Exceptionally preserved Early Cretaceous stalked crinoids from the Agrio Formation, Neuquén basin, west-central Argentina

Abstract

Crinoid beds were studied from the Valanginian beds of the Agrio Formation, in the Cerro Mesa area, located 20 km northeast of the city of Zapala (Neuquén Basin, Argentina). Deposits represent a mixed clastic-carbonate succession broadly interpreted as storm-influenced shoreface to offshore settings. Crinoid paleocommunities developed in association with large migratory subaqueous dunes (sand waves) and were preserved in a number of stacked crinoid beds including a key surface with 20 specimens highly articulated. This is the first record of complete stalked crinoids from the Early Cretaceous of South America. Two distinct modes of occurrence have been recognized through a detailed taphonomic analysis. Well-preserved articulated specimens associated with rapid burial probably by avalanching processes and disarticulated specimens with more extensive periods of exposure before final burial. The specimens are provisionally assigned to the genus Isocrinus (Family Isocrinidae). However, some diagnostic characters remain unknown.