A well-preserved specimen of Stenopterygius quadriscissus (Reptilia, Ichthyosauria) from the Early Jurassic (Toarcian) of Yorkshire, UK and new insights into species delineation in the genus

Despite abundant and productive Toarcian ichthyosaur-bearing localities in continental Europe, in the UK, Toarcian ichthyosaurs may often be fragmentary and / or too poorly preserved for positive identification. Here, we describe a nearly complete ichthyosaur from the Toarcian (Lower Jurassic) Whitby Mudstone Formation at Ravenscar near Whitby, Yorkshire, England. Preliminary identification supported referral to the genus Stenopterygius; however, the hind limb and girdle were not preserved, creating difficulties for species determination.

To better undertake comparisons, we revised Toarcian species of the genus Stenopterygius, resulting in the recognition of S. longipes as a senior synonym of S. uniter and Magnipterygius huenei as a junior synonym of S. quadriscissus. We also identified new metrics pertaining to the caudal skeleton and proximal fore-limb that appear useful for differentiating species when the skull and / or hind-limb is missing or damaged and, thus, applicable to a much greater number of specimens.

The Ravenscar specimen is consistent with S. quadriscissus and represents one of the youngest stratigraphic occurrences of that species and the first definitively confirmed from the Toarcian of Yorkshire and probably the UK. Our revision resulted in a re-appraisal of species-level diversity in Stenopterygius in the Cleveland Basin and throughout the European Toarcian epicontinental basins, with many of the specimens previously identified as S. triscissus now confidently attributable to S. quadriscissus.

These results will facilitate a detailed understanding of vertebrate palaeobiogeography and habitat use in well-studied Jurassic epicontinental basins.