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

["Jaime A. Headden" <qilongia@yahoo.com>]

Jura (pristichampsus@yahoo.com) wrote:

<I'll leave the answer to this question to more qualified individuals. Offhand,
though, I think coelurosaurs are believed to have had hard shelled eggs.>

  Correct me if I am wrong, but dinosaurian eggshell is of the hard type due to
the presence of the mammilary layer coupled with the calcitic squamous layer,
forming definite structure absent in most turtle and croc shells. This is
present in birds, and only dinosaurs share the "avian" condition of eggshell of
a calcium carbonate (calcite) shell; that of turtles, for example, is
aragonitic.

  In Ji et al., 2004 (_Nature_ 432:572) they identify an eggshell surrounding
the embryo of a pterosaur which is described thusly:

  "The thin shell is brown-to-dark-brown and contrasts with the matrix
   surrounding the eggshell. The eggshell appears to be very thin (about 0.25
   mm; Fig. 1, inset) and has no lamination in its microstructure. We did not
   detect the inner mammillary layer or the outer squamous layer formed of
   calcite crystallites that form the standard structural components of the
hard
   eggshells of dinosaurs (Zelenitsky et al., 2002, _Cretaceous Ressearch_
   23:297-305; Zhao, 1978, _Vertebrata PalAsiatica_ 16:213-221) and of some
   modern amniotes (Carpenter, 1999, _Eggs, Nests, and Baby Dinosaurs_ [Indiana
   Univ. Press]). Neither the part nor the counterpart of this pterosaur
   eggshell show any of the sharp fractures that are typical of broken, hard
   eggshells."

  [References interpolated]

  Meanwhile, Chiappe et al., 2004 (_Nature_ 432:571-572) identify eggshell
associated with adult and juvenile remains of *Pterodaustro* as bearing "[a]
smooth, carbonatic material of biotic origin." They further describe the
fossil:

  "Although the eggshell is somewhat weathered, it is extremely thin -- about
30
   microns in thickness. Scanning electron microscopy reveals that it consists
   of a single layer of calcite, with crystals radiating from a basal point and
   forming V-shaped eggshell units, similar to the eggshell of brief
   communications archosaurs (including birds), whose units radiate from an
   organic core (Board & Sparks, 1995, pg. 71-86 in _Egg Incubation_ [Cambridge
   Univ. Press]). Preserved crystals near the nucleation centres also exhibit
   the blade shape that is characteristic of most archosaurian lineages."

  It seems that the hard calcite would prevent the shell from being as flexible
as Ji et al's "leathery" shell, and this is the key point about avian eggshell
and the similarity in most archosaurs, including dinosaurs.

  Cheers,

Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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