WHAT IS A PYRGOCYSTID? NEW EVIDENCE FROM AN EARLY ORDOVICIAN EDRIOASTEROID FAUNA FROM MOROCCO

Pyrgocystids have long been an enigmatic group of edrioasteroids known from Early Ordovician through Middle Devonian faunas worldwide. This clade includes a number of genera of small, highly turreted edrioasteroids with a small oral surface and an exaggerated pedunculate zone. Three factors have lead to a fundamental confusion about their basic high-level taxonomy. First, the small oral surface is fragile and readily disarticulates relative to the pedunculate zone. Consequently most species are described from peduncular material only. Second, several taxa have spines, obscuring details of the oral surface including the ambulacral system, hydropore, anal pyramid and oral rise. Third, no described specimen is preserved that exposes the interior of the oral surface to show the nature of the oral frame and floor plate system. These factors result is a poor understanding of pyrgocystid anatomy and phylogenetic placement. New material from the Lower Ordovician (Arenig) Upper Fezouata Formation of Morocco has none of the aforementioned issues. Specimens are preserved as high quality natural molds in shale concretions with some specimens preserving both part and counterpart. Latex casts of these specimens reveal several important anatomical features previously unknown in pyrgocystids. Pyrgocystids have a peripheral rim system that is identical in detail to isorophid edrioasteroids, the hydropore and gonopore are located in the proximal right CD interambulacrum, the floor plates imbricate distally, and the cover plates have passageways, but lack intrathecal extensions. These features strongly suggest lebetodiscinid affinities. Most unusual, however, is the nature of an ambulacral system bearing three series of plates unlike other edrioasteroids that bear only two series. Previous reports suggest the pyrgocystid ambulacral system lacks ambulacral floor plates but bears a derived set of plates, termed hood plates, that articulate to the ambulacral cover plates (Guensburg and Sprinkle, 1994). In fact, ambulacral floor plates are present as a distally imbricating uniseries bearing the food groove to which hood plates articulate. These in turn articulate to a single biseries of ambulacral cover plates. All of these features suggest that pyrgocystids are highly derived lebetodiscinid edrioasteroids.