Talk:Archosaur

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[edit] Image

With the Postosuchus image gone, I found a pretty nice looking shot of a crocodilian on stock.xchng (though a good photo incorporating both a croc and a bird would be ideal), but I'm not sure of the species. Maybe somebody who's up on crocs can fll in the caption.The Thagomizer 18:30, 27 January 2006 (UTC)

[edit] Crocs are birds?

I'm a layman, but this sentence bothers me: The only groups of archosaurs to continue through to the Tertiary and, ultimately, to the present day, are the birds, which are descended from the dinosaurs and crocodylians, which include all modern crocodiles, alligators, and gharials.

Birds are descended from Dinosaurs exclusively, are they not? The crocs and dinosaurs split off long before the first bird/dinos showed up. Saying that the only archosaurs are birds is rather silly considering that crocs are still extant.

Again, I'm just a (paleonerd) layman, so am I interpreting this correctly? --Shubbell 09:14, 15 August 2006 (UTC)

The sentence is poorly constructed, I'll clarify it M Alan Kazlev 03:19, 16 August 2006 (UTC)

[edit] Advantages; metabolism

The article says "Their 'reptilian' metabolism seem to have given them a clear advantage over the mammal-like therapsids..." This is ambiguous as it may give the impression that a sluggish metabolism was an advantage. One could argue that endotherms would be at a disadvantage in hot, dry conditions, but some parts of the Triassic world must have been cool; and since modern monotremes have lower, more variable body temperatures than most placentals (and some placentals such as tenrecs have unusually variable temperatures), one has to assume that therapsids also had lower, more variable body temperatures and therefore the heat would not have placed them at a major disadvantage. I remember from Kermack & Kermack's book on the mammal-like reptiles that living diapsids (lizards, crocs, birds, etc.) excrete uric acid while mammals excrete urea. Uric acid can be excreted as a paste while urea requires penty of water to keep it fully liquid. If Permian-Triassic diapsids excreted uric acid and mammal-like reptiles excreted urea (as seems likely), the diapsids would have had better water conservation and this was probably a major advantage in the arid interior of Pangaea.

It would be interesting if someone could add good material on archosaur metabolic rates. I think dinos had high metabolic rates (see my comments on Wikipedia's "dinosaur" page), and crocs, although cold-blooded, have features normally associated with high metabolic rates: a palate which enables them to breathe while eating (palates were the first stage in synapsids' evolution towards endothermy - Kermack & Kermack), a mechanism to pump the lungs while moving, and a four-chambered heart (but small, running at lower pressure and with a by-pass which converts it to 3-chambered while diving, presumably to conserve oxygen). Either dinosaurs and their close ancestors developed high metabolic rates in 10-20M years although the synapsids took about 65M years, or earlier archosaurs had fairly high metabolic rates and crocs are secondarily cold-blooded. The latter hypothesis is consistent with the fact that Triassic crocs were slim and leggy (e.g. Terristrisuchus) and look like cursorial predators (e.g. dogs). An abstract which was recent taken offline by CMNH (I got it from Google's cache) supports the view that crocs were originally endothermic, and I'm inserting a copy at the end of this post. I have been unable to find other good material online about archosaur metabolism. Can anyone else do better?

Philcha 18:02, 5 October 2006 (UTC)

6th International Congress of Comparative Physiology and Biochemistry: 23.5. A case for endothermic ancestors of crocodiles at the stem of archosaur evolution

Seymour, R.S. Department of Environmental Biology, University of Adelaide, Adelaide 5005, SA, Australia roger.seymour@adelaide.edu.au

In living endotherms, there is a functional nexus between high rates of metabolism, high systemic blood flow rates, and high systemic (but low pulmonary) blood pressures produced by thick-walled, four-chambered hearts. The hearts of crocodiles are equivocal: they are four-chambered, but are relatively small and feature low blood pressure and a capacity for right-to-left shunting. Recent crocodiles are definitely ectotherms, but there is paleontological, embryological, anatomical and physiological evidence that their ancestors may have been highly active, endothermic predators. By invading an aquatic, ambush predator niche, one descendent line may have become ill-suited for endothermy and reverted to ectothermy, which may have prevented their extinction at the end of the Mesozoic. Endothermy in stem archosaurs has implications for the metabolic status of both saurischian and ornithischian dinosaurs.

(Seymour's web page http://www.ees.adelaide.edu.au/people/enviro/rseymo01.html indicates that the full paper appeard in Physiol. Biochem. Zool. 77: 1051-1067)