Saturday, 19 November 2011

SVP 2011 - Theropods

Time for theropods and about time too I hear you say. No matter whoever tries to convince you otherwise, theropods are without doubt the coolest animals ever to walk the planet and, of course, they were strongly represented at SVP.

Did change in theropod skull morphology occur due to the type of prey animals that shared their environment? Richard Bykowski of Indiana University has attempted to answer this question by combining the morphological data taken from the maxillary bones of multiple theropod specimens with the estimated relative abundance of other dinosaurs - including multiple prey animals as well as other theropods.
It appears that there is slight variation in maxilla morphology when the abundance of coexisting animals varies. However, faunas dominated by sauropods display the greatest change whereby the skull becomes more elongate, displays a flatter ventral border and a thickened ramus of the maxilla where it ascends. Other results indicate a lengthening of the snout where there are increased populations of marginocephalians and thyreophorans, and theropods, that shared their environment with larger populations of both ornithopods and other theropods, developed deeper and stronger skulls.
The sauropod result is interesting bearing my recent post about titanosaurs and carcharodontosaurs and Bykowski suggests that the typical carcharodontosaur skull layout is well suited to “slash and run” techniques which, again, I alluded to in the same post. Nice work by the author and I must point out that I have only skimmed the surface of what is obviously an intense and thorough study but I have to say that I would have thought that a sampling bias, in either direction, would probably affect results.
Any material from the mid-Cretaceous that transcends the early to late Cretaceous faunal changeover is rare and when it consists of new theropod material as well then it is doubly important. Derek Main et al have revealed new theropod specimens from the Arlington Archosaur Site in north Texas which comprises of a fauna that inhabited the Cenomanian coastal delta plains and comprise of multi-sized taxa, from the small to the large.
What makes this fauna so special is that it includes both basal tetanurans and dromaeosaurids which are representative of both early and late Cretaceous communities - transitional. Why were basal tetanurans, which originated from Gondwana, still around at this point? It seems that there was a global semi-arid climate that extended from Texas right through to Morocco and Niger and is indicative that these tetanurans preferred this drier environment   and this also explains their presence in this transitional mid-Cretaceous fauna.
Study of this unique fauna continues and it is hoped that it will shed more light on both palaeogeographic and palaeoclimatic questions during this crucial period in the Cretaceous. Of particular interest is how Gondwanan land faunas made the dispersal across the Berentsian land bridge and when this occurred since the Arlington fauna suggests an earlier crossing than initially thought.
Ornithomimus is one of best known ostrich-like dinosaurs and it is good to know that the holotype, Ornithomimus velox, is indeed a confirmed and valid taxon. Leon Claessons et al have reappraised the type material, which comprised of a distal tibia, partial pes and a partial manus, and performed a detailed examination which was enhanced by the fact that the material had been properly prepared and freed from matrix for the first time in over one hundred years.

Comparing the type material with specimens of Ornithomimus edmontonicus has shown the two genera to be similar but O. velox has slightly longer metacarpals although the pes of O. edmontonicus are longer overall but not a as robust. There is also an unusual development in the metatarsals and that is metatarsal II is longer than metatarsal IV which no other ornithomimid displays.
But the authors point out that a specimen from the Campanian Kaiparowits Formation of Utah has a similar condition and since there is a ten million year age gap between it and the purely Maastrichtian O. velox then it is apparent that O. velox is valid. Incidentally, the fossils from the Kaiparowits display morphological differences that infer that there are at least two as yet unnamed taxa from this formation.
Michael D’emic has featured a few times in recent posts and this time, with lead author Keegan Melstrom, both from the University of Michigan, have reported on a partial skeleton of a juvenile Acrocanthosaurus atokensis that has been recovered from the Cloverly Formation of Wyoming – the first time that this allosauroid has been identified in this formation.
The specimen has provided further data revealing that these theropods also grew fast at similar rates to tyrannosaurids and would reach adult size somewhere between 20 – 30 years old. Acrocanthosaurus represents one of the basal Gondwanan forms that was referred to earlier in the post and appears to have been widespread leading up to the early/late Cretaceous transitional stage.
A few years ago I was privy to some research being carried out on spinosaurids and baryonychinines and was told, in no uncertain terms, that it was certain that Suchomimus would be sunk into Baryonyx because they were irrefutably the same taxon. Well in 2011, that appears to no longer be the case. Nizar Ibrahim and Paul Sereno have revealed detail s of some new Suchomimus material which appears to contradict that theory.
The material was recovered from Middle Cretaceous exposures in Niger and includes a complete snout and, for the first time, some braincase material. There are several features, both cranial and postcranial that, according to the authors, settles the argument once and for all that Suchomimus and Baryonyx are, in fact, distinct genera – now where have I heard that one before?
It will be interesting to hear the response from the opposition camp and, as usual, we will all have to wait for the paper. Somehow, I suspect that this is not the end of this saga just yet. On the same poster the authors report on new spinosaur material from the early Upper Cretaceous deposits of Morocco and by comparing this new material with some “original” material from Egypt, they suggest it is likely that there was only one species of Spinosaurus that existed throughout the Cenomanian of Africa.
The quotation marks on “original” are my additions since, according to the abstract, the comparison was with “........the type material of Spinosaurus from Egypt.” And yet it is generally accepted that all of this material was destroyed by allied bombing during the war – so where did this material come from? It will be interesting to find out what this compared material actually is and again we must wait for the paper – the devil is in the detail.

