Cheirocystis fultonensis, a new glyptocystitoid rhombiferan from the Upper Ordovician of the Cincinnati Arch—comments on cheirocrinid ontogeny

Abstract

A new glyptocystitidoid rhombiferan, Cheirocystis fultonensis, is described from the contact zone between the Point Pleasant Formation and lower Kope Formation exposed 50 km south east of Cincinnati, Ohio. This species, the second known glyptocystitidoid rhombiferan from the Cincinnatian Series and the youngest known species of Cheirocystis, shows significant suture modification where pectinirhombs are placed. Growth of large pectinirhombs along vertical sutures results in an unusual bowing-out of thecal plate sutures that is also seen in the related species Cheirocystis anatiformis. A juvenile specimen shows that ontogenetically the lateral shared ambulacra become less pronounced, pectinirhombs are added until the eight standard positions are expressed, and the periproct becomes proportionately smaller with age.