Binocularity and tyrants (was RE: How would Tyrannosaurus approach a Triceratops?)

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

Greetings,

Lots of postings on this topic over the weekend, so some general comments:

*Possible tyrannosaurid binocular vision was proposed initially because tyrants 
in general, and T. rex (including Nanotyrannus) have
exceptionally broad backs of their skull, resulting in a more forward 
orientation of the orbits. As Jaime noted, the particulars of
the cheek region enhance this forward orientation.

*Unlike someone posted (with sufficient caveats, to be sure!), tyrannosaurids 
do NOT have narrower snouts than most theropods. Far
from it! They have about the broadest snouts of any theropod (only real rival 
are some abelisaurids), and T. rex is broadest of all
the tyrants. Typical large theropods (big coelophysoids, ceratosaurids, 
carnosaurs) have slab-sided skulls with much narrower
braincases: perhaps not as narrow as the cast of Carcharodontosaurus, but 
narrower than the cast of Giganotosaurus (both casts have
some problems). A good articulated Allosaurus or Ceratosaurus skull will show 
you what I'm talking about.

*Jura suggested that there was a different orientation of the heads of various 
theropods to allow better forward vision. Indeed!
This issue is addressed in part in the Bakker et al. paper in which 
Nanotyrannus was named, and in a presentation by Kent Stevens at
SVP several years ago (that will hopefully see print someday soon). These 
papers worked from the details of the basicranium (Bakker
et al.) and models of the fleshed out skull (Stevens), but in principle an 
enterprising researcher with access to big theropod
skulls and a good CAT scan system could look at the orientation of the 
semicircular canals.

                Thomas R. Holtz, Jr.
        Senior Lecturer, Vertebrate Paleontology
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
                Building 237, Room 1117
                College Park, MD  20742

http://www.geol.umd.edu/~tholtz/
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