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Re: Alioramus



Tim Donovan wrote:

> P. Currie mentioned a broad nuchal crest, downturned occiput and other
> features which suggest Alioramus is most closely related to Tarbosaurus,
> Daspletosaurus and T.rex. Of course, the skull of Alioramus is much longer
> and lower, but he pointed out that a long, low skull is a juvenile feature.

Wait a second, I want a little more clarification on this. When you say
the skull shape is a 'juvenile feature' are you suggesting that the
existing specimen(s) of _Alioramus_ *are* juveniles, perhaps of an
already known other species? Or are you thinking that they are adults,
but the species arose from other tyrannosaurs through some kind of
neoteny?

FWIW, I don't know how out of date it is, but this is what Paul said
about _Alioramus_ in PDW:

'the lower jaw ... is much slimmer, straighter, longer in the dentary,
and less advanced than the tyrannosaurs discussed below. The upper jaw's
large maxillary bone is also lightly built, with a larger preorbital
depression. In these respects this is one of the least advanced
tyrannosaurs. Otherwise, what is known of this genus and species is
typical tyrannosaur in design, and looks rather like a small
_Albertosaurus_.'

This gives me the impression that he thought it fell pretty much in
between _Aublysodon_ and its close relatives (which he places in a
separate subfamily at the root of Tyrannosauridae) on the one hand, and
the _Albertosaurus_/_Tyrannosaurus_ ('Tyrannosaurus' in this case
including not only _T. rex_ but _'Daspletosaurus' torosus_ and
_'Tarbosaurus' bataar_) clade on the other. From the way he orders
things, though he does not explicitly use cladistic terminology, this is
what I kind of see as the 'tree' he constructed:

Tyrannosauridae
 |-Aublysodontinae
 `-Tyrannosaurinae
     |-_Indosuchus raptorius_
     |-_Alioramus remotus_
     `-advanced tyrannosaurines
         |-albertosaurs
         |   |-_Alectrosaurus olseni_
         |   |-_Albertosaurus_
         |   `-_Nanotyrannus lancensis_
         `-_Tyrannosaurus_

IOW: _Alioramus_ and _Indosuchus_ at the base of the main subclade, with
_Albertosaurus_ and _Tyrannosaurus_ as the advanced members. He's not
clear on whether he things _Indosuchus_ or _Alioramus_ is the more
primitive, or if he thinks either of them might have been a direct
sister group to the albertosaur/tyrannosaur clade.

But if _Alioramus_ is really more closely related to the 3 big
_Tyrannosaurus_ species and is either a juvenile or a case of neoteny,
then that could provide a false impression that it was a more primitive
tyrannosaurine. Has anyone ever done a direct comparison of these two
hypotheses (or something close to them) to see if one can be supported
better than the other?

-- 
--Sean
http://www.livejournal.com/users/spclsd223/