Aristocystites, a recumbent diploporid (Echinodermata) from the Middle and Late Ordovician of Bohemia, ČSSR

Abstract

The Ordovician diploporid Aristocystites Barrande lived recumbently on the seafloor but never evolved marginal frame plates or flexible tesselated surfaces. Flume studies on 1:1 scale models clearly indicate that this genus lived attached by an aboral holdfast with the aboral end of the theca facing into the current. The oral end, with its paired brachioles, fed in vortices generated in the lee of the theca. Optical current velocities for feeding probably ranged from 6 to 18 cm/sec. Growth of thecal plates and diplopore structures originated in galleried stereom. Secondary thickening occurred by growth of labyrinthine stereom. Aristocystites probably pressurized its gut as is suggested by the massive anal pyramid. Peristaltic waves through the gut would have enhanced body-fluid circulation and circulation through the diplopores. Aristocystites shows no signs of simplification in the ambulacral area. The transversely oriented, bilateral, ambulacral system is considered to be primitive. Other species synonymized with Aristocystites bohemicus Barrande from the Caradocian Zahořany Beds of Bohemia are A. desideratus, A. grandisculum, A. idealis, and A. rudis. Aristocystites metroi Parsley and Prokop, n. sp., is described from the upper Zahořany shale facies and the Bohdalec Formation from the Prague area.