Morphologic and Systematic Revision of the Solute Maennilia estonica (Homoiostelea, Echinodermata) from the Upper Ordovician of Estonia

Maennilia estonica Rozhnov and Jefferies was first described as a stem-chordate but restudy shows it is an unusually large homoiostelean echinoderm. Its feeding structure, an erect ambulacrum, extending exothecally from the peristomial frame plates, bears a large internal tunnel that opens directly into the body cavity. This type of feeding appendage is now recognized to be unique to homoiosteles. It bears no evidence for water vascular system impressions adjacent to its food groove. The theca has poorly developed marginal plates and a narrow rim that, in contrast to some other homoiosteles, does not extend over either lower or upper thecal face. Maennilia appears to have inhabited the deeper portions of a near-shore environment in limey muds adjacent to a linear zone of bryozoan and microbial bioherms.