A taxonomic reassessment of Ichthyosaurus communis and I. intermedius and a revised diagnosis for the genus

Abstract

Ichthyosaurus communis De la Beche & Conybeare, 1821 and I. intermedius Conybeare, 1822 have been considered synonymous by some researchers, but distinct species by others. The distinction between the two species was originally based on tooth morphology, which has been shown to vary ontogenetically as well as within an individual. Subsequent literature of the nineteenth century did not propose adequate diagnostic features to distinguish the two species. In addition, illustrations suggest that both species were confused with I. breviceps, which was defined over 50 years later. The type specimens of both species are missing, making the problem unresolvable. However, a neotype was designated for I. communis to retain the widely used species name and to stabilize the taxonomy. The species can be recognized by a symmetric, triangular maxilla with long processes and a large, broad, triradiate lacrimal, as well as a unique combination of other characters. The neotype shares numerous features with the I. intermedius ‘type figure’, including the morphologies of the maxilla, prefrontal, lacrimal, jugal and postorbital, such that the two are nearly indistinguishable. Thus I. intermedius must be considered a synonym of I. communis. Ichthyosaurus communis can be identified from the Lyme Regis-Charmouth coast, west Dorset; the ?Whitby coast of Yorkshire; and probably from Street, Somerset. The stratigraphical range of I. communis is at least lower Hettangian to lower Sinemurian.