Reassessment of ‘plesiosaurus’ megacephalus (Sauropterygia: Plesiosauria) from the triassic-jurassic boundary, UK

Abstract

The holotype specimen of ‘Plesiosaurus’ megacephalus Stutchbury, 1846 (BRSMG Cb 2335) was destroyed in an air raid on Bristol during WWII. The specimen comprised a complete skeleton from the Triassic-Jurassic boundary of Street-on-the- Fosse, Somerset, UK. Plaster casts of the skull and right forelimb produced prior to its destruction, in conjunction with historical photographs and original descriptions, provide three-dimensional anatomical data for a modern systematic diagnosis of the taxon. A previously proposed neotype specimen (LEICS G221.1851) is therefore regarded as a referred specimen. Detailed comparison with other Lower Jurassic plesiosaurians demonstrates that ‘Plesiosaurus’ megacephalus is a distinct taxon, separate from Rhomaleosaurus sensu stricto and Eurycleidus arcuatus, and so a new genus is erected to accommodate it: Atychodracon gen. nov. The clarification and description of Atychodracon megacephalus is a vital step towards understanding the diversity and resolving the systematics of large-bodied plesiosaurians from the Triassic- Jurassic boundary.