The Alioramus
monograph has provided a comprehensive and meticulous description of specimen
IGM 100/1844 – Alioramus altai. The comparisons
throughout with other tyrannosauroids have provided more insight into what is
actually the rather complex systematics of the group but still leaves questions
unanswered.
One thing that is certain, however, is that A. altai is a derived tyrannosaurine
tyrannosaurid for sure. This may be glaringly apparent to some but the
monograph takes any lingering doubt out. But is Alioramus a genus in its own right or perhaps, as some have
maintained, a juvenile Tarbosaurus?
There are a few issues here.
As the authors state, the elongate skull of Alioramus appears to be diagnostic of
the taxon and I would tend to agree with this. We know that all tyrannosaurids
went through significant morphological change throughout ontogeny but what
evidence there is suggests that the skull of Alioramus is not aberrant and that the skull length is a genuine
diagnostic feature. Certainly, using the classic skull/femur ratios of Currie
(2003b) and comparing the skull proportions of Alioramus with other tyrannosaurids appears to support this
conclusion.
Looking at the lengthened skull, it may be that we make
things more complex than they really are and ignore the obvious. I understand
this, however, and we must always make our assumptions based upon the evidence.
After all, let us not forget the amount of ontogenetic change there is within
ceratopsids by way of example. This can be extreme so it would not be that much
of a surprise if Alioramus turned out
to be another ontogentic stage of
Tarbosaurus or maybe another, as yet unknown, tyrannosaurid – no matter how
unlikely.
But, and for the sake of argument, I believe that this is
very unlikely. These beds are amongst the most heavily sampled (and poached)
beds in the world and it seems improbable that there is yet another large
tyrannosaurid waiting to be discovered. There is, however, a pretty complete
growth series of Tarbosaurus bataar
and there are indeed specimens of juveniles similar in age to A. altai – estimated to be nine years
old at time of death. None of these are truly comparable as Tarbosaurus increases skull depth
throughout ontogeny and it is hard to visualise Alioramus as a growth stage of this very well sampled taxon. I
support the authors in considering the elongate skull of Alioramus as diagnostic.
In addition, and to bolster their argument, the authors
highlight morphological disparities of the maxilla, postorbital, surangular,
tibia and jugal when comparing these elements from both Alioramus and those of Tarbosaurus
specimens of comparable size thus increasing the likelihood of generic
separation.
Are A. altai and A. remotus the same animal? This would
seem likely and yet the authors are happy to keep the two taxa separate pending
further research. There are similarities in the two specimens with the most
obvious being the elongate skull and both specimens are immature individuals
although A. remotus appears to be a little
older. There is enough detail, however, to confirm that A. altai is indeed Alioramus.
However, the holotype of A.
remotus is somewhat poorly preserved when compared to A. altai thus proper comparison is difficult. The authors list
several differences between the two specimens but the differences in preservation
between the two is significant and is the primary reason why the authors chose to
keep the two specimens generically separate for now.
A. altai also displays
various ontogenetic characters that support its juvenile status and the authors
compared these with specimens of both Gorgosaurus
and Tyrannosaurus. Interestingly, and
perhaps unsurprisingly, these shared features aptly demonstrate that these
tyrannosaurids shared a common ancestor and that these same ontogentically
driven morphological changes are not altered even when taxa change the entire
shape of the skull.
The elongation of the skull itself is interesting since it
demonstrates that skull morphology in tyrannosaurids, whilst based upon a
rather consistent morphological blueprint, could actually be diversified
without compromising what are considered standard tyrannosaur biomechanics. In
addition, the skull displays eight cephalic horns as opposed to the usual
tyrannosaur count of six and the authors suggest that these were probably for
intraspecific communication and recognition as is generally considered for all
derived tyrannosaurids.
Assuming then that Alioramus
altai is indeed a valid taxon then what sort of animal was it and where did
it fit in the Late Cretaceous environment of Nogon Tsav? Well, for me, one of the most interesting
points here is that it appears we have yet again evidence for sympatric large
carnivores in the palaeoenvironment – Alioramus
and Tarbosaurus. So we should really
put to bed any lingering doubts that sympatric large theropods are a rarity
since there are now multiple examples to cite. It is only the latest
Maastrichtian of North America that there appears to be the one genuine example of an
environment dominated by one large carnivore – Tyrannosaurus rex.
