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RE: Shedding archosaurs?

On the endothermy of crocodilian ancestors, see


Physiological and Biochemical Zoology 77(6):1051-1067. 2004. 
C 2004 by The University of Chicago. All rights reserved.
DOI: 10.1086/422766

Evidence for Endothermic Ancestors of Crocodiles at the Stem of Archosaur

Roger S. Seymour1 Christina L. Bennett-Stamper2 Sonya D. Johnston1 David R.
Carrier3 Gordon C. Grigg4 

1Department of Environmental Biology, University of Adelaide, Adelaide,
South Australia 5005, Australia; 2Center for Biological Microscopy,
University of Cincinnati, 3125 Eden Avenue, P.O. Box 670521, Cincinnati,
Ohio 45267-0521; 3Department of Biology, University of Utah, Salt Lake City,
Utah 84112; 4Department of Zoology, University of Queensland, Brisbane,
Queensland 4072, Australia

Accepted 11/19/04

Abstract: Physiological, anatomical, and developmental features of the
crocodilian heart support the paleontological evidence that the ancestors of
living crocodilians were active and endothermic, but the lineage reverted to
ectothermy when it invaded the aquatic, ambush predator niche. In
endotherms, there is a functional nexus between high metabolic rates, high
blood flow rates, and complete separation of high systemic blood pressure
from low pulmonary blood pressure in a four-chambered heart. Ectotherms
generally lack all of these characteristics, but crocodilians retain a
four-chambered heart. However, crocodilians have a neurally controlled,
pulmonary bypass shunt that is functional in diving. Shunting occurs outside
of the heart and involves the left aortic arch that originates from the
right ventricle, the foramen of Panizza between the left and right aortic
arches, and the cog-tooth valve at the base of the pulmonary artery.
Developmental studies show that all of these uniquely crocodilian features
are secondarily derived, indicating a shift from the complete separation of
blood flow of endotherms to the controlled shunting of ectotherms. We
present other evidence for endothermy in stem archosaurs and suggest that
some dinosaurs may have inherited the trait.

This paper was discussed at

Dr John D. Scanlon, FCD
Riversleigh Fossil Centre, Outback at Isa

-----Original Message-----
From: David Marjanovic [mailto:david.marjanovic@gmx.at] 
Sent: 15 January, 2008 9:10 AM
Subject: Re: Shedding archosaurs?

>>  I thought turtles (like mammals) were outside of
>> the archosaur group.
> Barring one anomalous molecular study, this is
> probably true.

Oh yeah -- there are at least two molecular studies that have found this 
result. It's morphologically impossible, though.

> That is hardly conclusive. The only real argument for
> this based it on the apparent higher activity levels
> of prehistoric crocodyliformes, and the erroneous
> assumption that you need to be an automatic endotherm
> to be highly active.

Another argument is the high rate of evolution in the mitochondrial genome, 
but how closely such rates correlate to anything needs more investigation.