A new Tithonian ophthalmosaurid ichthyosaur from Coahuila in northeastern Mexico

Abstract

Historically, Mexican Jurassic ichthyosaurs have been referred to European representatives of the cosmopolitan family-level clade Ophthalmosauridae. Here, we describe one of the most skeletally complete Mexican ichthyosaur fossils discovered to date. This specimen was recovered from Late Jurassic (Tithonian) deposits of the La Caja Formation exposed near Gómez Farias in the Saltillo municipality of Coahuila, northern Mexico. The unique morphology of the external nasal opening, which is sub-divided by the dorsal process of the maxilla, an anteroposteriorly elongate lacrimal, and the presence of an elongate linear contact between the lacrimal and prefrontal identify these remains as a new genus and species—Parrassaurus yacahuitztli gen. et sp. nov. Other diagnostic character states include an extremely elongate prenarial rostrum, a proportionately large subnarial versus supranarial process on the premaxilla, anterior bordering of the supratemporal fenestra by the frontal, and an articular contact between the humerus and intermedium that separates the ulna and radius. Parrassaurus yacahuitztli represents the second platypterygiine taxon recorded from Mexico to date, and thus demonstrates a greater diversity of North American ophthalmosaurids than has been previously suspected. Jair Israel Barrientos-Lara* [j4ir@me.com] and Jesús Alvarado-Ortega [alvarado@geologia.unam.mx], Instituto de Geología, Universidad Nacional Autónoma de México, Circuito de la Investigación S/N, Ciudad Universitaria, Delegación Coyoacán, Ciudad de México, 04510 México.