The evolution of the rhinoceros

Rhinos can be traced back over some 50 million years, with a complex series of evolutionary paths throughout a sequence of geologic or evolutionary epochs (the term scientists use for these periods of time).

All rhinoceroses belong to the mammalian order, Perissodactyla (from the Greek perissos, meaning numbers odd, and daktulos, meaning a finger or toe). In other words, they are all odd-toed ungulates (ungulates meaning hoofed-animals), with the axis of symmetry of the foot passing through the central toe, a characteristic also known as mesaxonic. Other Perissodactyla include horses and tapirs, and their evolution began during the early Paleocene,or possibly even earlier in the late Cretaceous.

Rhinoceroses were a very diverse and abundant family of mammals and were the largest terrestrial mammals on all the northern continents from about 35 to about 20 million years ago. During this time they ranged over all ecosystems and exhibited a wide range of behaviour, with many different size and morphological adaptations.

The five species found today (white, black, greater one-horned, Sumatran and Javan) come from different lineages. The Sumatran rhino is thought to be the oldest and the most archaic form. As far as we can tell (fossil records with radioactive dating disagree with molecular DNA clocks), the five modern species probably originated at these approximate times in the past:

Sumatran 15+ million years ago
Black 4-10 million years ago
White 2-5 million years ago
Greater one-horned 2-4 million years ago
Javan 2-4 million years ago

The rhinoceros has a massive body and a large head with one or two horns, depending upon the species, placed in the middle of the frontal or nasal bone of the skull. The horn has a dermal origin. It is composed of compressed fibrous keratin. Rhinoceroses have a very elongated skull, which is elevated in the occipital part. They have a small braincase, and the nasal bone is clearly projected forward, beyond the premaxillae bone. Its surface is rough where the insertion of the horn takes place.

All the Perissodactyla, especially rhinoceroses, have or had lophodont teeth, in other words the teeth are formed by two transverse lophs of enamel. The dental formula varies between species. I 0-3/0-3, C 0-1/0-1, P 3-4/3-4, M 3/3 x 2 = 24-44. They could be grazers (eg Elasmotherium), which means their premolars and molars are hypsodont (high crown), or sub-hypsodont (e.g. the woolly rhino, Coelodonta antiquitatis); but most of them are browsers with brachydont teeth (low crown). These characteristics are directly related to the species’ environment. The hypsodont species could eat grass (a very rough food for the enamel), so lived in open habitat. On the contrary, the brachyodont species could only eat soft vegetable (such as leaves); consequently, they live in a more forested environment.

Frédéric Lacombat