Re: eggshell request

[David Marjanovic <david.marjanovic@gmx.at>]

----- Original Message -----
From: "david peters" <davidrpeters@earthlink.net>
Sent: Friday, June 24, 2005 4:00 PM

> Turtles retain them, Archosaurs retain them, both split off from 
> Lepidosaurs prior to tuatara.

Hard eggshell seems to be derived within turtles and within geckos, and in 
addition mammal eggs have soft shells as well, so it's probably more 
parsimonious to assume that soft-shelled eggs are the plesiomorphic state 
for amniotes, with calcite crystals appearing on top of the soft shell in 
sphenodonts, geckos and archosaurs (...and some pterosaurs?), and with 
calcite crystals "fusing" into hard shells in geckos and archosaurs (where 
the original soft shell below it is called membrana testacea... right?), 
while aragonite crystals appeared in some turtle clade and "fused" into a 
hard shell in some subclade of that.