Thursday, June 12, 2008

Were the Basal Archosaurs Endothermic?

As most of you know, I am rather interested in the mass extinctions of life on our planet. It's not so much the morbidity. It has a lot more to do with the fact that there's a massive scale murder mystery involved and how life deals with the die offs is really an exercise in world building. Or rebuilding. My interest in all this can be traced back to, of course, the perennial question of what killed the dinosaurs. Every kid that loved the dinos wondered what did the deed. However, that interest waned as I got older and got involved with other interests and projects: solars sails, supercomputers, rockets, architecture and gob smacking sized lasers. I probably would have just read a bit here and there on paleontology and paleoanthropology if not for Dr Benton's book, When Life Nearly Died. From then on, I'd been turned back to being intensely curious about mass extinctions and the worlds that begat them. If not for reading that, I wouldn't have started down the paleo path that has brought so many readers to my blog.

While I was researching my new interest, gobbling up any and all new books on the subject that I could afford, I came across what Dr Peter Ward had to say about crocodilian evolution while reading Out of Thin Air. I was more than a bit surprised and incredulous at the prospect that crocodiles' ancestors would have been endothermic and that the crocs lost that. I couldn't wrap my head around the idea that sometime, something had given up such a metabolic advantage. I had only encountered this idea in Ward's book and not having all the time in the world (or knowledge enough as yet) to read the literature, I was extraordinarily skeptical. Then I started encountering the idea elsewhere.

Part of my resistance also stemmed from the fact that of the two extent archosaur lineages, one (crocs et al) is ectothermic and the other (Aves) is endothermic. The other nearest retatives, the other diapsids (lizards, snakes and the tuatara), were ectothermic, so, therefore, in my mind, the obviously basal condition was ectothermy. It seemed plain. It seemed obvious. Why argue over the settled? Well, then along came science.

The first bit that started to defy my expectations was what Ward pointed out: the crocs and relatives have a heart that was designed much more so for an endotherm and later modified for their unique ecological role and the behavior, long submergence with minimal activity to facilitate ambush predation. I did initially dismiss this as merely a case of the crocs lineage having split off while the prerequisite traits for endothermy were being acquired before endothermy arose in actuality. Note: I never doubted that dinosaurs were endothermic. Birds are the closest thing and they're definitely hot blooded. Some people did. Others did not, and those that did not began to wonder just how to tell whether or not something is warmed blooded from the fossil record. Once they felt they had established that the dinosaurs were endothermic, they began to study further and further back on the archosaurian family tree to establish just where it did arise. Dr Kevin Padian and his merry bad of paleo pirates were doing exactly just that and with their latest paper maybe have converted me from a 'huh. that's what they think' to 'wow. they're probably right.' So what exactly was in that paper?

Padian et al went out and sampled bones to look for growth patterns for numerous basal archosauriformes. They sliced and prepared various fossilized bones. What they were looking for was how the bones were grown. Ectotherms' bones grow in a manner that looks not all that dissimilar to tree rings. Slice a crocodile's or lizard's or turtle's bone cross wise and you would see these rings, albeit only if you looked very closely. Endotherms' bones don't do this. They have a rather different, nearly homogeneous structure. They examined the bones from numerous archosaurs and relatives. They had hoped to find out what the basal condition for the archosaurs was, ectothermic or endothermic. Their results were quite interesting and, honestly, a little unexpected.

The authors looked at a total of twelve members of Archosauromorpha. There was one outlier, purposefully, from Rhynchosauria in the form of Scanphonyx. The remainder were from the Archosauriformes. The intent was that the authors could compare something more distantly related to the remainder of the archosauriformes and have some idea what the basal and derived conditions were. Of these twelve specimens studied, four had very strong indications of the layered growth ring patterns in their bones. One had a case where it appeared initially there was a fast growth phase and then developed layered growth approach. Another they stated was poorly preserved, but appeared to have layers of the same sort of tissue growth of what we would normally find in endotherms: ie, if I am understanding what they stated correctly, this was something odd and unique in the form of a compromise. One more specimen needed to be double checked, but it looked strongly like a purely endothermic growth pattern. The remaining six had purely nonlayered bone growth structure. So who was who?

The first four that were strongly layered were Scanphonyx, "Mandasuchus," Hesperosuchus, and Luperosuchus. The one that seemed to switch growth patterns was Chanaresuchus. The one that had the odd and possibly misunderstood by me growth pattern was Herrerasaurus. The one that needed to be double checked, but looked like it was not layered was Ornithosuchus. The remainder were "Teleocrater," Erythrosuchus, Euparkeria, Thecodontosaurus, and Lesothosaurus. Now take a moment and go back to look at the cladogram. Now wrap your brain around that one

There are some basic statements that can be made at this time. It appears that the basal condition for the ancestors of archosauromorpha were ectothermic. This is a common, basal characteristic of all amniotes and the endothermic condition is derived: we knew that. The question here was when endothermy arose in archosaurs and their relatives. The answer to that is that it needs more study because the archosaurs and their ancient relatives were out having an adaptive radiation party during the Triassic. We knew that too. However, it appears that they were doing this metabolically and in their growth patterns as well.

It looks as though the archosauriforms were on the cusp of endothermy for some time and crossed over fairly early in their evolution if "Teleocrater" and Erythrosuchus are any indication. Yet if you look closely you will see that in some cases later, this condition was lost. Pseudosuchia, frex, seems to have largely dumped it. The authors concluded that only the Ornithodirans, pterodactyls and dinosaurs, were to eventually keep the derived metabolic condition.

The authors do warn that their sample is small and there needs to be more research done. There need to be more samples from specimens of the same critters they studied - they warned that "Mandasuchus" had different bone deposition patterns in different bones and that some of the specimens they had were not well preserved, rex, Herrerasaurus - and that there needs to be more specimens of different genuses in the archisauriformes to be be studied. I would argue that more from the broader archosauropmorpha need to be studied to make sure that Rhynchosauria is not the exception rather than the rule for more basal members of the lineage. If their hypothesis holds up, then there are some interesting implications.

The first is that endothermy was an innovation in the archosaurian line after the Permian Extinction that arose some time during the Triassic. One hypothesis was that endothermy was a reaction in both the therapsids and archosaurs to the events leading up to and during the PT Extinction. That doesn't appear to be the case. The therapsids picked this up prior to the Permian Extinction. The causes for these lineages adopting this metabolic change are for different reasons. That's a big deal.

Second, it really did happen, the crocs went from a warm blooded to cold blooded metabolism. As hard as it was - and, in some ways, still is - to accept, the evidence really is mounting. The archosaurian ancestors hit on the warm bloodedness and then dumped it as they adapted into a mode of life where it was unnecessary. The final confirmation of this would be if there were pseudogenes associated with metabolic functions. We ought to know relatively soon since crocs are getting their genome sequenced now.

Finally, yep, the dinosaurs were warm blooded. People just need to get over that fact. Their ancestors were. Their daughters - aves - are.

Deal with it:

images blatantly stolen from Scott, Zach, Mark, and wikipedia. The cladogram is from the paper (Padian 's Paleo Pirates et al) itself.

1 comment:

Zach said...

Herrerasaurus' "unique" mode of growth--fast initial growth followed by slow growth--is fairly diagnostic for endotherms, but moreso for dinosaurs in general.

Also, I think that while crocs take a long time to hit adult size, they have a definate "stopping" point. Snakes and lizards keep growing their whole lives, albeit very slowly, to the point where my oldest gecko is 10+ years old and doesn't look like he's grown at all for the past 5. But he's still shedding his skin every two months or so, so he must be getting bigger.