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

[Phil Bigelow <bigelowp@juno.com>]

On Tue, 21 Jun 2005 21:41:29 -0300 Matias Soto <soto@adinet.com.uy>
writes:
> > > Anyway, I have a follow up question to Chris's. How do
> > > we tell the differences between leathery and hard, in
> > > a fossilized egg?
> 
> And if there is a flat surface in a fossilized egg? (this is the 
> case of
> *Tacuarembovum oblongum* Mones, 1980)
> Does this mean something?
> 

SEM (scanning electron microscopy) can readily distinguish "leathery"
fossilized eggs from "hard shell" fossilized eggs.  "Hard shell" eggs
have tightly interlocked calcite crystallites, with only a little
interstitial protein forming the matrix.  "Leathery" eggs are mostly
protein.  IIRC, there is a continuum between leathery eggs and hard shell
eggs, with the abundance of calcite crystallites being the variable.

Regarding the flattened side on the _Tacuarembovum oblongum_ egg, try
this experiment:  Take a chicken egg and place it in a pan with a couple
millimeters of vinegar in the pan.  Wait a day and remove egg and let it
dry out.  The bottom of the egg shell will be flattened.  Meaning that
even hard shell eggs can display this feature (groundwater can be acidic,
particularly if the egg is sitting in a puddle of rotting vegetation).

"Hard shell" fossilized eggs are often composed of original material. 
"Leathery" fossilized eggs must either be replaced by minerals in order
to be preserved, or they must be mummified (in the presence of water, the
protein will rot away).

For more details, read the book _Dinosaur Eggs and Babies_, edited by K.
Carpenter and a couple other people whos names escape me.

<pb>
--