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SVP 2012 - Theropods Part 1

Sunday, November 25, 2012 15:01

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There is no doubt that theropods are probably the most popular dinosaur group of all and, knowing most of you like I do, then it is the theropods that are of the greatest interest whenever there is new research and information revealed. So without further ado, we can proceed. I always feel that ornithomimosaurs do not get as much attention as they deserve but they are bang in the news at the moment because of the newly revealed feathered ornithomimosaurs from Alberta and they also received some attention at SVP this year.

Ornithomimosaurs have long be known as “ostrich mimics” but how true a comparison is this with today’s extant palaeognaths – especially when it comes to the crania? By digitally reconstructing three ornithomimosaur skulls (Garudimimus, Struthiomimus and Ornithomimus), Andrew Cuff and Emily Rayfield, both of the University of Bristol, have produced some interesting results.

By utilising a combination of techniques including virtual muscle attachment data, adding hypothetical beaks to the rostra and studying finite element models and then, by comparing them to both ostrich and other theropods, has indicated that the sutures of the skull were key in minimising pressure to the skull. The beaks too also help in reducing strain and the different morphologies, especially of those beaks that are more extensive, help compensate for the loads.

More derived ornithomimosaurs reveal lesser muscle loads but still strain similarly to more basal forms. Compared on a like for like basis, the skulls of the advanced forms buckle quicker than the more primitive forms. The authors take this as indication that derived ornithomimosaurs may have been changing or varying their diets.

Not surprisingly, the skulls of ornithomimosaurs differ greatly from other theropods – in this study with Allosaurus and Coelophysis. The authors speculate that this may simply be a feature of herbivory evolving in ornithomimosaurs and in other theropods and the repeated evolution of beaks in Theropoda would simply have been a natural progression.

I mentioned earlier the feathered ornithomimosaurs from Alberta and a bone bed from the Upper Cretaceous Horseshoe Canyon Formation in the same province has been providing interesting histological data to Thomas Cullen, of Carleton University in Ottawa, and colleagues. The bone bed, the first of its kind in North America, contained the remains of three individuals and multiple leg elements from these specimens were histologically examined.

As is often the case with histological studies of non avian dinosaurs, indications are that growth was rapid, lines of arrested growth are equispaced and there is fibrolamellar tissue present. At time of death, however, the larger individual reveals growth to be slowing up thus indicating the onset of maturity. Age for the two smaller individuals is estimated to be between two and three years old whilst the older specimen may be approaching four years of age.

Because the lines of arrested growth are constant throughout growth the authors suggest that postcranial long bones may be used as good indication of age – particularly in smaller theropods and, if they are correct in their hypothesis, then we may already have a wealth of information in our collections that enables us to determine growth patterns in different taxa – especially if the overall body size of a specimen can be ascertained.

Now I have mentioned on this blog on more than one or two occasions that the Upper Jurassic Morrison Formation of North America has many surprises still left up its sleeve and that there were many new taxa still to be described. However, this next one may seem surprising since it appears that we have yet another large bodied allosauroid in residence in what already appears to be an ecosystem already well represented in that department.

The specimen was recovered from McElmo Canyon in Montezuma County, Colorado and is late Tithonian in age. There are multiple elements represented including cranial, axial and appendicular and these have been examined by Sebastian Dalman, of Fort Hays State University, and colleagues with particular attention made to the cranial bones which were CT scanned and then digitally reconstructed three dimensionally.

This has enabled the authors to highlight characters in the premaxilla, maxilla and quadratojugal that warrant the erection of a new taxon. This animal is a large allosauroid comparable with other sympatric theropods in the same formation such as Allosaurusand Saurophaganax although, when compared with elements of similar sized Allosaurus specimens, is clearly a more massive animal.

Early phylogenetic analysis indicates the specimen is well placed within Allosauroidea and is closely related to Allosaurus, Fukuiraptor and Neovenator.

