Wednesday, January 31, 2007

Ward's Surprising Crocodile Evolution

Peter Ward's most recent book, Out of Thin Air, has a lot of weighty scientific hypotheses contained in it. The whole work requires some serious guts to put out like he has. There are a lot of testable hypotheses there. Two of the hypotheses that I have issues with are that endothermy evolved as a response to a hypoxic atmosphere and what he has to say about crocodiles. The endothermy argument I am going to have to mull over more before posting. However, what he has to say about the evolution of crocodiles is a bit easier to mull, chew on, and spit out. Note th following, wrt to the crocs.

One of the most enduring scientific debates of the past two decades has been about the metabolism of dinosaurs. Were they endotherms, ectotherms, or so massive that neither applied? The arguments have gone back and forth, based on evidence as disparate as bone structure, oxygen isoptopes from dinosaur fossils, and reputed preator-prey ratios. Here it is proposed that endothermy originated as an adaptation to low atmospheric oxygen. If that was the case, endothermy should have evolved in multiple lineages near the end of the Permian. We have seen that such evidence exists for the late Permian therapsids, the lineage leading to mammals. But what of the other groups of Permian reptiles, the diapsids and anapsids? There is no evidence one way or another about the anapsids, but this is not the case for the other large reptilian group, the diapsids (or archosaurs) - the ancestors of crocdiles, dinosaurs, and many other lineages. Until recently most arguments about this lineage have rested on evidence from the modern crocodile group. It is generally agreed that crocodiles are archosaurs belonging to a lineage dating back to the late Permian. According to most phylogenies, this Permian group was also ancestral to the dinosaur-avian lineage and that the fundamental split into separate crocodile and dinosaur-avian lineages took place in the middle to late Triassic. Therefore, late Permian and early Triassic archosaurs were ancestors to both later lineages. So when might endothermy have evolved, if it did at all?

The large number of extant crociles are all ectotherms, and because of this it has been theorized that if endothermy evolved anywhere in the archosaur lineages other than in birds, it did so only in dinosaurs. According to this phylogeny, then, endothermy evolved after the crocodile-like lineage split off from the dinosaur-bird lineage. The former group, known as the crurotarsans, evolved into a number of very successful and common taxa of the middle to late Triassic, including the crocdile-like phytosaurs, the wholly terrestrial aetosaurs, and the carnivorous rauisuchians. Endothermy evolved somewhere on the other great branch, known as archosaurs, the lineage leading to dinosaurs and birds. We know that birds are warm-blooded, and in recent decades there has been a great deal of research and speculation as to whether the ancestors of birds, the saurischian dinosaurs, were themselves endothermic. Several camps of dinosaur specialists have formed around this fundamental question about dinosaur metabolism. One group includes Jack Horner, Robert Bakker, and A. de Riqules arge that dinosaurs were endotherms. More recently, however, a new faction has come forward suggesting that dinosaurs and even the earliest birds were all ectotherms and that endothermy in birds did not arise until at least the Cretaceous Period.

Recently a new hypothesis has been put forward by the same group that argued that the late Permian archosaurs had a four-chambered heart and were at least primitively endothermic [a page earlier in the book, but I don't have time to type in the whole fscking book - WB]. This idea has arisen from the recognition that, like the contemporaneous therapsids, the late Permian and early Triassic archosaurs had a more upright posture with legs beneath the body, rather than sprawled to the side in lizard-like fashion. The skeletons of both groups suggest an active life style of high mobility. In this model, all the basal archosaurs had warm blood. But later, perhaps in the middle Triassic, the crocodile and crocodile-like lineages returned to a largely aquatic lifestyle, and re-evolved ectothermy while maintaining the crocodilian four-chambered heart. The argument here is that ectothermy was thus secondarily re-evolved in this lineage for a simple reason: ectothermy aids diving by enabling the animal to stay underwater longer. [emphasis added - WB] Large size also favors diving and breath holding. For every order of magnitude body mass increases, diving time is doubled. Another adpation to diving is blood "shunting", where oxygenated blood is mixed with less oxygenated blood ruing dives.