A very popular talk, and discussed already in the palaeoworld by Brian Switek at Dinosaur Tracking and Stu Pond over at Paleo Illustrata, Alicia Cutler et al looked at why and how the classic theropod death pose occurred whereby the head and neck are stretched and arched back almost over the sacrum and the tail stretches cranially over the head and neck.
 Since most fossils discovered this way are found in sediments laid down in an aqueous environment, the authors decided to test the theory that submersion would inflate the elongate muscles that lie along the long axis which would shorten them thus creating the familiar contraction. Utilising some novel controlled experiments by submerging plucked chickens in water have provided the authors with startling results.

Astonishingly, once submerged, there is an almost instantaneous reaction by the head and neck muscles that pulls the head back – in some cases almost identically to the positions found in theropod dinosaurs. To make sure of their findings, controlled experiments with chicken carcasses in a dry environment were performed but with no noticeable distortion in over three months.
The team were both surprised and delighted by the results and suggest that movement was also helped by the fact that skull pneumaticity was a contributing factor due its relative lightness. Contraction continues after the initial submersion but only to a slight degree. For me, this is science at its best and I love the nature of these experiments and its simplicity and the team are to be highly commended.

Tyrannosaurs did not feature too much this year – indeed only there were only three technical sessions and one poster this year - of which three refer to T.rex, but the one of most interest, to me, is Thomas Carr’s continued work within Tyrannosauridae. This time Carr focussed on the sequence of development (heterochrony) that led to evolved features (synapomorphies and autapomorphies within tyrannosaurid phylogeny.
Because several species of tyrannosaurids are represented by almost complete ontogenetic life histories it was possible, by using cladistic analysis, to compare these stages and look for evidence of heterochrony. The author did indeed identify heterochrony, especially within tyrannosaurines, highlighting the maxillary fenestra as evidence and suggests that this kind of study is indeed ideal for identifying the morphological processes arrived at via heterochrony.

References

Bykowski, R. 2011. Biting Off More Than They Can Chew: A Geometric Morphometric Approach to Theropod Feeding Ecology. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp81. 

Carr, T. 2011. A Comparative Study of Ontogeny Between Derived Tyrannosauroids: Evidence for Heterochrony. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp84.

Claessens, L., Loewen, M. and Lavender, Z. 2011. A Reevaluation of the Genus Ornithomimus Based on New Preparation of the Holotype of O.velox and New Fossil Discoveries. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp90.

Cutler, A., Britt, B., Scheetz, R. and Cotton, J. 2011. The Opisthotonic Death Pose as a Function of Muscle Tone and Aqueous Immersion. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp95.

Ibrahim, N and Sereno, P. 2011.New Data on Spinosaurids (Dinosauria: Theropoda) from Africa. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp130.

Main, D., Noto, C. and Scotese, C. 2011. New Theropod Material from the Cretaceous (Cenomanian) Woodbine Formation of North Central Texas: Paleobiogeographic and Paleoecological Implications. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp150.

Melstrom, K. and D’emic, M. 2011. Acrocanthosaurus atokensis (Dinosauria: Theropoda) from the Cloverly Formation of Wyoming: Implications for Early Cretaceous North American Ecosystems. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp157.








3 comments:

Fabrizio said...

Please, can you tell me where's the link where i can download the pdf file containing all of these abstracts?
I don't mean this one
http://www.vertpaleo.org/meetings/documents/SVP11Abstracts_Front_000.pdf

Mark Wildman said...

Hi Fabrizio,

The abstracts aren't generally available (yet) unless you are an SVP member but if you want to leave an email address I would be glad to forward the document to you.

Fabrizio said...
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