Sunday, 4 December 2011

SVP 2011 - And Finally.....


This is the last of my SVP 2011 reports and I’m very grateful for the positive feedback that I’ve received from so many of you. This last batch of entries represents research across a variety of disciplines but are all equally interesting.

It appears to me that ichthyosaurs are undergoing something of resurgence in popularity and those of the Cretaceous are proving to be not quite as elusive as they were once thought to be. Valentin Fischer has been actively studying these fascinating animals and following on from his recent paper in the Journal of Paleontology, where he announced the new taxonSveltonectes insolitus, he presented more data detailing that ichthyosaurs were still very diverse throughout the Early Cretaceous.
In particular ophthalmosaurids continued to be a very successful group right through the Early Cretaceous. It was initially thought that they had rapidly declined after their initial radiation during the Middle Jurassic but new fossils from Europe and Russia (including Sveltonectes) clearly dispel this theory. It is now apparent that typical Late Jurassic ophthalmosaurs shared the seas of the Eurasian archipelago with typically more derived ichthyosaurs from the same clade such as Platypterygius and that these taxa also appear to be well adapted to fill different niches in the ecosystem.
Staying with marine reptiles, only this time plesiosaurs, and some interesting morphological study of the plesiosaurian body shape by Courtney Richards et al. Comparative studies of three cryptoclidid plesiosaurs– Tatenectes, Cryptoclidus and Muraenosaurus – were performed using a combination of measurements and photographs to correctly position the various bones, such as the vertebrae and ribs etc, which enabled the accurate structural depth and width shape to be attained.

A.Muraenosaurus B. Cryptoclidus
C. Tatenectes (from O'Keefe et al 2011)
To supplement this detail, the vertebrae were also analysed to see if the morphology of the centrum is relevant to the curvature of the spine. Those vertebrae that are most rhomboidal correlate with those areas of the spine that curve the most while those that are less rhomboidal correlate with sections of the spine that are flatter and/or curve less.
Running this data through the computer and subjecting the various structures to lateral rolling suggests that the shallow draft plesiosaurs such as Tatenectes and Cryptoclidus were well suited to shallow waters while those with a deeper draft, like Muraenosaurus, were far more adapted to an open ocean deep water existence.
This is interesting because of its relevance to the Oxford Clay Formation, which has featured heavily in this blog throughout. The sea where the Oxford Clay was deposited was both warm and shallow and conditions were such that vertebrate remains fossilised well and are relatively abundant. Muraenosaurus is a very common genus from these deposits and this does not tie in with the above findings so it will be interesting when this data is formally published. I wonder what the author’s interpretation of deep water is in relation to the Oxford Clay Sea? 

