Morphological disparity in the evolution of the ophthalmosaurid forefin: new clues from the Upper Jurassic of Argentina

Abstract

Jurassic and Cretaceous marine deposits worldwide show that ophthalmosaurid ichthyosaurs are major components of marine tetrapod communities for almost 76 myr. One of the major features characterizing this clade is the complexity and diversity of their stylopodium–zeugopodium morphology. Upper Jurassic deposits of the Vaca Muerta Formation in north-west Patagonia (Argentina) have yielded the richest Tithonian ophthalmosaurid records from Gondwana. Here, we present a new ophthalmosaurid from this lithostratigraphic unit, Sumapalla argentina gen. et sp. nov., recognized based on several unique features of the skull roof, forefin, scapular, and pelvic girdles. On phylogenetic analysis it was recovered as a basal member of Platypterygiinae. The peculiar morphology of the forefin of the new taxon indicates a previously unnoticed morphological diversity in the ophthalmosaurid appendicular skeleton and provides an excellent opportunity to test its range of variation. For this purpose, we implemented a holistic approach, using 2D geometric morphometric analysis focused on the humerus, and disparity analysis of the zeugopodium. Our results reveal a long duration of one humeral morphotype characterizing Arthropterygius spp. and closely related taxa. Furthermore, we found a peak of zeugopodial disparity during the Kimmeridgian, followed by a decrease by the end of the Jurassic, and then a recovery by the Early Cretaceous.