Medullary Bone Distribution in Archosaurs

[Guy Leahy <xrciseguy@sbcglobal.net>]

Bone. 2007 Jan 11; 
Do egg-laying crocodilian (Alligator mississippiensis)
archosaurs form medullary bone?

Schweitzer MH, Elsey RM, Dacke CG, Horner JR, Lamm ET.

Department of Marine, Earth, Atmospheric Sciences,
North Carolina State University, Raleigh, NC 27695,
USA; North Carolina Museum of Natural Sciences,
Raleigh, NC 27695, USA.

It is beyond question that Mesozoic dinosaurs, like
Aves and Crocodylia, are archosaurs. However, within
the archosaurian clade, the origin and distribution of
some major features are less clear, particularly with
respect to reproductive physiology. Medullary bone, a
highly mineralized, bony reproductive tissue present
in the endosteal cavities of all extant egg-laying
birds thus far examined, has recently been reported in
Tyrannosaurus rex. Its presence or absence in extant
crocodilians, therefore, may shed light on the timing
of its evolutionary appearance. If medullary bone is
present in all three taxa, it arose before the three
lineages diverged. However, if medullary bone arose
after this divergence, it may be present in both
extinct dinosaurs and birds, or in birds only. If
present in extinct dinosaurs and birds, but not
crocodilians, it would indicate that it arose in the
common ancestor of this clade, thus adding support to
the closer phylogenetic relationship of dinosaurs and
birds relative to crocodilians. Thus, the question of
whether the crocodilian Alligator mississippiensis
forms medullary bone during the production of eggs has
important evolutionary significance. Our examination
of long bones from several alligators (two alligators
with eggs in the oviducts, one that had produced eggs
in the past but was not currently in reproductive
phase, an immature female and an adult male) shows no
differences on the endosteal surfaces of the long
bones, and no evidence of medullary bone, supporting
the hypothesis that medullary bone first evolved in
the dinosaur-bird line, after the divergence of
crocodilians from this lineage.