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Re: Alioramus



Sean Carroll (sean@kua.net) wrote:

<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?>

  It's been suggested before, such as by Carr, and Holtz, that the
juvenile features of advanced tyrannosaurs are reflected in the adult
features of primitive tyrannosaurs. It was likely the case in other
dinosaurs for which sampled primitive and advanced animals show the same
advancement of adult structure. This is largely explained by
Rozhdestvensky in the monograph on ontogeny, and is also seen in
*Protoceratops* (see Matthew and Brown, 1925) where the juvenile skulls
resemble *Bagaceratops*, enforcing the latter's primitive nature even in
adult material.
 
<...this is what Paul said about _Alioramus_ in PDW:
 
"... 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.>

  Currently, *Indosuchus is a "long-snouted" abelisaurid "ceratosaur", and
Chatterjee apparently has a partial skeleton that has yet to be described,
catalogued in the ISR, Calcutta.

  However, the most current analysis on Tyrannosauridae is that of Holtz,
2001:

  Holtz, T.H., Jr. 2001. The phylogeny and taxonomy of the
Tyrannosauridae. pg. 64-83 in Tanke and Carpenter (eds.) Mesozoic
Vertebrate Life (NRC Research Press and Indiana University Press
[Bloomington & Indianapolis].)

  This analysis is based on a cladistic matrix of 111 characters and 17
taxa and one all-0 outgroup "taxon." It produces following tree:

--Tyrannosauridae
   |--Aublysodontinae
   |   |--Aublysodon molnari
   |   |--Kirtland aublysodontine
   |   `--Alectrosaurus olseni
   `--Tyrannosaurinae
       `--+--Alioramus remotus
          `--+--Gorgosaurus libratus
             |--Albertosaurus sarcophagus
             `--+--Daspletosaurus torosus
                |--Two Medicine tyrannosaurine
                `--+--Tyrannosaurus bataar
                   `--Tyrannosaurus rex

  Alioramus is a tyrannosaurine on the folowing features: nasal rugosities
present, adult frontals with the caudal end expands laterally, and the
frontal expression of the supratemporal fossa occupies the caudolateral
half. It is restricted from the other tyrannosaurines by: a high tooth
count, well above 13; smooth dorsal surface of the postorbital surface;
the supraoccipital is expressed on the rim of the foramen magnum; the
vertical depth of the caudal dentary is only half as deep or less that of
the vertical depth of the symphysis; there are at least 16 dentary teeth,
not 15 or less; teeth are ziphodont rather than incrassate (cross-section
greater than 60% wide mediolaterally as long craniocaudally). Several
apomorphies restrict *Alioramus* from other tyrannosaurs: contact between
dorsal surface of the lachrymal and postorbital is present in lateral
view, without any intergrowth, which also occurs in juvenile *Tarbosaurus
baatar*; the nuchal crest rostrocaudal thickness very thick, the dorsal
margin is rugose (adult feature), also seen in *Tyrannosaurus rex*; a
double row of five vertical blades; a prootic rostral expansion on the
lateral braincase is present; and the trigeminal foramen is in the
prootic. The latter three do not occur in other taxa.

<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?>

  Hurum and Currie, and Hurum and Sabath are or have done work on the
issue of *Tarbosaurus*, plus Carr did and is working on work on both new
tyrannosaurs and tyrannosaur ontogeny, and Holtz continues to work on
tyrannosaur phylogeny. Many things to expect in this year and the next. So
far, any allusion to the juvenile or synonymous nature of *Alioramus* has
occured outside of the literature or has been made in limited comments
without much support at the time; Paul aptly reflected the small, gracile,
slender anatomy of the skull of *Alioramus* as being like juvenile
*Albertosaurus*, which included at the time his paradigm of the CMNH skull
of *Nanotyrannus*. On the other hand, a basal, valid nature of *Alioramus*
has been recovered in phylogenies which has undergone at least some
objective work through cladistic machine. Based on recent work, the second
tree above may be misleading as *Aublysodon* has been suggested as
belonging to abnormal teeth of *Tyrannosaurus* or *Daspletosaurus*;
Judithian teeth may belong to *Gorgosaurus*. This suggests that the basal
phylogeny is:

--Tyrannosauridae
   |--Alectrosaurus olseni
   `--Tyrannosaurinae
       `--+--Alioramus remotus
          `--other tyrannosaurines

  Holtz also included *Siamotyannus* in his matrix, and it came out basal
to all other tyrannosaurs; no non-coelurosaurs were included in the
matrix, which makes this matrix difficult to test the theory of a
carnosaur relationship for *Siamotyrannus*. He also included
*Shanshanosaurus*, but this taxon has been indicated to possess several
juvenile features and is probably a juvenile specimen of *Tarbosaurus*
(Dong and Currie, 2002).

  Cheers,

=====
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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