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3 1/2 vs. 4 chambered hearts

Mike and Eric,
A quick e-mail for today. Although I disagree with Eric in using ectothermy or endothermy as a reason for dividing classes (it's just too speculative and unreliable), I do agree for other reasons that a paraphyletic (actually semi-paraphyletic) Reptilia should be recognized. It is quite simply all basal amniotes, with the paraphyletic removal of two derived holophyletic clades (Mammalia and Aves). And it also has the advantage that almost everyone (except strict cladists) still use such a traditional Reptilia.
But what most interests me right now is the evolution of the chambers of the heart. Is it not possible that ALL reptiles except Ornithodira had 3 1/2 chambered hearts (including crocodyliforms) and that true four-chambered hearts (other than the separate evolution in the mammalian lineage) only evolved in Ornithodirans----perhaps all ornithodirans (including pterodactyls?), or just the dinosaur-bird clade, or possibly even just certain branches of dinosaurs (including those that gave rise to birds).
Is there any reason at all to assume that all archosauromorphs had four-chambered hearts, and crocodyliformes reverted back to a 3 1/2 chambered heart? This doesn't seem very parisimonious to me, unless I am unaware of evidence that would make it more parsimonious. A four-chambered heart in birds, plus all (or some) dinosaurs makes more sense, and all other archosauromorphs with some version of 3 1/2 hearts.
----Ken Kinman
From: "T. Mike Keesey" <tmk@dinosauricon.com>
Reply-To: tmk@dinosauricon.com
To: ELurio@aol.com
CC: dinosaur@usc.edu
Subject: Re: [Re: [Re: Non-serpentine lacertids (was RE:WHAT'S GOING ON?)]]
Date: Wed, 5 Jul 2000 21:54:14 -0400 (EDT)

On Wed, 5 Jul 2000 ELurio@aol.com wrote:

> Calling birds reptiles is invalid for two reasons, reptiles in the tradtional
> sense is a very different beast than a bird, i.e., reptiles are ectothermic
> and birds are not, reptiles have three and a fraction chambered hearts, and
> birds have four chambered hearts....we can go on and on.

One other comment -- many fossil taxa of traditional Reptilia likely did
have four-chambered hearts, since the crocodylian condition is supposed to
be derived from a four-chambered heart. This includes at least
part of non-crocodylian Pseudosuchia, all of non-avian Ornithosuchia
(including traditional Dinosauria) and possibly more basal
Archosauromorpha as well.

The recent _Thescelosaurus_ find may be direct evidence of this.

It is also arguable whether certain members of traditional Reptilia
(Therapsida, Ornithodira) were ectothermic.

T. Michael Keesey <tmk@dinosauricon.com> | AIM <Ric Blayze> | ICQ <77314901>
My Worlds
The Dinosauricon <http://dinosauricon.com
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