Re: Egg questions (was: Re: Allosaur baby faces)

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

----- Original Message -----
From: "Jura" <pristichampsus@yahoo.com>
Sent: Wednesday, June 22, 2005 2:43 AM

> --- Christopher Taylor <ck.taylor@auckland.ac.nz>
> wrote:
>
> >     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
> > Hesperornithiformes and
> > _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.

Certainly.

> > 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 
Fm...).

> 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... 
:-S

A few fossil crocodile eggs are known. There's *Krokolithus* from Messel or 
Geiseltal or so.