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RE: Earliest North American Tyrannosaurids



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> no go
>
> I was curious, which of the Tyrannosaurid species actually have
> the honour
> of being the first to be North American? And this was the time
> before they
> were the apex predator, so who was the top predator back then? Any
> facts/speculations that can give me a better idea on this rather murky
> aspect of Tyrannosaurid history? Most people are rather sure of
> the era of
> Tyrannosaurus rex dominace supreme, but they all had to start somewhere,
> whatever happened up in the north Americas to the other large theropod
> designs that were once common fare?

Well, Late Jurassic _Stokesosaurus_ is turning out to be the oldest known
tyrannosaurOID, and is North American.  Being in the Morrison, there were at
least four (_Allosaurus_, _Torvosaurus_, _Ceratosaurus_, and _Marshosaurus_;
this takes the position of maximal "lumping" and excludes other named taxa
related to the above) theropod taxa which were larger than it in the same
community.

TyrannosaurIDAE is used in a much more restricted sense by the Terrible
Tyrant Taxonomy Team (Holtz, Brochu, Carr, Williamson).  Our revisions of
the taxonomic defintions are forthcoming.  In this sense, the oldest
currently named North American tyrannosaurids would be those present in the
Oldman Formation (_Daspletosaurus torosus_ and specimens probably referrable
to _Gorgosaurus libratus_); there is older material from the Eagle Sandstone
(Early Campanian), such as the missing type of "Ornithomimus" _grandis_ and
isolated teeth in various pre-Campanian Late Cretaceous beds, starting with
the Mussentuchit Member of the Cedar Mountain Fm.  However, this material
cannot definitely be placed within Tyrannosauridae following our definition,
although it might well belong there (or be anatomically very close to that
condition, even if just outside of it).

The problem here stems from the big stratigraphic gap in North American
terrestrial record.  We have very good records for the Aptian-Albian in
Oklahoma, Utah, Wyoming and Montana (and heck, even stuff in Maryland),
where _Acrocanthosaurus_ or a similar form is the Big Guy, and various other
theropods (including _Deinonychus_) are the smaller fry.  No definite
tyrannosauroids recorded from this community yet.

Then we have almost no good records until the mid-Campanian Oldman Formation
and the lower Two Medicine Formation.  By that time tyrannosaurids had
evolved into giants and were the top dogs..., er, dinos.  Indeed, from that
point onward in North America there are no non-tyrannosauroid (and, save for
_Daspletosaurus_, no non-tyrannosaurID) large theropods known.

So somewhere in the gap from the latest Albian to the mid-Campanian this
ecological shift took place.  That gap is being filled (the Mussentuchit
Member of the Cedar Mountain is at the beginning of this gap, and contains
"_Alectrosaurus_"-like teeth; the Moreno Hills Formation from the Turonian
of New Mexico and the "bloat-and-float" dinosaur carcasses of the Niobrara
Chalk have some good fossils, but sadly no big theropods yet; and isolated
tyrannosaur bones and teeth are found higher up in the West).

Hope this helps.

                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
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796