One of the symposia this year featured the evolution and ecology of terrestrial ecosystems of Campanian Laramidia. Currently, research in this particular field is incredibly popular and there is enough data around to fill a series of volumes on its own. There are a couple of presentations that I have opted to highlight and one was presented by Mark Loewen et al and discussed the evolution of theropods and how it was affected by geographical boundaries.
In recent years there have been more fossil discoveries from southern Laramidia to complement the already heavily sampled northern fossil fields. Because theropods are less likely to be affected by climatic differences and any change in flora, they were chosen to see if they provide evidence of faunal endemism. Despite the fact that all major groups of theropods are represented from north to south there is no doubt that there were different species represented in different isolated endemic faunas. Tyrannosaurids, as an example, were used to explain that, although they were all large predators that hunted, more or less, the same prey, they still evolved into distinct clades.
Add to this the fact that all these animals all display patterns of diversification and dispersal that are characteristic of those that exist in isolation. These findings seem to confirm what has been suspected for some time now, namely that there was some form of geophysical dividing barrier between north and south Laramidia restricting any obvious faunal exchange. However, just what this barrier was still has to be determined.
From Sampson et al 2010
The second presentation came from, more or less, the same research team – this time with Scott Sampson as lead author. At the time that the Western Interior Seaway split the North American continent into two, Laramidia was isolated for around 25 million years. It has always been assumed, more often than not, that because Laramidia and the corresponding formations of Asia display the same clades of dinosaur, the origins of these clades in North America are as a result of a dispersal event from Asia.
Timing of such an event has been problematic since the Asian formations still have to be satisfactorily radioisotopically dated and now new fossil evidence from the Campanian rocks of the Wahweap and Kaiparowits Formations of southern Utah throws the original dispersal theory into doubt. Specimens of both tyrannosaurid and ceratopsids have been recovered that have been established as the earliest representatives of their kind and this, in addition to stratigraphic, phylogenetic and biogeographic analysis, suggests that it was, indeed, the landmass of Laramidia that was probably responsible for the origins of many taxa in Asia.
The authors suggest that this is unusual since most dispersal events are usually biased in as much that the bigger landmass normally provides the greater faunal exchange and this is particularly true of Cenozoic events. However, Laramidia was a comparatively small landmass compared to the Asian continent and this stands out as being a truly unusual event. There are so many unanswered questions regarding Laramidia that it will continue to attract increased attention and research.
Another symposia this year was devoted to vertebrate diversity patterns and sampling bias and Matthew Carrano and Matthew Oreska, both of the Smithsonian Institute, demonstrated the importance of vertebrate microfossil bone beds. I’m a big fan of these extremely interesting fossil hot spots and the data they provide and, if you have never looked at microfossil sites, then you are missing out – I recommend Julia Sankeys’ work if you are looking for somewhere to start.
The Cloverly Formation of Wyoming and Montana has been worked for around 75 years now and since collecting techniques tend to concentrate on macrovertebrate remains it appears that the overall diversity of the this formation has been overlooked. Reassessment and analysis of the microfossil sites have now added considerably to our knowledge of the animals that once lived in this area during the Early Cretaceous.
The authors report that freshwater sharks, crocodilians and mammals have all been identified with, as is the norm with this kind of analysis, bony fish and crocodilians dominant. But there are significant remains of dinosaurs, amphibians and turtles as well and even after taking proportional representation and other sorting biases into consideration, the results stand up to scrutiny. This is reminiscent of later Hell Creek microsites and is indicative of a wet, aquatic and semi-aquatic environment.
Microfossil bone beds continue to add to our understanding of many palaeoenvironments and, provided the sampling technique is consistent and of sufficient quantity, they are very effective indicators of the different kinds of vertebrate taxa that can be identified in various formations throughout the world.
Now you may remember my teaser image of a rhamphorynchid pterosaur a little while back which will be featuring in my very next blog post and S. Christopher Bennett, of Fort Hays University, has suggested that Rhamphorynchus muensteri may have actually displayed a cranial crest. Although crests for non-pterodactyloid pterosaurs are known, no specimen of Rhamphorynchus has been found with structures preserved that are indicative of a sagittal cranial crest– until now.
A large adult specimen appears to display a series of features on the skull roof that may have formed the base of a soft tissue crest that covers an area measuring about 25% of the skull. Interestingly the author suggests that the majority of R. muensteri fossils are juveniles since there are over 100 specimens in depositories all over the world and not one fossil displays the same features or, indeed, evidence of a crest. 
Image by Ryan Somma
This is a very strange and problematic. Preservation of these pterosaurs from the fine grained limestones of such places as Solenhofen is spectacular and specimens of both invertebrates and vertebrates often display the fleshy outlines of wings and body shapes and yet there is not one fossil of R. muensteri that displays any signs of a crest. Even if all the specimens were juveniles, surely there would be one specimen that was at least starting to develop a sagittal crest?
This lack of evidence, for me, is troubling but then the author may correct in his assertion that all the specimens are indeed juvenile. But what would cause such a preservational bias? I’m not too sure but it will be interesting to read the paper on this one and I will look forward to its publication in the future.
And now for my final piece from this year’s SVP and features the ankylosaur Pinacosaurus. Victoria Arbour (of Pseudoplocephalus fame) and Phil Currie, both from the University of Alberta, report on bone beds in Bayan Mandahu, China and Alag Teeg in Mongolia and describe how multiple skeletons that were more or less aligned in close proximity and preserved upright in a miring situation may be indicative of a social group.
Both sites mainly contain the remains of juveniles but at Alag Teeg there is also one large specimen that is certainly an adult. The preservation of the remains is good and taphonomic analysis suggests that this was not groups of animals perhaps gathering together around a dwindling pool in a drought situation. The remains in drought assemblages tend to have their remains scattered and are often badly trampled.
The authors suggest that the ankylosaurs at both bone beds were probably mired in a fairly short space of time by sandstorms or even an alluvial fan. It is possible that the juveniles were accompanied by an adult or adult animals to provide some protection from predators since their osteoderms were yet to develop as well as their tail clubs, which had only partially developed. This scenario suggests that the adult animal(s) were able to pull themselves out of the sand whilst the juveniles could not although obviously, at Alag Teeg, one adult did not escape. It will be interesting when other bone beds are found to see if further evidence and analysis can also suggest the possibility of a social structure in ankylosaurs.

References

Arbour, V. and Currie, P. 2011.Taphonomic filters of age groups on the ankylosaurid dinosaur Pinacosaurus. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp64.
Bennett, S.C. 2011. First evidence of a cranial crest in the pterosaur Rhamphorynchus muensteri. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp69.
Carrano, M. and Oreska, M. 2011. The importance of vertebrate microfossil bonebeds in understanding the fossil record: examples from the Cloverly Formation. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp84.
Valentin Fischer, Edwige Masure, Maxim S. Arkhangelsky & Pascal Godefroit (2011): A new Barremian (Early Cretaceous) ichthyosaur from western Russia, Journal of Vertebrate Paleontology, 31:5, 1010-1025.
Fischer, V. 2011. New ophthalmosaurids from Europe and Russia broaden the biodiversity of Early Cretaceous ichthyosaurs. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp110.
Loewen, M., Zanno, L., Irmis, R., Sertich, J. and Sampson, S. 2011. Campanian theropod evolution and intracontinental endemism on Laramidia. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp146.
O'Keefe, F. Robin, Street, Hallie P., Wilhelm, Benjamin C., Richards, Courtney D. and Zhu, Helen (2011) . A new skeleton of the cryptoclidid plesiosaur Tatenectes laramiensis reveals a novel body shape among plesiosaurs. Journal of Vertebrate Paleontology 31:2, 330-339.
Richards, C., O’Keefe, R. and Henderson, D. 2011. Plesiosaur body shape and its impact on stability. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp178.
Sampson SD, Loewen MA, Farke AA, Roberts EM, Forster CA, et al. (2010) New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism. PLoS ONE 5(9): e12292. doi:10.1371/journal.pone.0012292

Sampson, S., Loewen, M., Irmis, R., Sertich, J. and Evans, D. 2011. Laurasian faunal interchange in the Late Cretaceous: the out of Laramidia hypothesis. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp185.



2 comments:

Henrique Niza said...

Thanks for these posts. They're really helpful for those of us that can't attend to the SVP.

Mark Wildman said...

It's a pleasure Henrique - thanks for the comment!

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