Sunday, 13 November 2011

SVP 2011 - Lungs, Legs & Rapid Growth

Contributions on dinosaur physiology were well represented at SVP and here are a few that I found interesting. How dinosaurs managed to regulate their body temperature has been debated ad infinitum and this will no doubt continue to be so and, regardless of whether they were endothermic or not, the problem of overheating and how they managed to avoid it has proven difficult to explain.

William Porter and Larry Witmer of Ohio University hypothesised that dinosaurs must have used a form of vascular physiology as a method for heat control. Using a combination of CT scanning and the Extant Phylogenetic Bracket whilst studying dinosaur fossils, and then running the data in a 3D environment, has provided some interesting preliminary results. By comparing these results with similar studies of extant animals, such as birds and reptiles, a vascular anatomical profile of three dinosaurs (Majungasaurus, Edmontosaurus and Diplodocus) was established.

In simple terms, similar osteological correlates were identified in these dinosaurs that are present in the extant taxa examined. This study is at an early stage but further investigatitive techniques, new methodologies and expanding the results by utilising more dinosaurian taxa may provide a more robust and demonstrable dataset.

Today’s birds and crocodiles are in possession of lungs that allow unidirectional airflow but what are the origins of this physiological adaption in archosaurs and is there evidence for it in Triassic and Jurassic archosaurs? Physical evidence of postcranial skeletal pneumaticity (PSP) does not fossilize but traces remain and can suggest the extent of any air sacs that were present which, in turn, may indicate an avian-like respiratory system. Paul Barrett et al used micro-CT scanning on multiple archosaur specimens from the Triassic and Jurassic to search for evidence of PSP - and the results were encouraging.

It seems likely that non-avian saurischians and pterosaurs did indeed possess advanced respiratory systems but the results are less conclusive for ornithischians and other archosaurs. In these specimens there are other features, however, that may be indicative of a somewhat less advanced form of respiratory system and the authors suggest this may also be indicative of an earlier origin for the system, not only in dinosaurs, but throughout the entire archosaurian lineage. Interestingly, it seems, like so many other evolutionary convergences, that different groups of archosaur evolved their own form of respiratory systems and these, in turn, were probably variations on a theme that developed unidirectional airflow, as well as specialised air sacs.

A quick mention now about Heinrich Mallison’s contribution that looks again at how dinosaur speed is calculated and discusses how our original perception of speed that was calculated by using Alexander’s froude number, is outdated and is compromised by far too many inconsistencies. Instead, Mallison uses modern digital models combining SIMM and computer aided design to reach some interesting conclusions. Already discussed at national level at Nature, Mallison is currently explaining his work at his own blog, dinosaurpaleo, and I strongly advise you to visit – great stuff.

Of course, one of the pre-requisites in working out speed and locomotion in dinosaurs is being sure that your limb posture is correct in the first place but things are seldom that easy in vertebrate palaeontology. Forelimb posture in quadrapedal dinosaurs continues to stimulate debate with ceratopsians often at the forefront of discussion but the problems are much broader than that. Where and how do you start to sort this problem out?

Well you need radii, lots of them, 380 in fact and then you have to perform a painstaking morphological study and quantify them before comparisons can be made – and this is what Collin Vanburen and Matt Bonnan of Western Illinois University have done. The sample included radii from mammals and sauropsids, as well as dinosaurs, and this provided the ability to test the theory that, if dinosaurs had an erect limb posture similar to mammals, then it should follow that the shape of the radius would be similar as well.

However, the results suggest that dinosaurs and mammals are not alike at all – in fact they are actually more similar to non-erect sauropsids. And ceratopsians were hugely different from mammals, which substantially challenges the suggestion that they were able to run and charge like extant rhinos. The radii of sauropodomorphs, typically of the group, resemble neither radii from both mammals and sauropsids and have their own distinct morphology which suggests that they may have held their forelimbs erect but in a very specialised manner.

The authors conclude by stating that it seems likely that the majority of quadrapedal dinosaurs held their forelimbs in a non-erect posture and that it is best not use the mammal analogy as a basis for further study, whether assessing dinosaurian speed, gait or posture.

The study of bone histology was at the very forefront of the dinosaur revolution back in the seventies and continues to yield yet more data in support of active, high metabolic dinosaurs. Some of the latest information is provided by Koen Stein and Martin Sander of the Steinmann Institut fr Geologie in Bonn, Germany and concerns different cellular density in fibrolamellar bone.

Cellular density is measured by studying something known as osteocytes and these exhibit distinct patterns and morphologies that allow them to be quantified. By comparing the osteocytes of mammals and saurischians dinosaurs, including both sauropods and large theropods, it was possible to compare the two and then expand the analysis by also including extant poikilotherm ectotherms.

Saurischian dinosaurs have a much higher osteocyte density than mammals of comparable size and, likewise, mammals have a much greater density than ectotherms. Since mammals are high metabolic endotherms then the study suggests that dinosaurs had different ways of building fibrolamellar bone. High osteocyte density that is so much greater than ectotherms also provides powerful evidence for sustained active, high metabolic saurischians dinosaurs.

I have merely skimmed the detail of the top of this fascinating work and it provides the foundation for further studies of osteocyte density within the greater framework that is bone histology and physiology. And, of course, yet more evidence that supports the already huge amount of data that tells us that dinosaur were fast growing, high metabolic animals.

References

Barrett, A., Butler, R., Gower, D. And Abel, R. 2011. Postcranial Skeletal Pneumaticity and the Evolution of Archosaur Respiratory Systems. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp66.

Mallison, H. 2011. Fast Moving Dinosaurs: Why Our Basic Tenet is Wrong. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp150.

Porter, W. and Witmer, L. 2011. Vascular Anatomy and it Physiological Implications in Extant and Extinct Dinosaurs and Other Diapsids. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp176.

Stein, K. And Sander, P.M. 2011. Osteocyte Lacuna Density in Saurischian Dinosaurs and the Convergence of Fibrolamellar Bone in Mammals and Dinosaurs: Different Strategies to Grow Fast. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp199.

Vanburen, C. and Bonnan, M. 2011. Quantifying the Posture of Quadrapedal Dinosaurs: A Morphometric Approach. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp208.

1 comments:

Heinrich Mallison said...

cool - the dread word "Froude" :)

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