Hairy Museum of Natural History

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.

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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.

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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.

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The cover of this past week’s issue of Nature is graced with a restoration of a newly discovered gliding mammal from the Age of Dinosaurs. The associated article inside describes Volaticotherium antiquum—”the ancient flying beast”—known from a flattened skeleton from the Daohugou beds in northeastern China. The fossil preserves traces of a big blob of hair and soft tissue that the authors interpret as a patagium—a large area of skin supported by the limbs that, when spread open in midleap, enabled Volaticotherium to glide through the treetops. Such gliding has evolved independently several times among mammals, in flying squirrels, for example, and colugos, and, (presumably) among the ancestors of bats.

Volaticotherium, however, hails from well before any of these aerial upstarts. The oldest known gliding rodent is not a squirrel but an eomyid from the Late Oligocene (~25,000,000 years ago), and the earliest known evidence for flight in bats comes from the Eocene, 51,000,000 years ago. Volaticotherium is at least 70,000,000 years older than that, and could be even older. (The age of the Daohugou beds is controversial, but lies somewhere between the Early Cretaceous (~125,000,000 years ago) and the Middle Jurassic (~170,000,000 years ago).) If the older age is correct, this would mean that Volaticotherium was experimenting with aerial locomotion before the 150,000,000-year-old Archaeopteryx, the earliest known bird.

Further reading:

Meng, J., Hu, Y., Wang, Y., Wang, X., and Li, C. 2006. A Mesozoic gliding mammal from northeastern China. Nature 444, 889–893. Editor’s summary.

A larger image of the cover art by Chuang Zhao and Lida Xing is shown at National Geographic News.

12/19 Update: Much more information on Volaticotherium (and how its discovery fits into the grand arc of Mesozoic Mammal Studies) in this article by Trevor Dykes.

—Matt Celeskey.

File under: Cretaceous, Jurassic, Mammals, Recent Discoveries.

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An interesting new paper recently published online in the Proceedings of the National Academy of Sciences reports on Miocene (16–19 million year old) mammal remains from South Island, New Zealand. The fossils, which consist of two jawbone fragments and the end of one femur, are surprising for two reasons:

1. These are the first terrestrial mammals known from the island nation before the arrival of humans, excepting bats (who were able to fly there),
2. The remains are tantalizingly primitive, closer in developmental “grade” to mammals that lived during the Age of Dinosaurs than most of its Miocene contemporaries.

Put together, these facts suggest that New Zealand was home to a heretofore unkown lineage of mammals that split off from the rest of the mammal-bearing world during the Cretaceous Period, when New Zealand split off from eastern Australia and began its lonely journey across the South Pacific.

The discovery of a relatively recent member of a group believed to be long extinct implies what is known as a ghost lineage—a series of ancestors whose fossils have yet to be found (or possibly were never preserved). The concept of a ghost lineage is perhaps easiest to envision in a diagram that combines a timeline with a family tree:

So here we have a grotesquely simplified family tree of mammals from the Middle Jurassic to the present day. I’ve left out quite a few critters, and I may be a bit off on some of the branching dates, but for a rough outline, it should suffice. On the right, the major players in today’s mammalian fauna, the placentals and marsupials, which split apart from each other about 125,000,000 years ago. Moving down the family tree we run into the long extinct aegialodontids, dryolestoids, and multituberculates. According to the new paper, the branch leading to the new New Zealand mammals split off before the aforementioned groups, but after earlier mammals like eutriconodonts, monotremes, and docodonts.

The dotted lines represent ghost lineages (to which I’ve added little ghosts to reinforce the point). Provided the family tree is correctly interpreted, these are paths of evolution that most certainly existed despite gaps in the fossil record. There are undoubtedly gaps in this diagram that I’ve neglected to portray, but hopefully the main points are clear.

Islands seem particularly prone to ghost lineages, perhaps because their isolated faunas often contain species whose relatives have met with extinction elsewhere in the world. Monotremes, for example, are today only found in Australia and some nearby islands. These modern-day monotremes (three species of echidna and the platypus) are island-dwelling relicts of a group of mammals that started down their own path sometime in the Jurassic Period. Geographically isolated and only distantly related to other living groups, their fossil record contains several long gaps with ghost lineages filling in the blanks. Viewed in this light, New Zealand seems like quite a likely spot to uncover a mammal with a 60 to 140 million year gap in its pedigree.

One more note about ghost lineages. The HMNH has been suffering gaps in its own record of late, and for that I apologize. (An interesting admission is that we’ve had more visitors lately than during periods of regular posting, which I hope isn’t a commentary on my writing.) Hopefully this post will mark the end of this particular ‘ghost lineage,’ and, with any luck, the Hairy Museum will regain its typical, punctuated equilibrium as we head into 2007!

Further reading:

Worthy, T. H., Tennyson, A. J. D., Archer, M., Musser, A. M., Hand, S. J., Jones, C., Douglas, B. J., McNamara, J. A., and Beck, R. M. D. 2006. Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific. Proceedings of the National Academy of Sciences USA, published online; doi:10.1073/pnas.0605684103. PDF. Press release.

Afarensis has the story, as does Palaeoblog.

And Darren Naish notes that this find inspired two earlier stories on unexpectedly long-lineaged synapsids over at Tetrapod Zoology.

—Matt Celeskey.

File under: Mammals, Neogene, Recent Discoveries.

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