Abstract

Thunnosaurian ichthyosaurs were one of the most successful lineages of Mesozoic tetrapods in terms of adaptation to a pelagic lifestyle. Among them, the ophthalmosaurids are of particular interest. Their fossils document an early widespread distribution, including high northern and southern latitudes soon after they appear at the Aalenian–Bajocian boundary, becoming almost the only surviving lineage of ichthyosaurs for approximately 76 million years until their extinction at the end of the Cenomanian (Late Cretaceous). Evolutionary innovations that could have been involved in their remarkable evolutionary success are still poorly understood. Two regions of the skeleton, involved in two vital functions – breathing and swimming – depict notable variation among ophthalmosaurids: the narial region and the proximal forefin configuration (i.e., stylopodium-zeugopodium-mesopodium pattern). Here, we describe Catutosaurus gaspariniae gen. et sp. nov., a new Tithonian (Late Jurassic) ophthalmosaurid from the northwest of Patagonia, Argentina. The new taxon depicts a peculiar pattern of the narial opening and forefin. Catutosaurus has an ‘ophthalmosaurine-like’ skull with reduced cheek, triangular squamosal, slender and delicate snout and rounded tooth roots; and ‘platypterygiine-like’ forefin composed by the typical polygonal and tightly packed elements and a broad articulation between the stylopodium and zeugopodial row. Phylogenetic analysis retrieves Catutosaurus nested within basal ophthalmosaurids, at the base of a large clade named here as Panplatypterygiine. New information provided by Catutosaurus suggests that changes in the narial region and complexity of the stylopodium–zeugopodium articulation occurred independently among ophthalmosaurids and not in a stepwise manner along with the evolution of the clade.