Wednesday, 12 January 2011

Tail Fins for Plesiosaurs

Photo by LadyofHats

One of the most interesting presentations at SVP last year was Benjamin Wilhelms’ talk on osteological evidence that supports the existence of a tail fin in cryptocleidoid plesiosaurs. I often wondered if they had a tail fin or a similar fluke since, on the face of it, it seemed that evolution was missing a trick here, especially if cryptocleidoids were fairly active hunters and needed a boost of speed to intercept their prey.

The hypothesis that plesiosaurs had tail fins is not new. Wilhelm points out that Richard Owen in 1865 suggested that plesiosaurs may have had a tail fin due to lateral compression in the last ten caudal vertebrae of one specimen. I was particularly intrigued by the specimen referred to Dames (1895) as the holotype of Seeleysaurus guilelmiimperatoris, which clearly showed a dark outline in the caudal region attributable to a tail fin or similar, although this cannot be confirmed.

Working in the Oxford Clay, it occurred to me that I had never heard of any skin, fin or body impressions found with the multitude of specimens of marine reptile that had been recovered, although I’m sure there must have been examples. Fish, on the other hand, I know of a few examples and, the most recent, was a large (two metres) Asthenocormus sp. from Quarry 4 which was exceptional due to preservation of skin and fin impressions and a wonderfully preserved outline of the tail

Comparing two recently recovered specimens of plesiosaur, tentatively referred to Pantosaurus striatus from the Sundance Formation in Wyoming, with specimens of Cryptoclidus oxoniensis and Muraenosaurus leedsi, both from the Oxford Clay in England, Wilhelm has unearthed some pretty good comparative evidence that at least cryptocleidoid plesiosaurs did possess a tail fin.

These include variable morphology of the neural spines in some caudal vertebrae including increased height and distal ends which demonstrate increased length and that are more flared to increase attachment for cartilage which would have provided strength and stiffness to a tail fin.

Fascinatingly, some neural spines change direction and increase articulation which provides even more surface area for a larger cartilage attachment which tends to be situated below what appears to be the tallest part of the fin and would, again, have provided strength and support.

Toward the distal end of the tail, the caudal vertebrae gradually become laterally compressed as the centra reduce in width, as do the caudal ribs. This is also thought to demonstrate the presence of a tail fin.

Comparisons with extant taxa such as cetaceans, scombroid fishes and lamnid sharks suggest the tail fin could produce thrust, downward force and some ruddering capability for the plesiosaur and was almost certainly capable of independant movement.

So it does seem likely that cryptocleidoid plesiosaurs did have a tail fin. Whether this applies to all plesiosaurs is uncertain and this work does have some ramifications, particularly for elasmosaurids and, as Wilhelm points out, there is still much more research to do.

This is really solid work and I’m very impressed by it. If you want to learn more and to get the in depth detail, a copy of Wilhelm’s work is freely available here.

References

Dames, W. 1895. Die plesiosaurier der Süddeutschen Liasformation. Abhandlungen der Königlich Preussischen Akademie der Wissenschaften zu Berlin 1895:1–81.

Owen, R. 1865. A monograph on the fossil Reptilia of the Liassic Formations. Part 3. Sauropterygia. Monograph of the Palaeontographical Society 17: 1-40.

Wilhelm, B.C. 2010. Novel anatomy of cryptoclidid plesiosaurs with comments on axial locomotion. Ph.D thesis, Marshall University, Huntington, WV. USA

Wilhelm, B.C., O’Keefe, F. 2010. A new partial skeleton of Pantosaurus striatus, a cryptocleidoid Plesiosaur from the Upper Jurassic Sundance Formation of Wyoming. Journal of Vertebrate Paleontology, 30:1736–1742.


Reproduced from Wilhelm 2010.


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