NEW LATE MIDDLE EOCENE ANTARCTIC GONIASTERIDS (ASTEROIDEA, ECHINODERMATA) FROM LA MESETA FORMATION. IMPLICATIONS FOR THE ORIGIN, EVOLUTION, AND PALEOECOLOGY OF THE PENTAGONASTERINAE

Abstract

Two specimens of exceptionally well-preserved Antarctic Eocene goniasterids are described. The first one, Billaster blakei gen. et sp. nov., is characterized by a dense granulation on the actinal surface with some spherical granules arranged into a rosette-like structure, enlarged antepenultimate superomarginal plates—in three out of five arms—corresponding to two inferomarginal plates, and granules and pedicellariae pits on marginal and abactinal plates. The second specimen, Buterminaster elegans, is characterized by a terminal plate—enlarged into a subtrigonal single plate covering completely the abactinal part of the arm tip—and granulation on the surface of actinal and marginal plates as well as along the periphery of actinal, abactinal, and marginals plates. Based mainly on the presence of surface granulation, Buterminaster is here considered a valid genus and not a synonym of Pentagonaster. The Eocene asteroids are associated with well-preserved bryozoans, brachiopods, a single polyplacophoran, echinoids, and serpulids, among other marine organisms inhabiting hard or firm substrates. Their exceptional preservation is attributed to rapid burial. The relatively diverse Eocene asteroid genera are quite different from modern Antarctic goniasterids but share many similarities with extant taxa outside Antarctica. Billaster shares many characteristic features with the extant genera Eknomiaster and Pawsonaster, which are restricted to New Caledonia-Kermadec and Tropical Atlantic regions (up to Uruguay), respectively, and are considered stemward genera of the Pentagonasterinae clade. Therefore, these well-preserved asteroids are significant to elucidate the origin, evolution, and dispersal of the group.