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Re: Egg questions (was: Re: Allosaur baby faces)
----- Original Message -----
From: "Jura" <firstname.lastname@example.org>
Sent: Wednesday, June 22, 2005 2:43 AM
--- Christopher Taylor <email@example.com>
Which reminds me of another question I was
thinking of asking - does
anyone know if coelurosaurs laid hard-shelled eggs
like modern birds, or
leathery-shelled eggs like other reptiles? What is
the earliest branch on
the avian stem that we can be sure laid a hard egg?
Hard-shelled eggs are known all over the Dinosauria: hadrosaurs, sauropods,
theropods. Crocodile eggs seem to be intermediate, judging from the
contradicting statements I've read...
I ask because I was
wondering if well-known stem birds such as
_Ichthyornis_, which are commonly (albeit perhaps
mistakenly) assumed to be
biologically much the same as modern birds, can be
assumed to lay a hard egg.
Considering how difficult it is to imagine
_Hesperornis_ moving at all
on land, could it have even produced live young?
The loss of a hard eggshell seems to be impossible... there's some
discussion on this in the archives...
Incidentally, not all reptiles lay leathery
eggs. Most tortoises are hard shelled egg layers,
along with most geckos.
(This is certainly convergence, as shown by the disparate phylogenetic
distribution of hard shells, the very different microstructures, and the
different materials -- dinosaurs use calcite, turtles with half-hard and
hard shells use aragonite.)
Anyway, I have a follow up question to Chris's. How do
we tell the differences between leathery and hard, in
a fossilized egg?
That's very simple. Soft shells are organic material and therefore simply
don't fossilize except under very special circumstances (upper Yixian
Furthermore, have there been any fossil eggs
of crocodilians, or squamates, or anything that isn't
a dinosaur; which might be used for comparison.
Being soft-shelled, squamate eggs (apart from the occasional gecko egg) are
extremely rare. I've seen a photo of what's said to be a natural cast of a
Miocene snake egg, composed of IIRC sandstone. Uh... yes, the shape fits...
A few fossil crocodile eggs are known. There's *Krokolithus* from Messel or
Geiseltal or so.