RE: Orbital bars

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Waylon Rowley
<
> Jordan Mallon wrote:
>
> > The topic of orbital bars has recently come up on a
> > message board I frequent.  I understand _T. rex_ had
> > them, as did_Carnotaurus_, and maybe a few others
> > that I'm not aware of.
>
> I probably don't have a complete list either, but I
> know that _Acrocanthosaurus_, _Carcharodontosaurus_,
> _Giganotosaurus_, _Abelisaurus_, _Majungatholus_, and
> _Albertosaurus_ can be added. In fact, I remember
> seeing a specimen of _Albertosaurus_ where it looked
> like the suborbital bar had been shifted far ventrally
> and contacted the base of the lacrimal.

Actually, that would be _Gorgosaurus_ (or, at least, _A. libratus_): that is
one of the diagnostic features of adults of this taxon.

In _A. sarcophagus_ there is a sheet of bone that heads off lacrimal-ward,
but it peters out before reaching the other side.  I strongly suspect that
there was a cartiligenous sheath in this position, which was partially
ossified in _A. sarcophagus_, but fully ossified in adult _A. libratus_.
NOTE: the Field Museum "Albertosaurus" (a _Daspletosaurus_ specimen) shows
the primitive condition for tyrannosaurids.

Oh, and larger _T. bataar_ specimens show it, as well.

> > But what's the significance of these structures?
> > Have any suggestions been put forth as to why they
> > might have arisen?
>
> I believe someone once suggested that they helped
> support the eyeball. The suborbital ligaments also
> attach there. Hope I've been of help.

Dan Chure wrote a short paper on this subject, published in the Gaia
theropod volume (and NO, I have no recent information on ordering it.  Wish
I did, folks!).

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
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