Indohyus restoration by Carl Buell
Meet Indohyus
The image above, created by artist Carl Buell, shows a charming little animal called Indohyus, about the size of a modern raccoon, that lived some 50,000,000 years ago in what is now northern India. Indohyus is a member of the Raoellidae, an obscure, extinct family of hoofed mammals closely related to the artiodactyls (even-toed hoofed mammals, currently represented by pigs, hippos, camels, deer, etc.) Raoellids are only known from Eocene-aged (56–34 million year-old) fossils from southern and southeastern Asia.
Like any good group of obscure, extinct mammals, the raoellids were primarily known from fossil teeth. Until this week, that is, when a team of scientists led by Hans Thewissen described new cranial and postcranial fossils of Indohyus in the journal Nature. These new fossils are helping to provide a more complete picture of raoellid appearance, life habits, and possible relationships.
She is heavy, she’s my sister
As the restoration above shows, Indohyus was a long-legged animal, with the characteristic “double-pulley” ankle that gives artiodactyls a little extra swing to their step. Yet despite it’s relatively graceful profile, the walls of the bones of Indohyus are much thicker than in most other mammals. This is an adaptation commonly seen in aquatic animals, where thick bones act as ballast—helping them move underwater without automatically floating to the surface. Thewissen et al. suggest that the heavy skeleton of Indohyus allowed it to walk along the bottom of rivers and lakes, possibly as protection from predators or to help it search for food. Analysis of isotopes within the fossils add some additional weight (no pun intended) to this aquatic hypothesis.
The semi-aquatic lifestyle of raoellids seems to have been the start of something very big. Key features in the skull of Indohyus led Thewissen and his team to the conclusion that raoellids were the closest known relatives (or sister group) to whales.
Indohyus by Carl Buell
The Telltale Involucrum
Indohyus shares several dental features with early whales, including a front-to-back arrangement of the incisors, high crowns on its back molars, and similar wear facets. But the clincher is a little thickened lip of bone on the inside of the middle ear cavity, known as the involucrum, which likely assists in hearing underwater. Until this week, only whales were known to possess this feature. But one of the new Indohyus skulls shows that this little raoellid had a lovely little involucrum as well (see it here, and be amazed).
Shaking the Tree?
We know from the fossil record that, back when whales had ankles, they had double-pulley ankles. But exactly where whales fit into the artiodactyl family tree has been a matter of some debate. Molecular studies showed a close relationship to hippos, but there is a 35 million year gap between the oldest fossil whales (50 million years old) and the presumed origin of the Hippopotamidae (15 million years ago). Some researchers have held up the pudgy, long-faced anthracotheres as relatives of both hippos and whales, but the middle ear of Indohyus is very strong evidence that the little, long-legged raoellids were the whales’ closest kin.
That still leaves the question of how raoellids are related to other artiodactyls. Thewissen et al. propose a phylogeny showing that, as raoellids are the sister group to whales, then whales + raoellids form the sister group to all other artiodactyls. This keeps a close relationship between the whales, raoellids, and artiodactyls (as shown by their similar ankles), but removes whales from a close relationship to any particular artiodactyl lineage, such as hippos or anthracotheres.
Still, the classification of cetaceans has changed quite a bit in the past 15 years or so, and it will be interesting to see what future studies and discoveries have to say on the topic.
Reference:
Thewissen, J. G. M., Cooper, L. N., Clementz, M. T., Bajpai, S., and Tiwari, B. N. 2007. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature vol. 450, 20/27 December 2007, pp. 190–195.
Elsewhere online:
And a big thanks to Carl Buell for graciously granting permission to reproduce his excellent art in this post!
—Matt Celeskey.
File under: Mammals, Paleogene.
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