New Early Cambrian eocrinoids from the Iberian Chains (NE Spain) and their role in nonreefal benthic communities

Abstract

The Early Cambrian echinoderm-sponge meadows, which colonized shallow carbonate-siliciclastic platforms, have received relatively little study despite their widespread occurrence in several regions of Gondwana. Their taxa are generally preserved disarticulated, reworked and deposited through the action of waves and storms. Silicified echinoderm-rich packstones from the uppermost Bilbilian limestones (latest Early Cambrian in age) of the Valdemiedes Formation (Iberian Chains, NE Spain) formed through growth and destruction of echinoderm-sponge colonies by storm-wave action. This fauna has been studied in thin section and after etching by acetic acid. Eocrinoid remains include the thecal basal-ossicle of Rhopalocystis? mesonesensis n. sp. and other indeterminate species. Echinoderm columnals, visible in thin section, are associated with sponge spicules, trilobites, calcite- and phosphate-shelled brachiopods, chancelloriid sclerites, and foraminiferans. This finding suggests that stemmed eocrinoids were already present on the western Gondwana margin at the end of the Early Cambrian.