What with yesterday’s Simolestes picture-of-the-day article I couldn’t resist but using – at last – this picture. It was taken by Mark Witton at the Oxford University Museum and depicts the immense pliosaur mandible OUM J.10454, a specimen that comes from the Kimmeridge Clay of Cumnor, Oxfordshire, and was acquired by the museum some time between 1880 and 1888. William J. Sollas (1849-1936) had intended to describe the specimen in the year that he died, but its proper debut in the literature didn’t occur until 1959. The lovely lady posing next to the specimen is Claire, Mark’s sister (at left, you can see the flippers of the Nessie model): for Mark’s original go here.
The Cumnor mandible was first assigned to the pliosaurid species Stretosaurus macromerus by L. B. Tarlo (1959), a taxon which Tarlo regarded as representing a distinct genus because he thought that it had a unique scapula morphology. However, the ‘unique scapula’ later turned out to be a not-so-unique ilium and L. B. Halstead – who is the exact same person as L. B. Tarlo you understand – then sunk Stretosaurus into Liopleurodon (Halstead 1989). Noè et al. (2004) showed that the referral of mandibles like, and including, the Cumnor specimen to Liopleurodon is incorrect, and that all the material referred to the species macromerus should instead be regarded as part of Pliosaurus, a taxon which has a longer mandibular symphysis than Liopleurodon. The actual story is far more complex than this, and even after all this we still can’t be sure that the Cumnor specimen really belongs to P. macromerus as Tarlo/Halstead thought.
Anyway, the big deal about the specimen is how big it is: it’s 2.8 m long and, when complete, would almost certainly have exceeded 3 m. Tarlo (1959) wrote of this specimen that ‘without doubt it belongs to the largest pliosaur ever recorded, somewhat exceeding the size of the Cretaceous Kronosaurus‘ (p. 51): a consequence of this assertion is that P. macromerus has at times been stated to have been the largest marine reptile of all time (e.g., McWhirter & McWhirter, 1974) [it's since been outclassed in that regard by an ichthyosaur: the
21 27 23 m long Shonisaurus sikanniensis]. Near-complete pliosaur skeletons indicate that the skull was about 17% of total body length, suggesting that the whole animal might have approached 18 m. As Naish et al. (2001) said: ‘The caveat, however, is that this assumes that this skull-to-total-length ratio remains constant throughout growth, and it may well not do’. And, yes, it’s specimens like this that led to the totally speculative 25 m given for Liopleurodon in the BBC’s Walking With Dinosaurs. Dave Martill and I do have some new data on giant size in Jurassic pliosaurs, and I suppose one day we’ll get round to publishing it. To give you some idea how big some of the largest pliosaurs might have been, here’s another image I stole from Mark’s flickr site (from here): it shows a reconstruction of one of the paddles of a new giant pliosaurid from the Upper Jurassic of Mexico (see Buchy et al. 2003) as displayed at the Staatliches Museum für Naturkunde in Karlsruhe. Richard Hing acts as scalebar.
If you want to know more about the Kimmeridge Clay, there’s a ver 1 article on some of its fauna here.
Refs – –
Buchy, M-C., Frey, E., Stinnesbeck, W. & López-Oliva, J. G. 2003. First occurrence of a gigantic pliosaurid plesiosaur in the late Jurassic (Kimmeridgian) of Mexico. Bulletin de Societe géologique de France 174, 271-278.
Halstead, L. B. 1989. Plesiosaur locomotion. Journal of the Geological Society, London 146, 37-40.
McWhirter, N. & McWhirter, R. 1974. Guiness Book of Records. Guiness Superlatives Ltd., London.
Naish, D., Noè, L. F. & Martill, D. M. 2001. Giant pliosaurs and the mysterious ‘Megapleurodon’. Dino Press 4, 98-103.
Noè, L. F., Smith, D. T. J. & Walton, D. I. 2004. A new species of Kimmeridgian pliosaur (Reptilia; Sauropterygia) and its bearing on the nomenclature of Liopleurodon macromerus. Proceedings of the Geologists’ Association 115, 13-24.
Tarlo, L. B. 1957. The scapula of Pliosaurus macromerus Phillips. Palaeontology 1, 193-199.
– . 1959. Stretosaurus gen. nov., a giant pliosaur from the Kimeridge Clay. Palaeontology 2, 39-55.