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RE: no marine dinos/no viviparous dinos.
At 23.15 03/01/02 +0000, you wrote:
But would such physiological constraints hinder the evolution of
ovoviviparous dinosaurs?
------ Ken
****************************************
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).
In squamates we have a full trend from oviparity to full
viviparity Retention of the embrion sometimes implies transportation of
material from the mother to the embryo (even with an
eutherian-like chorioallanthoic placenta in extreme cases, e. g. the
skink Mabuya heathi, Blackburn et al., 1984) thus a reduction or
elimination of the eggshell.
In the ovoviviparous species Lacerta vivipara the embryo etains an
eggshell and it is capable of full development also outside the body of
the mother, but has to be put in a physiological solution, but even in
this case, the calcification of the egg shell is reduced . Thus we are
seemingly in the same situation.
Again, this is not hard evidence, but I think it is a good inference.
All the best,
Silvio
_
"The Wise Man is like a bamboo tree;
simple, upright, and useful, but hollow inside"
Lao Tzu
Silvio Renesto
Dipartimento di Scienze della Terra
Università degli Studi di Milano
Via Mangiagalli 34
I 20133 Milano
Italy
phone +39-02-58355511
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e-mail: renesto@mailserver.unimi.it
Silvio.Renesto@unimi.it
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