Saturday 1 November 2014

A Tethysuchian Crocodyliform from the Early Cretaceous of the Isle of Wight.

The Tethysuchians were a group of largely marine Crocodyliforms that are reasonably well known from the Middle and Late Cretaceous, with one group, the Dyrosauridae surviving the End Cretaceous extinction and surviving till the end of the Eocene. The origins of the group are obscure, but they appear to have diversified before the earliest fossils appear in the fossil record, suggesting that the group are somewhat older.

In a paper published in the Biological Journal of the LinneanSociety on 18 October 2014, Mark Young of the Institute of Evolutionary Biology at the University of Edinburgh and the National Oceanography Centre at the University of Southampton, Lorna Steel of the Department of Earth Sciences at the Natural History Museum in London, Davide Foffa of the School of Earth Sciences at the University of Bristol, Trevor Price Dinosaur Isle Museum, Darren Naish, also of the National Oceanography Centre at the University of Southampton and Jonathan Tennant of the Department of Earth Science andEngineering at Imperial College London describe a partial Tethysuchian Crocodyliform from the Early Cretaceous of the Isle of Wight.

The specimen comprises the anterior part of a right dentary, collected from Shanklin Beach on the Isle of Wight by a Mr Simmons who sold it to the British Museum in 1861; the specimen is now in the collection of the Natural History Museum, following the division of the museum. The specimen was described as the upper jaw of a Pliosaur by Richard Lydekker in 1889, but was recognised as coming from a Crocodyliform by Leslie Noé of the Universidad de los Andes, when visiting the museum in 1999; the pattern of tooth distribution and eruption in Crocodyliforms and Pliosaurs is quite different.

(Top) Anterior region of the right dentary in lateral view: (A) photograph; (B) line drawing. (Middle) Anterior region of the right dentary in ventral view: (A) photograph; (B) line drawing. (Bottom) Anterior region of the right dentary in dorsal view: (A) photograph; (B) line drawing. Young et al. (2014).

Young et al. conclude that the specimen belongs to a Tethysuchian Crocodyliform, but given the fragmentary nature of the specimen do not describe it to species level. The specimen is thought to originate from the Upper Greensand Formation, making it (93.9-100.5 million years years old), though an origin from the Lower Greensand Formation (125-100.5 million years old) cannot be ruled out. The Lower Greensand is predominantly very iron rich at Shanklin, where it is known as the ‘Ferruginous Sand’. This gives it a dark colour, different to the light colour of the matrix rock surrounding the specimen, however the Lower Greensand does contain lenses of lighter, calcium-rich material, and the specimen could come from one of these.

Map of the Isle of Wight showing Shanklin (A), with the geological column of formations exposed there (B). Photographs of the formations exposed near Shanklin, at Luccombe Chine (C), and Knock cliff (D). Young et al. (2014).

Young et al. note that the Shanklin specimen resembles a Drysosaurid, which if correct would make it the earliest member of the group. However the remains are too fragmentary for this to be asserted with any confidence; several other ‘early’ Drosaurid specimens have previously been described (though all post-date the Shanklin specimen), but these are also very fragmentary in nature, so that the origins of this significant group remain obscure.

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