Hairy Museum of Natural History

You could almost say, “Critters whose great big claws are their fingers”:

• Albertonykus and the Myrmecophagous Alvarezsaurs at When Pigs Fly Returns
• I, Priodontes, the tatuasu at Tetrapod Zoology

—Matt Celeskey.

File under: Cretaceous, Dinosaurs, Mammals, Neogene.

Comments on record: (0)

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:

• The Loom: Whales: From so Humble a Beginning…
• Laelaps: Shaking the Cetacean evolutionary bush: Indohyus and the origin of whales
• Greg Laden’s Blog: A Whale of a Missing Link : Indohyus
• Pharyngula: Indohyus
• Pondering Pikaia: Transitional whale ancestor unveiled
• National Geographic News: Whales evolved from tiny deer-like mammals

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.

Comments on record: (2)

From the illustration blog Drawn! comes word of The Daily Mammal, a blog where Jennifer Rae Atkins posts an original drawing of a different mammal every day. At this rate, she estimates she’ll make her way through all 5,000 or so species of living mammal in about 14 years.

This Saturday, she’ll make serious headway towards that goal by drawing 24 mammals in 24 hours as a fundraiser for Defenders of Wildlife. Donate at least $25, pick a mammal, and Jennifer will add it to her schedule on December 22. And she’ll send you the drawing! Support art, wildlife, and mammalian diversity all in one fell swoop. Not too shabby.

—Matt Celeskey.

File under: Mammals, Neogene, Webbery.

Comments on record: (0)

Anoplotherium skeletal diagram from copyrightexpired.com

Over at Catalogue of Organisms, Christopher has put up an interesting post about recently described bipedal adaptations in an extinct relative of camels, the tylopod Anoplotherium. Well worth a read!

—Matt Celeskey.

File under: Mammals, Paleogene, Recent Discoveries.

Comments on record: (0)

A couple of links from the Age of Mammals:

First up, Debby Cotter Kaspari shows off her excellent painting of the gigantic camel Megatylopus, enlarged to its mind-boggling life-size for an exhibit at the Sam Noble Oklahoma Museum of Natural History. Kaspari’s blog, Drawing the Motmot, is a must-read for anyone interested in the art of natural science.

Afarensis’ “Friday Primate” this past week was the Dryomomys szalayi, a Late Paleocene plesiadapiform from the Clarks Fork Basin of Wyoming. Plesiadapiforms were squirrely little tree-dwellers that evolved shortly after the dinosaurs went extinct. Their exact relationships have been hard to pinpoint, but the recently-described skeletons of D. szalayi and the contemporary plesiadapiform Ignacius clarkforkensis have led researchers to conclude that plesiadapiforms were true primates. This makes them some of the earliest known members of our own mammalian order, and should help shed some light on how primates first evolved. Additional resources about this story can be found at Anthropology.net and Palaeoblog.

—Matt Celeskey.

File under: Cenozoic.

Comments on record: (0)