RE: Dinosaur whodunit: Solving a 77-million-year-old mystery

[Dino Guy Ralph <dinoguy@sbcglobal.net>]

Dann Pigdon writes:

"The illustration has the pointed ends of the eggs facing down-slope.
Wouldn't the opposite be more stable?"

You would think so, but there were undoubtedly other considerations, for
this is how they were found in this specimen, and this is consistent with
_Troodon_ egg clutches from Montana as well as small theropod clutches in
Mongolia.  The following are a few of the advantages that such an
orientation may have conferred.    

1) For one thing, it would be easier to lay an egg pointed-end-first, as
this would require minimal dilation as the egg first begins to emerge.  This
is how kiwis lay their eggs, as you can see in x-rays.  In this scenario,
the eggs need not have been inverted by the mother after being laid.  

2) If the substrate was sufficiently compliant, the pointed end would have
been more readily planted than the rounder end, and it would have been
better suited to holding its position.  The impression of each egg's shape
in the mound surface may have helped to keep each egg in its place, but any
insertion of the pointed tip into the soil would have provided an extra
measure of security.   

3) The rounded end of each egg would have been better able to support its
share of the brooding parent's weight than the pointed end.

4) The rounded ends of the eggs would have been more comfortable for the
brooding parent to rest upon than the pointed ends.  

I'm no authority on the subject of dinosaur nesting behavior, and I hasten
to point out that there was a diversity of egg shapes and clutch
arrangements among non-avialian theropods.  These caveats aside, I have no
trouble seeing advantages to the pointed ends of the theropod eggs in
question facing down-slope, however counterintuitive this orientation might
appear to be. 

Dino Guy Ralph
Docent at the California Academy of Sciences
Dinosaur and Fossil Education
Member of the Society of Vertebrate Paleontology