Sunday 16 March 2014

A Leptocleidid Plesiosaur from the Early Cretaceous of the Isle of Wight.

The Plesiosaurs were Mesozoic marine reptiles that arose in the Late Triassic or Early Jurassic and persisted till the Late Cretaceous. They were related to modern lizards and snakes, but were fully aquatic, the largest species reaching 15 m in length. All species seem to have been strict carnivores, and at least one species gave birth to live young. Plesiosaurs are thought to have been fully aquatic from their first appearance; they are thought to have evolved from Nothosaurs, a group of semi-aquatic marine Reptiles. The term Sauropterygians is used to include the Nothosaurs and the Pliosaurs, plus the Pachypleurosaurs; Lizard-like aquatic Reptiles. The Sauropterygians are considered to be the sister group of the Squamates - Snakes and Lizards.

In a paper published in the Journal of Systematic Palaeontology on 11 December 2012, Roger Benson of the Department of Earth Sciences at the University of Cambridge, Hilary Ketchum of the University Museum of Zoology at the University of Cambridge, Darren Naish of the School of Earth & Environmental Sciences at the University of Portsmouth and the National Oceanography Centre at the University of Southampton and Langan Turner, also of the School of Earth & Environmental Sciences at the University of Portsmouth, describe a new species of Plesiosaur from the Early Cretaceous the Shepherds Chine Member of the Vectis Formation at Shepherds Chine, in Brighstone Bay on the south-west coast of the Isle of Wight.

The specimen is considered to be a member of the Leptocleidia, a poorly understood group of Cretaceous Plesiosaurs, of somewhat uncertain affinities. It is named Vectocleidus pastorum, where ‘Vectocleidus’ derives from a combination of Vectis, the Latin name for the Isle of Wight, and Leptocleidus, the first described species of the Leptocleidia, and ‘pastorum’ is Latin for ‘Shepherds’, a reference to the location where the specimen was found.

The specimen comprises three blocks, recovered separately by different collectors, upon which a partial post-cranial skeleton is preserved.

Vectocleidus pastorum; blocks one and two). In line drawing (B), grey shading indicates matrix. Abbreviations: caud. rib, caudal rib; cerv, cervical vertebra; clav, clavicle; cor, coracoid; dors, dorsal vertebra; hum, humerus; l., left; na, neural arch; pect, ‘pectoral’ (anterior dorsal) vertebra; phal, phalanx; r., right; scap, right scapula. Scale bars (in A) 100 mm. Benson et al. (2012).

The phylogenic position of the Leptocleidia is somewhat uncertain, as most known specimens are rather fragmentary; they are often considered to be Pliosaurs related to the Jurassic Rhomaleosaurids, or highly derived Plesiosaurs related to the Cretaceous Polycotylids. While Vectocleidus pastorum is known only from a single, incomplete, specimen, it is well enough preserved to exhibit clear Leptocleid affinities, as well as several features associated with highly derived Plesiosaurs. For this reason Benson et al. suggest that it supports the theory that the Leptocleids have a close affinity with the Polycotylids.

The suggested phylogenic affinities of the Leptocleids. Benson et al. (2012).

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