Monday, March 18, 2013

Feathered crocodiles?!

Although it’s not new, I just became aware of a study published some years ago on embryonic development and gene-mapping of the alligator which posits that the genes for feathers are present in alligators, but at an early point in embryonic development, feathers are suppressed and instead we get typical archosaurian scales.

Woah. Really? Granted, that’s a bit tantalizing and circumstantial, but the obvious implications are fascinating. Alligators and birds are about as far from each other on the archosaur family tree as you can get. In fact, since Archosauria is a crown group, by definition, it’s all animals related to crocodylians and birds back to their last common ancestor. Finding features associated with birds on a crocodilian suggests that all archosaurs had it.

We’ve had a lot of tantalizing evidence of this already. Lots of dinosaurs have been discovered with feathers. Although most of them are from therapod lineages close to the ancestry of birds, a few tantalizing ones are not—including (possibly) two ornithischians. Therapods and ornithischians sharing a feature suggests that all dinosaurs had it.

Of course, pterosaurs are also closely related to dinosaurs (there was a bunch of noise for a few years that they might be prolacertiform relatives, mostly propagated by David Peters, who’s work has gone increasingly off the reservation—to the point where it’s difficult to take seriously anymore. The hypothesis was intriguing for a while, but most folks now see it as solidly refuted.) Pterosaurs were “hairy”—they are known, at least in many species, to have had “pycnofibers”—hair-like body coverings that may (or may not) have been homologous with feathers or proto-feathers. If they are homologous (i.e., the same thing, evolved once rather than similar things evolved independently) then that pushes the original of proto-featherish structures further back down the ornithodiran family tree. But it doesn’t push it all the way to the base of the archosaurian family tree. The alligator gene suggests that, however, that's exactly where it belongs.

In addition to that, there is other evidence that circumstantially purports to show evidence of warm-bloodedness in “crocodile line archosaurs”—and warm-bloodedness and feathers go together (feather or any other fibrous coating would be maladaptive without warm-bloodedness. Similarly, warm-bloodedness along with small body size would probably be maladaptive without some kind of fibrous integument, like hair or feathers.) In addition to 4-chambered hearts, a diaphragm, secondary palate, erect limbs, active terrestrial lifestyle, and hepatic pistons, a number of “crocodile line archosaurs” appear to have lots of features associated with warm-bloodedness. It’s been suggested, and embryonic ontogeny seems to support this, that crocodilians themselves were initially warm-blooded, and as an adaptation to the aquatic ambush predatory role, they reverted to a cold-blooded physiology to better and more efficiently operate in that role.

But crocodile-like archosaurs with feathers? That’s a staggering idea. Even really progressive dino-guys, like Greg Paul who’s been illustrating feathered protodinosaurs like Lagosuchus for years never thought of making Terrestrisuchus or Hesperosuchus with feathers.

And, of course, it's the possibility of homology that makes it all the more intriguing.  If warm-bloodedness (and a filamentous integument that went along with it) evolved independently more than once amongst the saurians, well that's interesting, but that doesn't necessarily push the features back anywhere interesting on the family tree; after all, the development was independent and position on the family tree is irrelevant to that. Image (from Wikipedia) is of Poposaurus gracilis, a poposaur (from the crocodile-line radiation of archosaurs) that bears a striking convergence to later-appearing therapod dinosaurs.

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