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In a message dated 96-10-01 19:56:44 EDT, martz@holly.ColoState.EDU (Jeffrey
Martz) writes:

>      I thought crocodilians had four chambered hearts as well, and that 
> this was thus presumed to be a uniting fetaure for Archosaurs.  Are you 
> implying that tghis is covergence, and that "dinobirds" developed the 
> feature in association with bipedality?

(1) Crocodylians do have four-chambered hearts, but barely (you might say);
(2) the earliest crocodylians were slender-limbed, cursorial runners with a
fully erect stance, so as far as we know the present day's semi-erect
crocodylians arrived at this stance secondarily (and they still preserve, to
some extent, the elongate >carpal< elements of their cursorial
forerunners--no pun intended).

The four-chambered heart and its associated anatomical features are among the
synapomorphies that unite present-day crocodilians and birds, but we do not
yet know when the four-chambered heart appeared in Archosauria (except that
it surely occurred before the crocs and birds diverged). Without a
four-chambered heart, with which pulmonary and systemic blood circulation are
fully separated, elevation of the body on erect limbs is very unlikely. But
don't take my word for it. For details, check out this article

Regal, P. J. & Gans, C., 1980. "The Revolution in Thermal Physiology,"
Chapter 6 of Thomas & Olson, eds., 1980: 167:188.


Thomas, R. D. K. & Olson, E. C., eds., 1980. A Cold Look at the Warm-Blooded
Dinosaurs, AAAS Selected Symposium 28, Westview Press, Boulder, Colorado: xxx
+ 514 pp.