early tyrannosaurids

[bwitzke@gsbth-po.igsb.uiowa.edu]

     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 
     of tyrannosaurids."
     
     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?
     
     Brian Witzke
     Iowa City