Mass aggregation of Middle Miocene spine-coated echinoids Echinocardium and their integrated eco-taphonomy

Abstract

A unique "Fossillagerstatte" of spatangoid echinoids of the genus Echinocardium from the Middle Miocene (Badenian) sandy deposits of the Fore-Carpathian Depression, as exposed at Gleboviti (=Chlebowice) in the Ukraine, is characterised by a mass occurrence of tests often preserving their entire spine canopy, apparently unaffected by taphonomic filtering. These echinoids represent a new species, Echinocardium leopolitanum sp.nov., and are assumed to have had a similar mode of life as the extant, cosmopolitan species E. cordatum (PENNANT, 1777), i.e. relatively deep burrowing and confined to the sublittoral. Violent storms and/or storm-generated currents are held responsible for stirring up the sand and for bringing live specimens, of all ontogenetic stages, to the surface upon which followed deposition of a heavy-loaded sediment from which they could not escape. Thus, specimens are interpreted to have been buried alive, with all spines attached. Mass aggregation of tests occured either in patches laid down in vortical flutes on the current-swept seafloor, or within tabular scrolls of cross-bedded strata where they are locally imbricated. A functional analysis of the spines of Echinocardium leopolitanum sp.nov., and primarily of the large, triangular fan of plastron spines, suggests specimens to have been adapted to rapid burrowing throughout a weakly coherent and nutrient-poor sandy bottom. Ascribed to Echinocardium leopolitanum sp.nov. burrows, whose structure is comparable to, if not identical with, those of other Echinocardium species. The taxonomic potential of such burrows is discussed and it is suggested that names applied recently in ichnological analyses are in need of a modern revision.