THE TAXONOMIC UTILITY OF FOREFIN MORPHOLOGY IN LOWER JURASSIC ICHTHYOSAURS: PROTOICHTHYOSAURUS AND ICHTHYOSAURUS

The validity of the parvipelvian ichthyosaur genus Protoichthyosaurus was evaluated by examining the type material and recently recognized specimens. Protoichthyosaurus has a wide forefin with at least five primary digits, an anterior digital bifurcation in the forefin, a humerus that is nearly equal in width distally and proximally, and a coracoid with welldeveloped anterior and posterior notches, features it shares with the more common genus Ichthyosaurus. Previously, Protoichthyosaurus was synonymized with Ichthyosaurus because of an incomplete understanding of forefin variation in the latter taxon. However, Protoichthyosaurus has several features in the forefin that distinguishes it from Ichthyosaurus, including the presence of three elements in the distal carpal row, a contact between distal carpal 3 and the ulnare, a bifurcation of distal carpal
2 to form two elements in the metacarpal row, the proximal element of the bifurcation nearly separating distal carpal 2 from distal carpal 3, distal carpal 3 separating the intermedium from contact with distal carpal 4, and the absence of digit V. A phylogenetic analysis confirms that Protoichthyosaurus is the sister taxon of Ichthyosaurus and is nested in Ichthyosauridae. We recognize two species, P. prostaxalis and P. applebyi, sp. nov., but not P. prosostealis. Protoichthyosaurus is unequivocally from the Hettangian but might be present in the Rhaetian to lower Sinemurian. The genus is geographically widespread across the U.K.