Re: eggs with thin shells

[NEIL CLARK <NCLARK@museum.gla.ac.uk>]

Date sent:      Wed, 8 Feb 1995 22:19:30 -0500
Send reply to:  Crpntr@ix.netcom.com
>From:           Crpntr@ix.netcom.com (Kenneth Carpenter)
To:             Multiple recipients of list <dinosaur@lepomis.psych.upenn.edu>
Subject:        Re: eggs with thin shells

Neil Clark wrote: 


>Through compaction the calcite shell can be thinned in the direction of 
>maximum compaction resulting in the egg being thinner on the top 
>and bottom and thicker on the sides.
>->0<- (egg seen sideways on as I can't find the keys to do it the right 
>way - arrows show direction of compaction and the thinned part of the 
>egg shell).

Sorry, no the calcite of the shell would not thin through compaction, it 
would however, telescope in.  It is also important to note that shell 
thickness is not even all over the egg.  It is usually thickest on the 
lower pole or bottom, medium on the sides, and thinnest on top to allow 
the hatchling out (the French refer to the thin top as the "hatching 
window", a term I do not care for).


Kenneth Carpenter

 My reply now....  Yes the egg shell is thinned substantially by 
compaction (>25 percent), but not through decalcification directly.  
Impacting of quartz grains onto the eggshell surface has caused the 
calcite to migrate through pressure solution into the surrounding 
sediment.  This effectively thins the eggshell on the top and bottom  
(at least in the eggs I have dealt with).  This produces an un-
oxidised rim to the egg which is green as opposed to red (which is 
the colour of most of the surrounding sediment).  The sediment is 
cemented in these patches by a micrite (fine-grained calcite).

Neil




Neil Clark
Curator of Palaeontology
Hunterian Museum
University of Glasgow
email: NCLARK@museum.gla.ac.uk

Mountains are found in erogenous zones.
(Geological Howlers - ed. WDI Rolfe)