Monday, January 07, 2008

Indohyus: Awesome, but not a Whale


Blogging on Peer-Reviewed Research
A note on this restoration: It is tentative. Until I get the damn Wacom tablet working again, you'll have to do with this hairless, B&W version of Indohyus. When my technology decides to work for me, I will replace this shoddy line drawing with a much improved, fuzzy, colorful animal. But I'm going into blogging withdrawl, having not done so intently since Christmas. I apologize to all six of my readers, because I've been promising a post about Indohyus for awhile.

For those paleo-bloggers among you who have been living under a rock for the past few weeks, there's a new ancestral whale in town. You didn't think it could get much more basal than Pakicetus, did you? Honestly, it can't, but Indohyus (Thewissen, et al. 2007)* comes darn close. New fossil material from the quite old genus shows that the Raoellidae, the family to which Indohyus belongs, shares quite a few features in common with early and modern whales. As I will illucidate further on, this does not mean that Indohyus is actually a basal whale.

About the size of a racoon, Indohyus a lithe, gracile little animal. At first (or second, or third) glance, this raoellid looks absolutely nothing like the superficially crocodilian Ambulocetus or foxy Pakicetus. Truthfully, Indohyus has no modern analogue, which may be why the media has been so hard-pressed to compare it some extant animal for their readers. In a now infamous flub, Seth Borenstein of the Associated Press, in a stumbling attempt to explain what Indohyus looked like for those of us who couldn't just look at the accompanying image (by the excellent Carl Buell), suggested that it resembled "a long-tailed deer without antlers or an overgrown long-legged rat." Deer don't look much like rats, Seth.

Comical analogous snafus aside, the point is that Indohyus is suspiciously devoid, on the surface, of any features that would identify it as a potential cetacean ancestor. Probing deeper, however, Thewissen, et al. have found several subtle clues to its common ancestry.

First and foremost, little Indohyus' middle ear features an extra "wall" of bone called the involucrum, which helps whales hear underwater. Mammals are able to hear underwater thanks only to the vibrations of the skull bones (so our underwater hearing is severely dampened). Sound waves hit the whale's involucrum, and the waves are amplified and directed toward the ear canal. Modern cetaceans have managed to evolve melons and echolocation which further amplify sound waves via both the melon and the lower jaw. The involucrum is merely the first step in what will eventually be a very complicated method of hearing underwater. In a way, it's not a whole lot better than the basal mammalian condition--the involucrum merely amplifies the skull vibrations brought on by underwater sound waves.

In fact, Thewissen et al. aren't even that excited about the involucrum. Rather, they focus on the fact that Indohyus was an aquatic wader or bottom-walker. The bugger displays osteosclerosis, which is a fancy word for "thick limb bones." Osteosclerosis is not a unique feature to whales. Virtually all aquatic vertebrates evolved it due to the rigors of a marine lifestyle. In terms of bone thickness, Indohyus falls somewhere between hippos (which are bottom-walkers) and Ambulocetus (a swimmer). Because Indohyus' gracile limbs were not well adapted to a swimming lifestyle, Thewissen et al. interpret Indohyus as a bottom-walker.

The chemistry of Indohyus' enamel was studied to try and figure out its diet, but the authors could only conclude that Indohyus' diet must have been quite different from Pakicetus and Ambulocetus. Omnivory or herbivory are suggested, although the authors lean away from aquatic foraging toward the end. Thewissen, et al. suggest that Indohyus lived a lifestyle comparable to the muskrat, who spends most of its day in the water, coming ashore to sleep and eat its leafy greens (although the muskrats here in Alaska are strictly carnivorous).

Now, on to the phylogeny. I'm stealing Carl Zimmer's excellent cladogram, which shows exactly how Indohyus relates to whales (as far as we know) and why it's NOT on the direct cetacean lineage. Sorry, Carl!

What Indohyus and its raoellid cousins represent, kids, is an outgroup. Indohyus : Cetacea :: Silesaurus : Dinosauria. Need another example? Okay, I can do this all day. Herrerasaurus : Saurischia :: Indohyus : Cetacea. Want a non-dinosaurian comparison? Well, too bad. At any rate, Indohyus is too specialized to be a proper ancestral whale. Also, raoellids were around at the same time as the first cetaceans. Indohyus was not some ancestral whale. Rather, the Raoellidea shares a common ancestor with the Cetacea, and that common ancestor had an involucrum and was probably semi-aquatic by way of osteosclerosis.

The "Y" node on Carl's cladogram with Indohyus on one end and the branching Cetacea on the other is an unnamed clade. I humbly submit "Cetaceaformes" or some similar term.

The realization of Indohyus' cetacean features further calls into question the genetic evidence favoring a hippo/whale grouping. It may be that hippos still are related to whales, but in a (hippos + Cetaceaformes) way. We really need some basal hippo fossils. It could be that an aquatic nature was basal among some hypothetical hippo/Cetaceaformes ancestor (I don't even want to think about what that clade would be called), and that the invoculum is a novelty for Cetaceaformes. Now I'm rambling.

But that's Indohyus. An awesome little animal, to be sure, but I don't want people getting the wrong idea about its relationship to whales. It is NOT a whale, but an outgroup to whales.

* Was anyone else annoyed that Thewissen, et al. did not reference the species anywhere in the paper? According to the always-reliable Wikipedia, Indohyus consists of two species, I. major and I. indirae. It's possible, if not probable, that the two species are synonyms of each other, but I would have liked some explaination of Indohyus' taxonomy in the paper. It wasn't clear whether Thewissen, et al. was describing a brand-new creature (as the news items would lead one to believe), new material from an existing taxon, or a new taxon from somebody's basement.

2 comments:

Donald said...

Osteoschlerosis = pachyostosis? I'm not up on my terminology.

Anonymous said...

The osteoschlerosis is a kind of paquiostosi. Cortical bone thickness in secondarily aquatic tetrapods is commonly increased at the expense of the medullary cavity, a pattern called osteosclerosis.