Cystoid

Praepleurocystites watkinsi

• Ordovician
• Bromide Formation-Pooleville Member
• Geological Enterprises, Inc. Quarry, Criner Hills, Carter County, Oklahoma, USA

Size: 5.5 cm

The pleurocystitids are a subgroup of Rhombiferan cystoids that developed an unusual flexible, flattened theca with more prominent bilateral symmetry and two long brachioles. They presumably laid flat on the seafloor with minimal drag to feed on detritus, and the flexible theca may have aided in respiration. A representative form is Pleurocystites squamosous of the Ordovician Bobcaygeon Formation of Canada, where it is one of the most common echinoderm fossils found.

On the other hand, Praepleurocystites watkinsi of the Bromide Fm. is rather unique in possessing intermediate features between most other pleurocystitids and the likely stem group glyptocystitid ancestor (Parsley 1982). Unlike typical pleurocystitids, it has a relatively inflated and rigid theca with thick plates that bear strong ornamentation, making it superficially more similar to a glyptocystititid. Its three prominent pairs of respiratory pore rhombs likely made up for this lack of flexibility. Nonetheless, like most pleurocystitids it possessed a specialized ventral surface upon which it laid as well as two strong brachioles for feeding.