Early Jurassic <i>Opisoma</i> (Bivalvia: Astartidae) from Argentina: avant-garde of the tropical <i>Lithiotis</i> fauna in the Neuquén Basin

Abstract This contribution presents the analysis of previously undescribed material from Early Jurassic deposits of west-central Argentina. Opisoma is an astartid bivalve assigned to the subfamily Opinae. It is characterized by its robust trigonal hinge and the absence of lateral teeth. Wing-shaped bivalves are an informal, polyphyletic group characterized by shells strongly compressed in the anteroposterior direction, a case of morphological convergence that appears in several extinct lineages, including Opisoma , and persists today in the cockle Corculum . Although astartids typically show shallow infaunal habits, Opisoma developed an epifaunal or semi-infaunal lifestyle, maintaining the commissural plane perpendicular to the substrate, likely associated with photosymbiosis. During the Early Jurassic, reef systems developed in low-latitude marine environments of the Tethys and Panthalassa, dominated by large gregarious bivalves known as the Lithiotis fauna, within which Opisoma represented a subordinate component. The specimens studied here were analyzed using traditional techniques and computed tomography to provide new, robust evidence addressing the longstanding debate regarding valve orientation in Opisoma . The new species Opisoma romeroi is described. This Neuquén Basin record represents the southernmost occurrence known for the genus and significantly expands its paleogeographic distribution. UUID: http://zoobank.org/2f1c6c72-c6aa-4380-8a53-2ef764963e0c