Taxonomic analysis of the first ophiuroid remains for the Mesozoic of South America: the youngest record of the extinct genus Ophiopetra Hess

Abstract

The findings of articulated and/or nearly complete brittle stars in South America are restricted to Devonian, Permian and Cenozoic specimens. In this work ophiuroids are described from the Neuquén Basin, Patagonia, Argentina. These are the first remains of ophiuroids for the Mesozoic of South America. The specimens show a circular disc approximately 3.5 mm in diameter, and five long and slender arms that are 3 times longer than the disc diameter. Lateral arm plates are stout, subquadrangular to subrectangular with distal edge convex and proximal and lateral edges mostly straight. The height/length ratio is approximately 0.62 in proximal lateral arm plates and 0.30 in the distal ones. There are one or two horseshoe-shaped arm spine articulations per lateral arm plate. Dorsal arm plates are subtriangular with convex distal edge, acute proximal edge and straight to slightly convex lateral edges. Ventral arm plates are subrectangular with concave lateral edges, acute proximal edge and pointed to slightly convex distal edge. The lateral arm plates exhibited by the specimens described herein are similar to what is found in Ophiodoris Koehler and Ophioplax Lyman. However, the disc ornamentation differs from both taxa: both sides of the disc are densely covered with granules. We therefore place these specimens in the extinct genus Ophiopetra Hess, also taking into account similarities in the overall shape of the lateral arm plates. The specimens belong to a new species but are poorly preserved. A new name is not introduced, as it would be based on an incomplete diagnosis. The systematic position of Ophiopetra has been under debate, ever since the erection of the genus. At the moment, Ophiopetra is considered an ophiolepidid. In the context of the latest progress in ophiuroid systematics, and thanks to 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 from the Southern Hemisphere. It is also the first post-Jurassic record of the genus worldwide.