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Tyrannosauroids Imploding Further
The Tyrannosaur Implosion (TI) continues with new studies in
craniofacial anatomy of tyrannosaurids. The TI, first mentioned by George
Olshevsky over a year ago, implied that many taxa were related to one
another and could be synonymized. However, given the state of current
systematics, work by Carr (2000), Currie and Hurum (2003), Hurum, Currie
and Sabath (2003), and now Carr and Williamson (2004), the TI continues to
shrink taxa, more so than that implied by George Olshevsky by almost half.
Carr and Williamson assessed several skulls held at the LACM (Los
Angeles County Museum) which includes the holotypes of *Stygivenator
molnari* and *Dinotyrannus megagracilis,* taxa first suggested by Paul in
1988, then renamed by Olshevsky, Ford and Yamamoto in 1995. These taxa
show that if placed in series with *Tyrannosaurus rex* specimens in the
American, Los Angeles County, and Cleveland Museums of Natural History,
form a distinct series paralleling growth in albertosaurines (Carr, 1999),
which ontogeny being accepted even by those that question affinity of
*Nanotyrannus* to *Tyrannosaurus.* *Tarbosaurus* ontogeny is recapped, and
shows the same general series, arguing that *Stygivenator molnari* and
*Dinotyrannus megagracilis* are synonyms of *Tyrannosaurus rex.* On top of
this, the holotype of *S. molnari* shows that it is likely the youngest
specimen of *T. rex* so far described, smaller only than the holotype of
*N. lancensis.* Furthermore, the lectotype tooth of *Aublysodon mirandus*
appears to be a mesial maxillary crown, and data is presented to show that
serrations appear during ontogeny in the taxon, so younger skulls will
tend to have less denticulate crowns than older ones, and cross-section
variation from the first to the sixth-maxillary positions show degression
from a D-shaped, tricarinate crown to a more conventional tear-shaped,
bicarinate crown (tricarinate refers to the posterior ridge on the
premaxillary teeth of tyrannosauroids). Thus, *Aublysodon* is rendered a
nomen dubium, because it cannot be applied as a distinct taxon, solely due
to its absence of denticles or shape.
The implosion leaves us with the following synthesis from Carr (2000),
Currie and Hurum (2003), and Carr and Williamson (2004), including recent
taxa added for general opinions coming from me (the EK/LJ taxa from
Guiamarota and the Morrison, as well as the Weald form, are shown
generally "advanced" relative to *Dilong,* but this is subjective and
prone to change):
| |--Albertosaurus sarcophagus
| `--Gorgosaurus libratus
|--?Daspletosaurus n. sp.
| incl. Shanshanosaurus, var. Asian "spp."
|?-Alioramus remotus (syn. T. bataar?)
Carr and Williamson have several new species in prep, including two
western NA tyrannosaurs and one Alabama one, that will expand this tree.
But the clear extensive species of Asian tyrannosaur, most based only on
teeth, and thos from Canada and NW USA (*Deinodon,* *Aublysodon,*
*Tyrannosaurus,* *Tarbosaurus,* *Gorgosaurus,* *Albertosaurus,*
*Proedeinodon*) all appear to be nomina nuda in the same vein as *A.
mirandus* as explained in the paper, and while useful biogeographically,
they are useless biostratigraphically, or even for most specific faunal
comparisons, as it is nerly impossible to tell if the Asian
"tyrannosaurine" teeth are actually tyrannosaurine, or whether they
corellate with the late Maastrichtian (i.e., "Tyrannosaurus"), the early
Maastrichtian (i.e., "Tarbosaurus," "Daspletosaurus"), or even with the
early or late Campanian (i.e., albertosaurs and "Daspletosaurus"), which
may all be possible identities based on these general qualities of the
teeth. Incrassate structure of teeth and tooth-bearing bones as a
mechanical effect (various papers cover this topic, and I can summarize if
anyone wishes with refs) effect the use of these features phylogenetically
negatively, in that they are convergent through a dietary regime.
Given new data on *Dilong* and basal ornithomimosaurs (*Garudimimus* and
*Harpymimus*) the arctometatarsus is convergent, if highly detailed in
their similarities among two convergent clades, and thus the organization
of the limbs must be considered convergent and the design of little
phylogenetic value as far as inferring tyrannosaurs as "blown-up
ornithomimes." I will get to discussing Snively et al. (wonderful paper)
in a little bit, but my comments will be brief on on a few points, since
it's been adequately summarized in parts by others [Never do something if
someone else has already, and you knew about it].
Jaime A. Headden
Little steps are often the hardest to take. We are too used to making leaps
in the face of adversity, that a simple skip is so hard to do. We should all
learn to walk soft, walk small, see the world around us rather than zoom by it.
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
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