New morphological details revealed in well-preserved paracrinoids from reef facies in the Kimmswick Limestone (Upper Ordovician) of Missouri

New, well-preserved specimens of the paracrinoids Wellerocystis and Implicaticystis provide new morphological data. All specimens originate from reef facies in the Kimmswick Limestone (Upper Ordovician, Sandbian–Katian) at a single locality near St Louis, Missouri, USA. Wellerocystis is characterized by an ovoid theca largely composed of imperforate plates arranged in vertical columns lacking pore-structures but with fine granular sculpture; four recumbent branched uniserial ambulacra with up to seven branches in total; a mouth frame of four plates, one of which also contributes to the periproct frame; a sinuous hydropore; and circular gonopore. The stem is unknown; its facet is small and circular, similar to that of Platycystites. Implicaticystis is characterized by a circular, heteromorphic stem, ovoid theca composed of externally concave, perforate plates with foerstepores, internal pararhombs, and a mouth frame of three plates plus two lateral plates each bearing two facets for erect, uniserial, hemipinnate pseudoarms. Foerstepores connect to tubes that pass through the theca near plate sutures. Internal lamellae of pararhombs support thecal plates much as A-frames support ridged rooves. Erect versus recumbent and branched ambulacra evolved repeatedly in pelmatozoans so both are less useful in classifying paracrinoids than presence or absence of unique pore-structures. The sister group of paracrinoids could have included Columbocystis , rhipidocystids, and cryptocrinitids. Columbocystis is commonly mentioned in discussions in this context, but its asymmetrical facets suggest it had biserial feeding appendages, unlike uniserial paracrinoid appendages.