Re: Warm-Blooded debate

[Adam Yates <zooamy@zoo.latrobe.edu.au>]

On Wed, 18 Jun 1997 Dinogeorge@aol.com wrote:

> In a message dated 97-06-18 10:35:40 EDT, jwoolf@erinet.com (Jonathon Woolf)
> writes:
> 
> << Ectotherms have
>  no endurance because they can only generate energy anaerobically. >>
> 
> Too bad you haven't been able to find _A Cold Look at Warm-Blooded
> Dinosaurs_, because it contains a key paper by Regal and Gans on the role of
> the four-chambered heart in aerobic metabolism. Endothermy is only loosely
> connected with endurance in tetrapods; the four-chambered heart much more so.
> Since crocs >almost< have four-chambered hearts, and birds have them, we can
> use the Extant Phylogenetic Bracket to argue that all dinosaurs, which are
> phylogenetically between crocs and birds, had them. This, rather than
> endothermy--which developed much later, perhaps only in the theropod lineage
> that eventually evolved into birds--was necessarliy responsible for
> dinosaurs' erect stance and cruising ability.
> 

I think that there is good evidence that endothermy evolved twice within 
the Dinosauria. Once in Aves, perhaps as high in the cladogram as 
Ornithurae (that is the clade containing Hesperorinithiformes, 
Ichthyornithiformes and Neornithes but not Patagopteryx and 
Enantiornithes, isn't it?) and once in the Ornithopoda (probably 
restricted to the Dryosauridae). I find the evidence for endothermy in 
dryosaurids compelling. Unlike other nonavian dinos they have fast 
growing fibrolamellar bones that lack lines of arrested growth, at all 
stages of its ontogeny, this implies sustained fast growth without 
pauses. Dryosaurus was too small to have been a gigantotherm, though a 
continuously warm climate could be responsible. However a sample of 
"hysilophodont" femora (I suspect they may turn out to be dryosaurid) from 
the near-polar sites of Victoria show exactly the same histology. Could 
a smallish ectotherm maintain fast uninterupted growth in an environment 
that had an estimated mean annual temperature somewhere between 5 and -6 
degrees celcius? I don't think so. Interestingly enough the nonavian 
theropod, Timimus, from the same area shows prominant lines of arrested 
growth. So it is the ornithopods, not the theropods, which stand the best 
chance of including endotherms amongst the "traditional" (=non avian) 
dinosaurs. I would really like to know how  big the nasal passages of 
dryosaurids were! 

Adam Yates
"You can't bury love, you gotta dig it up"