[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: theropod-bird eggshells

Gerald Grellet-Tinner and Luis M. Chiappe have studied the correlation between
eggshell morphology and phylogenies based on comparative anatomy, and
Grellet-Tinner presented their results at the Florida Symposium on Dinosaur
Bird Evolution at the Broward County Convention Center in Fort Lauderdale,
Florida on April 7, 2000.  I quote the abstract from their talk, published in
_The Florida Symposium on Dinosaur Bird Evolution_, Publications in
Paleontology No. 2, Graves Museum of Archaeology and Natural History:


Luis M. Chiappe
Associate Curator, Vertebrate Paleontology
Natural History Museum of Los Angeles County   lchiappe@nhm.org

Gerald Grellet-Tinner
University of Texas   Geraldgt@mail.utexas.edu

New evidence is presented here confirming the theropodan origin of birds by
examining the evolution of eggshell structure throughout Archosauriformes.
Eggshells consisting of complex biopolymers and ionic crystals are formed under
genetic control and therefore contain information regarding the evolutionary
history of the egg layers (parents).  Thus, egg and eggshell carry
phylogenetically valuable information and are suitable for cladistic analysis.
This important conclusion is corroborated by the congruence found between
morphology-based phylogenetic hypotheses of paleognathous birds and a recent
cladistic analysis of these birds based on eggshell microstructural
characters.  Eggs and eggshells have been typically classified in parataxonomic
schemes, often they have been poorly diagnosed, and equivocally associated
to/with specific lineages of extinct organisms.  Recent spectacular discoveries
of dinosaur eggs containing embryos or eggs associated with brooding parents
have furnished data confidently anchor eggshell characters with specific taxa.
Unlike neosauropods and ornithopods, known theropodan eggshells possess two
macrostructural layers in radial view.  This innovation is expressed either by
an abrupt or a gradual layer between layer 1 and 2 (mammilary and squamatic
layers).  Furthermore, radiating acicular mammilae, a synapomorphy shared by
_D. antirrhopus_ and oviraptorids, is always associated with the abrupt
boundary character state.  To date, the rest of the theropodans are united by
having eggshells with a gradual interlayer boundary and blocky calcitic

The advent of avian eggshells is characterized by the accretion of a third
layer for neognathous birds and even a fourth one in the case of some
paleognaths.  The origin of the third and fourth zones has not yet been
determined, but it might very well lay within Avialae (not including Aves).
However, this speculation does not exclude a purely non-avian theropod origin.
Nevertheless, eggshell character optimization compared with already existing
cladograms proves most parsimonious and supports the theropodan hypothesis of
bird origins."

According to my notes, the presentation included a cladogram, consistent with
comparative eggshell microstructure analyses, which featured the following
Crocodylia, Ornithischia, Maiasaura, Neosauropoda, Oviraptorid, Deinonychus,
Troodontid, Avialae, Aves, Neognathae, and Paleognathae.

While I second the suggestion that you read Carpenter's _Eggs, Nests, and Baby
Dinosaurs_ (1999, Indiana University Press), I also direct you to the free
online article, "Eggs, Eggshells, and Nests" by K. E. Mikhailov, excerpted from
the very highly recommended book, _Encyclopedia of Dinosaurs_, (1997, Academic
Press), edited by Currie and Padian.  You can access this article at

Further free articles from _Encyclopedia of Dinosaurs_ may also be browsed at
<www.apnet.com/dinosaur/dinosamp.htm>.  Read them all for free, then buy the

Eggshell microstructure should be an important issue, and yet it has not been
addressed by opponents of the theropodan hypothesis of bird origins so far as I

I hope this helps.

-- Ralph W. Miller III       gbabcock@best.com