Thursday, 2 August 2012

More on Tyrants: Alioramus - Part 2


Alioramus remotus was first described by Kurzanov in 1976. The holotype (PIN 3141/1) was recovered from the Late Cretaceous Maastrichtian beds of Nogon Tsav in Mongolia and is very incomplete. A partial skull, a few cervical vertebrae, partial lower leg bones and bones from the pes are all that were found. As a result the very validity of Alioramus has often been questioned over the years with some authors suggesting the remains may represent a different ontogenetic stage of Tarbosaurus (eg Currie 2003a).
This new specimen of Alioramus (IGM 100/1844) is exquisitely preserved and very much more complete than the holotype. The skull is virtually complete but disarticulated and behind this there is what appears to be a complete series of cervical vertebrae. These are complemented by an array of both axial and appendicular elements and all together these have enabled the authors of this monograph to examine nearly every bone in depth.
The monograph is set out in a clear and concise manner. After the initial  introductions that set the tone for the paper, the next sections are set out exactly as you would expect with descriptions of the skull, axial and appendicular skeleton in that order. A contents section enables you to highlight a particular bone(s) with ease so that it is easy to navigate. After the descriptions come the usual discussion topics, conclusions and references. However, what sets this monograph apart from all others are the magnificent images that accompany the text. The photographer, Mick Ellison, took over 300 images of the specimen in RAW format which has provided unparalled clarity for this work.
I also have to mention the preparation of this specimen – it is absolutely magnificent. The preparator, Amy Davidson of the American Museum of Natural History, has worked on many specimens from Mongolia in the past and her familiarity and expertise with the fossils from this region shines through. I was fortunate to meet Amy and discuss her work with her on other specimens such as Shuvuuia deserti and Citipati osmolskae and I can tell you that her resolve and attention to detail with these specimens is second to none and Alioramus altai will have been exceptionally well prepared and every available detail exposed and preserved. It really is top drawer prep work.
So then to Alioramus altai. The most apparent difference between A. altai and other tyrannosaurids is the low profile elongate skull which is mainly a product of the elongation of the bones in the snout. The skull is also highly rugose and displays multiple horns which make for a very strange looking beast. It is mainly the shape of the skull that has led other palaeontologists to suggest that Alioramus may be a different ontogenetic stage of another tyrannosaurid – particularly Tarbosaurus. However, this is hard to quantify either way since although this specimen is superbly preserved the lack of substantial postcranial remains, just as with the holotype, makes clarification difficult.
But what makes Alioramus altai? Well there are many characteristics of note and, more importantly, perhaps several autapomorphies. For example, one interesting character is that the jugal and lacrimal form an articulation that appears to be unusual in tyrannosauroids. Despite displaying what appears to be the standard condition for this articulation, the ventral ramus of the lacrimal is relatively wide compared to other tyrannosauroids and may be unique although the authors recognise that this process is liable to erosion and may not be so well preserved in other tyrannosauroids as it is in A. altai.
Despite the fact that only the left jugal remains, it is very well preserved and displays a unique laterally projecting hornlet. This feature is not found in any other tyrannosauroid although, of course, it is likely that A. remotus also possessed this structure. However, the description by Kurzanov (1976) regarding this specimen is unclear.
The ectopterygoid displays a clear autapomorphic condition in A. altai.  A thickened ridge that separates twin fossae on the dorsal surface of the bone which, although may be present in other tyrannosaurids such as Albertosaurus and Daspletosaurus, is very much thinner or, indeed, absent in some cases. Intriguingly the authors still quantify this by suggesting that this thickened ridge may be an artefact of crushing but this does not appear likely to me and I suggest that this autapomorphy is real.
The splenial is located lingually on the mandible and reveals another unique condition. There is a large foramen situated anteriorly which demonstrates distinctive characteristics. It is elongate and narrow when compared to the usual circular foramen of other tyrannosaurids and is situated more or less horizontally which, again, displays a different orientation from other tyrannosaurids.
The splenial displays another likely autapomorphy. The dorsal flange of the splenial trends anteroventrally at a much shallower rate and is not is not as tall dorsoventrally as is found in other tyrannosaurids.
Unusually for a derived tyrannosaurid, laterally located teeth from both the maxilla and dentary in A. altai are narrow and recurved and resemble those found in other theropods as opposed to tyrannosaurids. This may be due to ontogeny since this specimen is clearly a juvenile but, for me, this is unlikely since other tyrannosaurids of similar age do indeed display teeth that are already thickening up as they approach the classic incrassate condition.
The cervical vertebrae display a few interesting characteristics. Firstly, the epipophyses are narrow and end in a sharp point and are considered autapomorphic in Alioramus by the authors although this may be revised since preservation of the epipophyses are rare and the condition in Alioramus may actually be a more common characteristic in other theropods than is currently known. Another possible autapomorphy is a posterior facing foramen located at the junction ventral of the postzygapophysis and dorsally of the supradiapohyseal fossa. Finally, the cervical vertebrae of A. altai are highly pneumatic and, when compared to other tyrannosaurids, the degree of pneumaticity displayed is almost excessive.
There are only three sections of dorsal vertebrae preserved and yet even here there may be a couple of unique characters present. It is possible that a fossa that leads into a foramen located ventrally to the hyposphene may be autapomorphic although the authors suggest that this may equally be an artefact or evidence of random pneumaticity. There is also a robust lamina present on the left side of one of the dorsals that partitions the posterior surface of the postzygapophyseal fossa. There is no lamina on the right side and this too may be an autapomorphic condition.
One clear autapomorphy is to be found in the only recovered dorsal rib. There is a clear pneumatic foramen on the proximal flange of the rib situated ventrally of the tuberculum and pnuematopores of this description are absent in all other tyrannosaurids.
So it can be seen that, on the face of it, there are many characters displayed in A. altai that are highly indicative that the taxon is valid but, equally, it can also be seen that many of the suggested autapomorphies are subject to various caveats. In part three we will look at what this all means with regards to the validity of Alioramus altai, its overall position within Tyrannosauridae and how this animal fitted into the Late Cretaceous ecosystem of Nogon Tsav.

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. 2003a. Cranial anatomy of tyrannosaurid dinosaurs from the late Cretaceous Alberta, Canada. Acta Palaeontologica Polonica 48: 191–226.
Kurzanov, S.M. 1976. [A new Late Cretaceous carnosaur from Nogon-Tsav, Mongolia]. Joint Soviet-Mongolian Paleontological Expedition Transactions 3: 93–104, [in Russian, English summary].

 

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