On the Origin Of Horses And Rhinos

The horse's hoof is in reality a single enlarged toe

The rhino and horse: two creatures which at first appear to be different. One is stocky with a limited skin palette comprising shades of grey, with one horn on its skull if it comes from either India and Java or two if it hails from Africa or Sumatra. The other is more finely built and by contrast has a distinct covering of fur, with a diverse colour palette.

Yet look at little more closely and a number of similarities become apparent. They are both quadrupeds, but the most prominent link is the feet.

Despite its size and weight the horse walks on a perpetual tip-toe, like some unwieldy ballerina. Its hoof is nothing more than a single wide, flat toe. The other toes have been reduced to stumps. In some mutant horses these present as small stumps visibly jutting out from the side of the leg. Rhinoceroses have three toes. The link comes from the odd number of toes (1 and 3) that each has; grouped under the name perissodactyla - or 'odd toed.' If we were to delve into their DNA, this degree of similarity would become even more apparent.

What is interesting is the geographical distribution of the perissodactyls. Horses and rhinos are endemic to Europe, Africa and Asia, While South America has its own type of perissodactylid - the pig-like tapir. Yet what accounts for this global distribution of this same group, but in different forms? The answer may lie with a team of researchers and a fossil discovery from a coal mine in Gujurat in India.

The skull of the 54.5 million year old perissodactylid
common ancestor Cambaytherium thewissi

It has previously been suggested that the common ancestor of different groups of perissodactylids originated on the Indian subcontinent. Alongside a treasure trove of mammal fossils dating from the upper Eocene epoch some 54.5 million years ago, 200 bones were found which belonged to a creature, which the researchers from the John Hopkins University dubbed Cambaytherium thewissi.

'Many of Cambaytherium's features, like the teeth, the number of sacral vertebrae, and the bones of the hands and feet, are intermediate between perissodactyla and more primitive animals,' said Ken Rose. 'This is the closest thing we've found to a common ancestor of the perissodactyla order.'

The story of the geographical diaspora is fascinating in itself. 'Around Cambaytherium's time, we think India was an island,' Rose continued. 'One possible explanation is that India passed close by the Arabian Peninsula or the Horn of Africa, and there was a land bridge that allowed the animals to migrate.'

Yet Cambaytherium is unique, suggesting that India was, at one point, isolated. An issue hindering further research is the use of heavy mining equipment which is damaging to the fossils found in the coal seams and may destroy additional evidence, but the team has found other mines from the same stratigraphic layer which may yield fossils from future excavations.