Jurassic and Cretaceous marine deposits worldwide depict ophthalmosaurid ichthyosaurs as major components of marine tetrapod communities for almost 76 million years. One of the major features characterising this clade is the complexity and diversity of their stylopodium-zeugopodium morphology. Late Jurassic deposits of the Vaca Muerta Formation in northwest 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. Phylogenetic analysis recovered it as a basal member of Platypterygiinae. The peculiar morphology of the forefin of the new taxon illustrates 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 occurred during the Kimmeridgian, followed by a decrease by the end of the Jurassic, and then a recovery by the Early Cretaceous.