Now we are all familiar with the great nest sites of sauropods and oviraptorids but those of other saurischians, particularly large theropods, whilst not unknown, remain relatively rare. The great nest sites of Portugal are an exception and a talk by Octávio Mateus, of the Universidade Nova de Lisboa, and colleagues have brought some amazing insights.

The nest, from the Upper Jurassic Lourinhã Formation in Paimago, Portugal contains the remains of approximately 100 eggs although erosion and the amount of eggshell present indicates the nest held many more. Also of interest is that the nest also contains crocodile eggs but we can only speculate as to their presence here.

There appears to be no clear structure to the nest but the best preserved eggs and embryonic bones are preserved in the centre suggesting that these were the first laid and were at a more advanced ontogenetic stage than those on the exterior of the nest. There are around 300 bones that have been recovered so far and this is despite the fact that the vast majority of the eggs are crushed.

The authors tentatively assign these embryos to Lourinhanosaurus antunesi since, although the bones are very similar to those of this theropod, the autapomorphies required to confirm identification were not preserved. However, there are sufficient differences in the remains to confirm that they are neither Allosaurus, Ceratosaurus nor Avityrannis. Nor does it seem that these eggs were laid by Torvosaurus, since the known nest, from the same formation, that was designated as belonging to this taxon are dissimilar in as much as the egg shell ornamentation is completely different and the embryonic remains are larger.

The authors point out that these embryos are very much fully loaded miniature versions of the adults and appear ready to become active predators from the moment they hatched. One other point of interest is that there are some very small patches of skin preserved in the nest and that these particular baby theropods display no evidence whatsoever of feathers or fuzz – only clearly defined scales – again highlighting the danger of assuming that all theropods were feathered to some degree.

Scott Persons and Phil Currie, both of the University of Alberta, have been continuing their research into the cursorial ability in theropods and this time looked at the relative length of limb bones measured against the enormous variance in overall theropod body size as well as the effects of allometry on limb proportions.

Sixty one different theropod taxa were studied that all had complete hind limbs. The authors confirm that distal limb bones are unaffected by allometry and compensate for this fact. This allowed them to recognise adaptions in limb proportions and, as a result, they were able compare theropods despite the extreme differences in size.

The essential result of this is that there is no evidence that limb proportions in theropods that change throughout ontogeny are different in any way from those that would be predicted by simply observing body size. Highlights of this study include that the relative limb proportions of Nanotyrannus, when compared to other tyrannosaurids including Tyrannosaurus, are so extreme that the authors suggest that Nanotyrannusis indeed a valid taxon (here we go again). There is also evidence that there was continuous adaptivity in improving cursorial ability in many theropod groups including troodontids and tyrannosauroids.

Of particular surprise, however, is that dromaeosaurids actually demonstrate a reduced ability to run which is quite the shock when you consider how we have all assumed them to be amongst the quickest and most agile of all the theropod dinosaurs.

More in Part Two.

References

Cuff, A.R. & Rayfield, E.J. 2012. Functional Mechanics of ornithomimosaur crania compared to other theropods. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 82.

Cullen, T.M., Ryan, M.J., Evans, D.C., Currie, P.J. & Kobayashi, Y. 2012. Multi-element histological analysis of an ornithomimid (Dinosauria) bone bed from the Horseshoe Canyon Formation, Alberta. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 82- 83.

Dalman, S., Paulina Carabajal, A. & Currie, P.J. 2012. A new large-bodied theropod dinosaur from the Upper Morrison Formation (Late Jurassic, Tithonian) of Colorado. . Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 84.

Mateus, O., Carrano, M.T. & Taquet, P. 2012. Osteology of the embryonic theropods from the Late Jurassic of Paimago, Portugal. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 137.

Persons, W.S. & Currie, P.J. 2012. Adaptive cursorial trends among theropod dinosaurs and an attempt to look beyond allometry. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 156 – 157.

2012-11-25 14:41:41

Source: http://saurian.blogspot.com/2012/11/svp-2012-theropods-part-1.html

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