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RE: Tyrannosaur growth and babies

Dwight Stewart wrote:

<How strong is the evidence for dimorphism in Tyrannosaurus rex? What
brought this to mind was a conversation I recently had with a friend
regarding this subject. That made me return to Dr. Horner's The
Complete T. rex to see what it had to say regarding potential
dimorphism. The book indicated that Dr. Bakker isn't sold on
dimorphism & (in fact) believes that the more robust specimens
represent a different species. If the only differences are size and
bulk, then the different species hypothesis doesn't seem likely to me.>

  If you can get your hands on Colbert (1989), Colbert (1990), Raath
(1990), and Carpenter (1990), I suggest you read them. They illustrate
vividly the various morphological distinctions between robust and
gracile forms in a group. In *Coelophysis* and *Syntarsus* (Colbert,
1990; Raath, 1990) these variations occur within a group, notably the
Tanzanian (Rhodesian) and Ghost Ranch populations, so specific
variation is definately out of the question, unless there was some
titanic struggle going on that ended everything (equally unlikely).
These variations can be separated into two distinct groups. Paul
(1988) helps to illustrate this with several skeletal reconstructions
and a few pages of text.

  Meanwhile, Carpenter (1990) does some work on morphological
variation between the skulls, necks, and ischia of *Tyrannosaurus*,
also compared in Horner and Lessem (1993) [the book you're reading].
He also separates the groups into robust and gracile forms, though the
skulls and pelvises do not exactly mesh in that what's robust in the
one is not robust in the other. Makes me think tyrannosaurs did not
exactly keep to the _morphologie rigeur_ when it came to their heads,
perhaps an environmental factor, taphonomy, growth stage, etc.

  As Larson recapitulated a few years ago, Sue and Stan most
definately concur with all the above's conclusions (I don't know much
on Bakker's angle) and add a new feature to the collection: placement
of the first chevron behind the pelvis.

  So there's a lot of support for the dimorphism angle, and I'm
certain there's more stuff to be had in other refs.

Jaime A. Headden

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