shell thickness - Bigelow; also pores

[Crpntr@ix.netcom.com (Kenneth Carpenter)]

Bigelow wrote:
>Regarding the discussion on whether the thickness of the ?T. bataar 
eggs is
>a diagenetic feature or is original structure:

>A 1992 paper by Karl Hirsch and Rolf Kohring (Jour. Vert. Paleo. 
12(1):59-65)
>describes some crocodillian eggs from the Eocene of Wyoming.  They note 
that
>the fossil croc. eggs are 65-68 mm long, and 0.6 mm thick.  
>From some quick math on my part:
>This thickness is _proportionally_ slightly over twice as thick as the
>_proportion_ in the ?T. bataar eggs, if that is the true T. bataar
>thickness.


Yes, and on average dinosaur eggshell is proportionally thinner than a 
comparable sized bird egg.

>  They noted no dissolution effects on the outer surface of the egg 
shell.  
>Prisms and plates of calcite on the inner surface of the shell does, in
>places, appear to be, in their words, "eroded". 

Yes, that is due to the embryo removing calcite from the mammillae.  
Eroded mammilae is typical
of hatched eggs.


>  The ?T. bataar eggshell may have experienced some degree of 
"telescoping" down
>to a smaller thickness; however, this doesn't seem to be universally so 
with
>all fossil eggs.  A blanket statement that dismisses egg thickness as 
anything
>unusual is probably premature.

You misunderstood me.  Telescoping is common in elongated eggs standing 
on end.  The result is
a shortening of length, it does not affect shell thickness.  I gave no 
blanket statement dismissing
shell thickness.  In fact, it is used in the diagnosis of egg taxa.


>It may relate to how the eggs were layed, buried,
>or even incubated. 

Perhaps, but less than you might expect.  Actually it is the gas 
conductance values of the egg
shells that provide the most clues to the nesting environment.


>Shell thickness is not just a side-show curiosity.

I never said it was.

>From personal experience, I have seen the effects of pressure solution 
on
>fossil mollusk shells in thin section, and the pitting, embedded quartz
>grains, and resultant neomorphism of prisms is unmistakable.  It should
>be easy to resolve this discussion regarding the ?T. bataar material.

Certainly, but I was not referring to diagenesis.  My understanding of 
Neil was that he was
stating that compaction would thin the eggs.


>it would be interesting to compare the shell pore density on modern
>crocodylian eggs, ostrich eggs, and the ?T. bataar eggs.  This may give 
us
>some clue as to what the _relative_ embryonic metabolisms are for these
>groups of archosaurs...at least, in my opinion>.  Oxygen uptake, you 
see...


Done a long time ago: Seymour, R., and Ackerman, R. 1980. Adaptations to 
underground nesting
in birds and reptiles.  American Zoologist 20:437-447.

See also:

Ackerman, R.  1980.  Physiological and ecological aspects of gas 
exchange by sea turtle eggs. 
American Zoologist 20:575-583.

Board, R.  1980.  Porosity of the avian eggshell. American Zoologist 
20:339-349 

Mou Y., 1992.  Nest environments of the Late Cretaceous dinosaur eggs 
from Nanxiong Basin,
Guangdong Province.  PalAsiatica Vertebrata 30:120-134.

Paganelli, C.  1980.  The physics of gas exchange across the avian 
eggshell.  American Zoologist
20:329-338.

Sabat, K.  1991.  Upper Cretaceous amniotic eggs from the Gobi Desert.  
Acta Palaeontologica
Polonica.  36:151-192.

Seymour, R.  1979.  Dinosaur eggs: gas conductance through the shell, 
waqter loss during
incubation and clutch size.  Paleobiology 5:1-11.

 Seymour, R., 1980.  Dinosaur eggs: the relationships between gas 
conductance values through
the shell, water loss during incubation and clutch size.  M moires 
Societe Geologique de France
139:177-184.