An early Miocene spatangoid assemblage on a submarine volcanic ash dune from Patagonia (Argentina)

A single 27 m2 spatangoid-dominated bed yielding 65 specimens from the Punta Entrada Member of the Monte Le ' on Formation was studied. Most of the fossils were preserved oral side down, with clusters of spines attached to the tests, showing no evidence of preburial transport. The deposit is interpreted as the result of sedimentation in a subtidal environment coeval with strong and continuous explosive volcanic activity, indicated by the significant amount of volcanic ash within the sediment. Echinoids assigned to Brisaster aff. B. iheringi De Loriol, 1902 colonized a distal part of the subtidal environment dominated by large-scale migrant bedforms. During dormant periods, it was characterized by low energy and low sedimentation rate, allowing colonization by several groups of infaunal organisms. Upon reactivation of sediment migration, a high sedimentary influx trapped the echinoids and prevented them from escaping.