RE: New tyrannosaur articles

[Tim Williams <twilliams_alpha@hotmail.com>]

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