Friday, 10 August 2012

More on Tyrants: Alioramus - Part 3


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:

Steve Brusatte said...

Cheers, Mark. I appreciate all of your kind words about the Alioramus monograph and am thrilled that you enjoyed it.

Best wishes,
Steve Brusatte

Mark Wildman said...

Thanks Steve - always a pleasure.

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