THE OLDEST RECORD OF RHENOPYRGID EDRIOASTEROIDS, FROM THE EARLY ORDOVICIAN FEZOUATA FORMATION OF MOROCCO, NORTH AFRICA

A new genus of rhenopyrgid edrioasteroid is described from the Fezouata Formation (Ordovician, Tremadocian/Floian) of Morocco, North Africa. Approximately 370 specimens occur as natural molds on a small concretionary siltstone slab collected as float. There is little size variation in the population and specimens preserve all skeletal aspects of the theca except for the hydrogonopore (yet to be identified); no trace of soft tissue structures is preserved.This single slab preserves all of the specimens known of this species despite extensive collections conducted over many years.

The rhenopyrgid is the oldest known member of the clade and has several atypical features that constrain character polarity. Interradial oral plates that dominate the rhenopyrgid oral surface are loosely sutured to the floor plates rather than tightly sutured or fused as in other rhenopyrgids. The suboral constriction is flexible, being composed of 6-7 irregular circlets of highly imbricated plates that span the suboral collar and the oral surface. The suboral collar is composed of two circlets of alternating and elongate plates. The pedunculate zone is irregularly plated, with some 150 circlets of plates; all other rhenopyrgids have about 50 circlets of four plates that alternate, forming eight regular columns. In the new taxon the basal coriaceous sac transitions into a multiplated holdfast.

The taphonomy is much like other faunal components in this zone of the Fezouata, i.e. an exceptionally preserved, mass occurrence of a single species, representing opportunistic colonisation dynamics. These are typically interpreted to have been buried by distal storm deposits. However, specimens in the rhenopyrgid slab do not show obvious current-alignment, and they occur through a thickness of sediment rather than on one bedding plane. They are also unusual in not having undergone thecal collapse, but are preserved three dimensionally in a concretionary bed. In many respects the situation is similar to recently described occurrences of Rhopalocystisinterpreted as mass transport and burial of dense occurrences of echinoderms, with most of the fine particles winnowed away to concentrate them. The superb preservation of fine details in this case results from the concretionary lithification, cementing them prior to decay-related collapse.