their front limbs, counter-balanced by their massive heads, their motion must have been excessively ponderous. That they were victim of a contemporary predator is unlikely because of their size.

Their generally placid demeanor and slow movement would have made them easy prey for a hunter and Stone-Age man was familiar with the woolly rhinoceros as is depicted in cave paintings such as those at Font-de-Gaume and other sites in France. Hunters in Sumatra, at the end of the last century, used sharpened stakes in pits dug in the rhinoceroses' habitual trails, and African tribesmen are also known to have killed rhinoceros from an early time in a similar manner with similar weapons. However, it was the great white hunters and explorers that first penetrated the interior of Southern Africa that severely depleted both species of rhinoceros.

After the Second World War, the preparation of land for agricultural settlement in East Africa caused the death of very large numbers of rhino. In India, the clearing of land for tea plantations was a principal cause of their decline, and even without direct killing, the clearance of woodland scrub would have been similar in its effect to the hypothetical change in vegetation which might have accounted for the decline of the large browsing species of the Oligocene.

The perissodactyls are an outmoded group, and humanity cannot prevent the eventual extinction of the rhinoceros, which is probably inevitable in geological time, and which is going on around us now, as it has been for many millions of years.

The Rhino species of Africa > The White Rhino, Ceratotherium simum The Black Rhino, Diceros bicornis

  
 

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