This latter view fits well with the history of the archosaurs. Modern crocodiles have four-chambered hearts, a trait associated with endothermy. Additionally, crocodiles came from ancestors that had an upright rather than sprawling posture. This upright posture is found today only in endotherms.

While the major radiation of the arly archosaurs took place in the Triassic, they were present in the late Permian strata. The oldest member of the group is Proerosuchus [aka Chasmatosaurus - WB] from the Karoo of South Africa. Its appearance coincides with the oxygen minimum, and it might be the first of its lineage to have been endothermic.
Out of Thin Air, pgs 151-2.

So, basically, crocs lose their endothermy when they go into the water. While I can't say that this didn't happen, I cannot think of a single instance of any other animal that has done this after returning to the water. Birds? Nope. Mammals? nyet. While crocs can hold their breath for over an hour (quick google shows up to two hours), I am hitting some pretty strong skepticism internally here. If its such a successful strategy, why don't we see more of terrestrial-turned-aquatic critters doing this?

I'm very skeptical.

I can say this might be testable with the extant archosaurs. If the ectothermy has been re-evolved, then there might be a chance there is genetic 'scar tissue' that indicates where it was regained and endothermy, erm, lost. Could we go digging through the crocodilian, bird, and perhaps other diapsid genomes to find the changes?

Maybe we can get Razib, Carlos, and Darren to comment.

Next time, thoughts on 'endothermy as a response to low atmospheric oxygen' and a scenario as pictured by Ward of 'active life style of high mobility' critters standing around doing nada, panting to conserve oxygen all the time because the altitudal compression gave sea level the same oxygen level matched 5k ft of our atmosphere.

A much later update (June 08): I wrote up a post about a paper on basal endothermy in archosaurs.

5 comments:

Anonymous said...

Birds and mammals have insulation, which makes it much easier to retain endothermy.

If the ur-crocs had nothing but scales, then a shift to exothermy seems more plausible to me. Looking at it from the other direction, at least three lineages of fish seem to have been trying to develop endothermy, but with only very partial and limited success... probably because of gill respiration (hard to stay warm when water is flowing through holes in you) but possibly also because they're not insulated.


Doug M.

Will Baird said...

Possible.

I am still unconvinced.


The insolation during the Triassic would have been less necessary than at our times. The crocs were also noted for their terrestrial forms during the Mesozoic. Did the land crocs ever go 'into the water?'

Additionally, isn't most of the insulation that mammals, or very aquatic birds like penguins, use in the water, well, fat? Crocs lay that on relatively well and had their been pressure to use like the mammals did as such instead of just as a food store. It'd be unique in the reptilians, in fact it'd be an interesting ATL branch in and of itself.

I think this needs to be turned testable somehow. hrm.

Stevo Darkly said...

One thing to point out: Aren't aquatic crocodilians unlike most birds and mammals that have returned to the water in that the birds and mammals are active swimmers and seekers of food, while the crocs are ambush predators that spend a lot of their time lying still, submerged, near the banks of rivers especially? Wouldn't the less-active hunting style of crocs fit better with a reduced metabolism, which would explain why crocs reverted to ectothermy while more-active aquatic birds and mammals did not?

Just a thought.

Will Baird said...

Ah! But Carl Buell (aka Olduvai George) has some of what the early whales were like, such as Ambulocetus natans, that it seems that some of the ancient whale family were pretty close to mammalian crocs. His renderings are based on consulting with the authorities in the field of archaeocetaceans.

Note: whales didn't lose their endothermy even when they really didn't need it in the Eocene.

Anonymous said...

Mr. Baird,

Is not the suggestion of a size variable in metabolics something that deserves speculation?

You can die of hypothermia in 85 degree water and if you look at animals like polar bears, scaling is directly related to external surface area vs. internal thermal volumes on a total immersion period basis (so that insulation can be 'rated' to a given conductance level from core to outer temperatures).

It would seem to me that you also may need to look at (any) differences between salt and any fresh water crocs as both base environmental (temp and salinity*), energy use (cruise vs. ambush) and specific hunting behaviors (depths and sprint condition for predation on other swimmers). All of which may change energy needs with type of adaptation.

Indeed, isn't there something out there on blood salts regulating body temperature through metabolic depression?


KPl.