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Endothermic Crocs in Nature



Has anyone else seen the article in 14 April's Nature (pgs 833-34) entitled 
"Warm-hearted Crocs"? I haven't seen it mentioned.

It's a report on an article written by Seymour et. al. and published in 
"Physiological and Biochemical Zoology", the conclusion of which states that 
crocodilians may have had active, endothermic ancestors. Taking what one can 
from 
the summary, the article seems to have a nice explanation for the mosaic 
characteristic of croc hearts; 4 chambered but with a shunting system (though 
unlike 
that found in other reptiles). The gist is that crocodilian ancestors, like 
the terrestrial nasties in the Triassic (eg. "Terrestrisuchus" and the bipedal 
"Saltoposuchus"), were endothermic and as such evolved a four chambered heart 
for pressure separation (lung and systemic). This has since re-evolved a 
shunting system that we see in modern aquatic crocs. It's stated that the 
terrestrial crocs of the Triassic are thought to have had an active lifestyle 
and show 
indications of having a diaphragmaticus to inflate the lungs, which would have 
allowed for higher rates of ventilation than contemporary crocs require. It 
was only in the Jurassic that we begin to see larger, fully aquatic crocs, 
which 
in turn are presumed to have adopted the same sit-and-wait strategy employed 
by modern crocs. Since the rate of heat loss is greater in water than in air, 
and since larger animals have higher metabolic rates, there would have been 
selective pressure against endothermy during the return to the water. 
Furthermore, animals employing lengthy dive times characteristic of ambush 
predators 
would have gained great benefits from having a shunting system capable of 
isolating the lungs, or increasing their relative prufusion. The article also 
goes on 
to say that mitochondrial DNA from Jurassic crocs shows a high rate of 
evolution.... roughly equivalent to that seen in the mitochondrial DNA in 
modern 
endotherms. All of this points to ancestral crocs with elevated metabolic rates 
above that possessed by crocs today.

There is also a short discussion dealing with the question of the lack of 
respiratory turbinates in modern crocs, and their apparent lack in ancestral 
forms. The article states that the absence of turbinates could mean that it is 
possible that the four chambered heart is a characteristic of an active 
lifestyle, rather than of endothermy perse.... (I have issues with that, but 
why waste 
the time when one can simply read past DML discussions pertaining to 
turbinates.)

The article concludes with stating how evidence from birds and crocs, which 
bracket dinosaurs, is very useful. If ancestral crocs had high resting 
metabolic rates, and birds (in a nut shell) have high resting metabolic rates, 
then 
there is a clear implication that dinosaurs did as well.

As a side bar...... I'd like to know some opinions on chapter 28 of the new 
"Dinosauria"... "Physiology of Nonavian Dinosaurs" by Chinsamy and Hillenius, 
where they conclude "active, dynamic, but ectothermic dinosaurs".  Of course, I 
could just read Padian and Horner's article in the following chapter, but I'd 
like to hear some opinions from the peanut gallery if anyone would like to 
offer some.

Kris
http://hometown.aol.com/saurierlagen/Paleo-Photography.html