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RE: New tyrannosaur articles



Andrew A. Farke wrote:

Diversity of late Maastrichtian Tyrannosauridae (Dinosauria: Theropoda) from
western North America
THOMAS D. CARR, THOMAS E. WILLIAMSON
Zoological Journal of the Linnean Society; Volume 142, Issue 4, Page 479

Abstract:
The tooth taxon _Aublysodon mirandus_ was reinstated following the collection of nondenticulate tyrannosaurid premaxillary teeth from late Maastrichtian deposits in western North America. A small skull from the Hell Creek Formation of Montana (the 'Jordan theropod', LACM 28471), that was associated with a nondenticulate premaxillary tooth, was referred to _Aublysodon_ and the diagnosis was revised to include cranial bones. However, the 'premaxillary' tooth of the specimen is actually a maxillary tooth. The small size of _Aublysodon_ crowns, and evidence that some denticles develop late in growth in theropods, indicates that the nondenticulate condition represents immaturity. Therefore, _Aublysodon_ is a nomen dubium. The Jordan theropod was recently designated as the type specimen of _Stygivenator molnari_. A tyrannosaurid from the Hell Creek Formation of Montana (LACM 23845) was first referred to _Albertosaurus_ cf. _A. lancensis_ and then later became the type specimen of _Dinotyrannus megagracilis_. On the basis of shared derived characters and a quantitative reconstruction of the growth series of _Tyrannosaurus rex_, the type specimens of _S. molnari_ and _D. megagracilis_ are juvenile and subadult specimens of _T. rex_, respectively. There is currently evidence for only one tyrannosaurid species in the late Maastrichtian of western North America: _T. rex_.


The authors conclude (p.519): "Based on the evidence presented herein, it is most parsimonious to identify LACM 28471 as a small juvenile _T. rex_ and LACM 23845 as a subadult _T. rex_ instead as different genera. _Aublysodon_ is an invalid taxon; the skeletal characters ascribed to it are either plesiomorphic among Tyrannosauridae or growth-related. Likewise, _Stygivenator molnari_ and _Dinotyrannus megagracilis_ are invalid. Although Carpenter has suggested the Tornillo Formation of Texas has a large theropod that is not _T. rex_, it is referable to _T. rex_ (Carr & Williamson, 2000)."

Thus, Carr and Williamson (2004) officially sink _Stygivenator_ and _Dinotyrannus_ into _Tyrannosaurus_ (although, one could argue that neither _Stygivenator_ and _Dinotyrannus_ were ever valid in the first place, due to the peculiar way both genera were described - but I won't go into that here). Carr and Williamson also reiterate that _Nanotyrannus_ is also a juvenile _T. rex_.

The article includes some very nice figures, including a growth series of the _T. rex_ skull.


Tim