RE: no marine dinos/no viviparous dinos.

[Silvio Renesto <renesto@mailserver.unimi.it>]

At 12.38 02/01/02 -0600, you wrote:

>  >      It seems to me that dinosaurs simply never had the chance to evolve
> >into that niche because of competitive exclusion from other marine reptiles
> >that got there first.

> >     Birds and turtles may have been too specialized to evolve marine
> >vivipary, but some early dinosaurs may have had the potential to develop it
> >if only other marine reptiles hadn't beat them to it.

> With all due respect:
>  We do not  forget physiology and its constraints. Turtles and 
> crocodylian embryos obtain most of the calcium for the development of 
> their skeleton from the egg shell (Packard et al. 1977),  while 
> lepidosaurs do not (Oelofsen 1978). Thus the lepidosaurs can eliminate 
> the shell and become viviparous (lots of lizards and snakes) while 
> turtles and crocodilians cannot , and no viviparous birds also (Blackburn 
> and Evans 1986).
This is not obviously hard evidence, but a good circumstantial one. If 
neither crocs nor birds can be viviparous, it seems to me  a good 
phylogenetic bracket speaking against viviparity in dinosaurs.
All the best,

                                        Silvio Renesto










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Silvio Renesto

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