Since then, the artiodactyls and perissodactyls have remained as two distinct groups of ungulates. But if they are all plant-eating mammals with hooves, what sets them apart?
Artiodactyls are a very diverse group of ungulates characterized by having an even number of toes, bearing most of their weight on two middle ones. This group includes most of the hoofed mammals, from giraffes and deer to pigs and hippos. The remarkable inclusions are whales and dolphins, that descended from small deer-like animals.
Perissodactyls – the rhinos, tapirs, and horses – have done something different. Instead of using the two middle toes to support their weight, they use mainly just one. As with artiodactyls, the remaining toes have been reduced in varying degrees and in extreme cases, such as the modern horses, were all completely lost.
This means that during their evolutionary history, artiodactyls and perissodactyls both emerged from a small hoofed species that gave rise to two lineages, one that relied mostly on two toes and another that used mainly the middle one for support. But given that horses, tapirs and rhinos look so different from each other, how did their common ancestor look?
Fossils of extinct perissodactyls are actually very common, and scientists were able to piece together most of their history with a lot of detail. We know that horses, for example, emerged from tiny forest-dwellers like the fox-sized, multi-toed Eohippus, and their progressive loss of toes is well documented. The same story could be seen in the first tapirs, that lacked a trunk and the earliest rhinos that looked more like ponies and didn’t yet have the signature horns.
All of the earliest perissodactyls had been found in North America, dating back to around 50 million years ago. That led paleontologists to believe that the family appeared on that continent, until the discovery of Cambaytherium in 2005.
Cambaytherium was a pig-sized herbivorous animal that lived around 54.5 million years ago. It still had all five of its toes and while it may not have been an actual perissodactyl, it was probably closely related to the first true ones and can be used as a model for their appearance.
It had several features linking it to horses and rhinos, like the fused bones of its lower jaw. But it also had a suite of more primitive characteristics that make it similar to much more archaic mammals like Phenacodon, an early sheep-like creature that lived not long after the dinosaur extinction. Cambaytherium then serves as a bridge between the two families.
The remarkable thing about Cambaytherium, however, is that it was found in India, far away from North America. And at the time it lived, India was still an island slowly cruising through the Indian Ocean before eventually colliding with Asia. The finding of this transitional animal rewrote the history of how perissodactyls appeared and spread around the world.
Scientists now believe that the ancestors of Cambaytherium lived around the Arabian region and, together with other small animals like early primates and rodents, managed to access the Indian island before it actually collided with Asia. These early ungulates then used the isolated India as a haven for their evolution. When it finally integrated with the Asian continent, Cambaytherium and the first true perissodactyls could proliferate until they reached North America, where an explosion of diversity gave rise to the precursors of our familiar odd-toed ungulates.
Cambaytherium is one among many remarkable fossils that made paleontologists rethink what they knew about the history of some animal groups, and it shows us that animals that look totally different from each other today can all be descended from the same great-grandfather. In this case, it made us realize that rhinos are basically stout, rotund unicorns.
Rose, K. D.; Holbrook, L. T.; Rana, R. S.; Kumar, K.; Jones, K. E.; Ahrens, H. E.; Missiaen, P.; Sahni, A.; Smith, T. (20 November 2014). "Early Eocene fossils suggest that the mammalian order Perissodactyla originated in India". Nature Communications. 5 (5570). doi:10.1038/ncomms6570
S. Bajpai; V. V. Kapur; D. P. Das; B. N. Tiwari; N. Saravanan; R. Sharma (2005). "Early Eocene land mammals from Vastan Lignite Mine, District Surat (Gujarat), Western India". Journal of the Palaeontological Society of India. 50 (1): 101–113.