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The first articulated remains of ophiuroids for the Mesozoic of South America are described from the Lower Cretaceous of Neuquén Basin, Argentina. The taxonomic analysis allows the assignment of the material described herein to the extinct genus Ophiopetra. The specimens belong to a new species, but considering the poor preservation, a new name is not introduced, as it would be based on an incomplete diagnosis. Certain characteristics (e.g., the diameter of the disc, the width/height ratio of the vertebrae) suggest that these ophiuroids are paedomorphic specimens. In light of the latest classification of the Ophiuroidea, and new insights on the spine articulation microstructure of Ophiopetra lithographica presented herein, a transfer of Ophiopetra to the family Ophionereididae within the order Amphilepidida is proposed. This material expands the palaeogeographic record of this genus, since it represents the first remains of Ophiopetra described in the Southern Hemisphere. It is also the first Cretaceous record of the genus worldwide. RESUMEN Se describen los primeros registros fósiles de ofiuroideos articulados del Mesozoico de América del Sur, provenientes del Cretácico Inferior de Cuenca Neuquina, Argentina. El análisis taxonómico permite asignar el material descrito al género extinto Ophiopetra. Los especímenes pertenecen a una nueva especie, pero no se propone un nuevo nombre porque, debido a la pobre preservación, estaría basado en una diagnosis incompleta. Ciertas características (e.g., el diámetro del disco, la relación ancho/alto de vértebras, etc.) sugieren que estos ofiuroideos son especímenes pedomórficos. Se propone la transferencia de Ophiopetra a la familia Ophionereididae del orden Amphilepidida teniendo en cuenta la última clasificación de ofiuroideos y nuevas evidencias sobre la microestructura de las articulaciones espinales de Ophiopetra lithographica presentadas en este trabajo. Este material expande el registro paleogeográfico del género, ya que es la primera descripción de Ophiopetra en el hemisferio sur. Además, es el primer registro cretácico del género a nivel mundial.

Asterozoans are free living, star-shaped echinoderms which are important components of benthic marine faunas worldwide. Their fossil record is, however, poor and fragmentary, probably due to dissarticulation of ossicles. In particular, fossil asteroids are infrequent in South America. A new species of starfish is reported from the early Valanginian of the Mulichinco Formation, Neuquén Basin, in the context of a shallow-water, storm-dominated shoreface environment. The specimen belongs to the Astropectinidae, and was assigned to a new species within the genus Tethyaster Sladen, T. antares sp. nov., characterized by a R:r ratio of 2.43:1, rectangular marginals wider in the interbrachial angles, inferomarginals (28 pairs along a median arc) with slightly convex profile and flat spines (one per ossicle in the interbrachials and two per ossicle in the arms). Both the sedimentologic features of the bearing bed, and the taphonomic signature point to a rapid burial. Considering the presence of well-developed fascioles, it is likely that the specimen was already half buried at the beginning of its taphonomic pathway. This record represents the oldest finding of Astropectinidae in the Southern Hemisphere and the first record of the genus Tethyaster for the Lower Cretaceous of South America.