A previously unrecognized specimen of Protoichthyosaurus prostaxalis, LEICT G142.1991, from the Lower Jurassic of Barrow-upon-Soar, Leicestershire, UK, includes an almost complete three-dimensional skull that provides new information on the configuration of the skull roof. The position of the pineal foramen (between the frontals and the parietals) and an elongated internasal foramen in a depression along the midline of the nasals are clearly shown. The maxilla makes up a significant portion of the external naris ventral margin, an unusual character for the genus/species. This reflects intraspecific variation, not evidence of a new taxon. The specimen enables comparisons of skull roof morphology with Ichthyosaurus and Stenopterygius, two common Early Jurassic taxa. In particular, the position of the pineal foramen is similar to Stenopterygius, but distinguishes Protoichthyosaurus from Ichthyosaurus. The lack of a frontal–prefrontal contact and the posteriorly wide nasals distinguishes Protoichthyosaurus from Stenopterygius. We also present a revised reconstruction of the skull roof morphology of Ichthyosaurus. Three additional specimens of Protoichthyosaurus are referred to the genus: another partial skull, referred to P. prostaxalis, and two isolated forefins, identified by their unique morphology.

Ichthyosaurs, a lineage of extinct Mesozoic marine reptiles, have garnered attention in both the palaeontological and developmental literature for the unique limb morphology seen in derived genera. These morphologies include an increase in the number of phalanges per digit (hyperphalangy) and in the number of digits (hyperdactyly), but most interestingly also a shift in element identity. Elements distal to the stylopodium acquire characteristics of mesopodial elements, such as a rounded, nodular shape and a loss of perichondral bone on the anterior and posterior surfaces. Here, we examine numerous aspects of the loss of proximodistal identity in ichthyosaur limbs including phylogenetic progression of the loss of perichondral bone, histology and microstructure of the elements retaining perichondral bone in derived taxa, and correlates of intraspecific variation in degree of perichondral bone reduction in a derived ichthyosaur, Stenopterygius quadriscissus. Results show that loss of limb element identity occurred progressively over ichthyosaurian evolution, and the notches seen on the anterior surface of limb elements in derived ichthyosaurs are homologous to the long bone shafts in terrestrial tetrapods. Variation in the number of notches in S. quadriscissus can best be explained through delayed ossification of the anterior perichondrium, indicating a heterochronic component to the loss of identity. We propose a developmental mechanism – gradual expansion of the polyalanine region of HoxD13 over ichthyosaurian evolution – to explain the progressive loss of limb regionalization as well as the heterochronic delay in perichondral ossification.