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Teruo Mizukami posted (5/1) a note on a probable premax tooth of a
tyrannosaurid from the Lower Cretaceous of Japan, and further noted
proposed tyrannosaurid material from the Lower Cretaceous of Thailand.
These early occurrences, as suggested, "may prove this Asia[n] origin
Kirkland subsequently (5/2) noted tyrannosaurid material from the
basal Cenomanian of Utah, and proposed this as the "earliest date for
opening land connections to Asia through Alaska."
These notes brought to mind some other cryptic notes of possible North
American tyranosaurids from the Late Jurassic-Early Cretaceous. Britt
and Stadtman (1996, p. 24A, SVP abstr) noted "possible tyrannosaurid"
material from the basal Cedar Mountain of Utah (= Early Cretaceous,
probably Barremian). Is this correct? Does anyone have additional
comments on this occurrence? If true, the tyrannosaurids may have
been in North America a very long time.
A large fragmentary tooth from the Dakota Formation of southeastern
Nebraska on display at the Nebraska State Museum is labeled a
"tyrannosaurid." This occurrence is mid-Cretaceous, likely late
Albian (or possibly Cenomanian). Has anyone out there looked more
closely at this specimen? Is it?
Finally, the late Jurassic (Morrison Fm) Stokesosaurus has been
suggested by some to share affinities with tyrannosaurids, and some
have tentatively placed it within the Tyrannosauridae. Chure (1994,
in his paper describing Koparion) briefly suggested that
Stokesosaurus' reference to the Tyrannosauridae was strengthened by as
yet undescribed new material. Can anyone shed more light on this
material and what it means?
If Stokesosaurus is part of the tyrannosaurid lineage, the origin of
the group would likely be North American, not Asian. The other noted
Early Cretaceous tyrannosaurid-like material from North America would
be consistent. Or is this stuff all misinterpreted?
The co-occurrence of true hadrosaurs in the Early Cretaceous of both
Asia and North America may mark the initial phases of faunal exchange
between the two continents. Likewise, the appearance of the Asian
triconodont mammal genus Gobiconodon in the Cloverly Fm (Aptian-lower
Albian) of the western Bighorns further suppports an Asian-North
American faunal exchange beginning during the Early Cretaceous.
Could the tyrannosaurids have originated in North America and migrated
to Asia in the Early Cretaceous? What are the pros and cons?