The Weymouth Bay pliosaur
The purchase of the skull of an enormous pliosaur from the Kimmeridge Clay of Weymouth Bay by the Lottery Heritage Fund was formally announced on 27th October 2009. The purchase came after a long and complicated series of negotiations, and when the specimen is prepared it will form a major new display in the Dorset County Museum in Dorchester.
This is an impressive specimen, even in its unprepared state. It is huge - the length of the skull from the tip of the snout to the back of the parietal ridge is 2.1 meters. Scaling from the lateral reconstruction of Pliosaurus by Taylor and Cruickshank (1993) this gives an overall lenght of about 2.4 m. By any standards, this is an enormous animal.
I have had a few hours to study the specimen, and any conclusions reached at this stage are tentative. However, it looks most likely that this is of the genus Pliosaurus. The overall proportions of the skull look about right, the teeth are trihedral in section, flat and smooth on one side and heavily ridged on the two others sides. The lower jaw symphysis is unfortunately missing, but matching the jaw to the skull suggests that it was fairly elongate. The specimen appears to be significantly more robust than either of the Westbury specimens of Pliosaurus brachyspondylus (Taylor & Cruickshank 1993, Grange et. al. 1996). As both are crushed this may be an artifact of preservation although the depth of the lower jaw in particular suggests that this is a real difference, possibly due to the larger size and ontogentically older age of the new specimen. The palatines appear to be much more robust than those of Pliosaurus brachyspondylus. This may also be an ontogentic or preservational difference. Until the specimen is fully prepared it's taxonomic status will be uncertain.
The post-cranial anatomy of large pliosaurs in general and Pliosaurus in particular is not known reliably, so any estimates of the overall length of the animal need to be treated with some caution. Tarlo (1959) reconstructed the large Callovian pliosaur Liopleurodon with an overall length seven times that of the skull. This would give an overall length of 16.8 m. However, specimens of geologically rather younger pliosaurs preserved in articulation (Acosta et. al. 1979) suggest that the overall length was closer to five times the skull length, giving an overall length of 12m. Until we have a better knowledge of pliosaur post-cranial anatomy, any estimates of length are rather speculative.
Estimating the weight of such an animal is even more problematic and depends on many assumptions. Even with a full skeletal reconstruction, soft-tissue reconstruction is largely a matter of assumptions. A very rough estimate can be made by measuring the displacement of pliosaur models and assuming a density slighly higher than that of water. This gives a weight of between 7 and 15 tons.
Serious research into this important specimen will only be possible when preparation is under way. Watch this space for news!
References
- Taylor, Michael Alan and Cruickshank, Arthur R I; 1993; Cranial anatomy and functional morphology of Pliosaurus brachyspondylus (Reptilia: Plesiosauria) from the Late Jurassic of Westbury, Wiltshire; Philosophical Transactions of the Royal Society of London; 341 pp.399-418.
- Grange, D. R., Storrs, Glenn W., Carpenter, S. and Etches, Steve; 1996; An important marine vertebrate-bearing locality from the Lower Kimmeridge Clay (Upper Jurassic) of Westbury, Wiltshire; Proceedings of the Geologists Association, London; 107 pp.107-116
- Tarlo, L. B. (1959); Stretosaurus gen. nov., a giant pliosaur from the Kimeridge Clay
- Acosta-A., Carlos Eduardo, Huertas-G., Gustavo and Ruiz-C., Pedro M.; 1979; Noticia Preliminar Sobre el Hallazgo de un Presunto Kronosaurus (Reptilia: Dolychorynchopidae) en el Aptiano Superior de Villa de Levia, Columbia; Lozania (Acta Zoologica Columbiana); 28 pp.1-10