The authors suggest that the elongate skull, the un-tyrannosaurid
like teeth, the lightly built and heavily pneumatised skeleton all point to
Alioramus filling an alternative predatory niche when compared to other
tyrannosaurids. This makes a lot of sense when comparing Alioramus with juvenile specimens of other tyrannosaurids and there
is a growing consensus amongst tyrannosaur workers that juvenile animals
utilised speed and agility to take down smaller prey than the adults and that
their feeding habits consequently changed throughout ontogeny (eg Tsuihiji et
al 2011). Regardless of this, Alioramus
is certainly different to mainstream tyrannosaurids and, as the authors point
out, could not employ the classic “puncture-pull” technique employed by other
tyrannosaurids.
Ultimately, we have to remember that both A. altai and A. remotus, regardless of taxanomic affinities, are only
represented by the two specimens discussed and they are also both juveniles. An
element of caution must be taken in this case because we do not have anywhere
near a complete specimen or, more importantly, an adult animal to compare with.
In addition, the holotype of A. remotus is in serious need of
preparation and needs a substantial redescription. There are other skulls in
existence but, as the authors point out, these are in private hands. The skull
I have featured throughout this series of posts is also in private hands although
only a small portion is actual bone.
Until there are more specimens available, we still have to
take a cautious view regarding this enigmatic tyrannosaurid. However, with the
evidence presented in this monograph, I am happy to currently accept the validity
of Alioramus altai and look forward
to finding out more about this intriguing animal.
The Alioramus
monograph is a triumph and sets the standard for all future monographs to
follow. Granted, 197 pages of anatomical
detail may be deemed excessive but that would do this fine work a great
injustice. You can think of it, if you like, as a car maintenance manual where
every part is stripped down and photographed and
described in detail – it really is very similar.
Even if your knowledge of the anatomical details of
tyrannosaurs is limited, I implore you to take a look and learn because that is
what it is all about. I needed extra
time to come to terms with the descriptions of the cervical vertebrae of Alioramus – there were lots of going
backwards and forwards and rechecking the images but you get there in the
end. You will learn.
For me, the monograph actually highlights what an odd
tyrannosaur Tarbosaurus is – quite
primitive in fact. Quite often the aberrant morphologies here are not displayed
by Alioramus but by Tarbosaurus and will make for some
interesting discussions that are coming up in the next few months and, I
believe, has palaeogeographical implications regarding the origins and
radiation of Tyrannosauridae as a whole.
Read this monograph!
Footnote
As is often the way with my posts, and whilst I have been
writing this particular one, the Society
of Vertebrate Paleontology have
released their program for their annual meeting in October this year and, lo
and behold, we have another Alioramus
altai session that looks at pneumaticity in the skull followed a little
later by Thomas Carr’s look at cephalic ornamentation in tyrannosauroids. No
doubt more details will come to light about Alioramus
and I can genuinely say that I am really looking forward to them.
References
Brusatte, S.L., Carr, T.D., and Norell, M.A. 2012. The
osteology of Alioramus, a gracile and long-snouted tyrannosaurid (Dinosauria:
Theropoda)from the Late Cretaceous of Mongolia. Bulletin of the American
Museum of Natural History, 366,
1 – 197.
Currie, P.J. 2003b. Allometric growth
in tyrannosaurids (Dinosauria: Theropoda) from the
Upper Cretaceous of North America and
Asia. Canadian Journal of Earth Sciences
40: 651–665.
Tsuihiji, T., et al. (2011). Cranial
osteology of a juvenile specimen of Tarbosaurus
bataar from
the Nemegt Formation (Upper
Cretaceous) of Bugin Tsav, Mongolia. Journal
of Vertebrate
Paleontology 31: 497–517.
2 comments:
Cheers, Mark. I appreciate all of your kind words about the Alioramus monograph and am thrilled that you enjoyed it.
Best wishes,
Steve Brusatte
Thanks Steve - always a pleasure.
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