Paulicystis Frest & Strimple, 2011

Genus Paulicystis Frest & Strimple in Frest et al.,

2011 Type species: Paulicystis densus Frest & Strimple in Frest et al., 2011,

by original designation, from the Massie Formation, Sheinwoodian lower Silurian), Napoleon, Ripley County, Indiana, USA.

Species included: Type species and Paulicystis sparsus Frest & Strimple in Frest et al., 2011.

Emended diagnosis.—Holocystitids with six inter-radial orals and eight facetals, four facets for recumbent ambulacra that extend below the facetal circlet, food grooves along O:O sutures, humatipores below smooth surface of tumid thecal plates in circlets of up to 20 plates, attachment area large (simplified from Frest et al. 2011: 83).

Remarks.— Paulicystis ( Figs. 19 View Fig , 20 View Fig ) has a barrel-shaped theca with a large attachment area, several circlets of smooth tumid plates with deep sutures below the surface of which the humatipores lie ( Fig. 19A View Fig 1 View Fig ). The rounded mouth is surrounded by the usual six interradial orals, with a facetal circlet of eight plates around that. Four of the facetals bear the adoral ends of elongate concave grooves that extend radially beyond the facetal circlet ( Fig. 19 View Fig ). The grooves are divided along their length and cross-wise ( Fig. 20 View Fig ) indicating that the attached ambulacra were biserial. An oval hydropore lies across the O1:O6 suture and a circular gonopore in plate F1 above the rounded anus ( Fig. 19A View Fig 3 View Fig ). Humatipores are well-developed with four to six tangential canals. According to Frest’s diagrams ( Frest 1983: 525, fig. 77C, D; Frest et al. 2011: 81, fig. 45), pairs of perpendicular canals from different humatipores are united by the same set of tangential canals. The attachment area appears to be characterized by a central heptagonal plate, surrounded by a closed circlet of seven basals ( Frest 1983: pl. 10: 9; Frest et al. 2011: pl. 20: 9; Thomka and Brett 2014a: 182, fig. 4).

Paulicystis ( Figs. 19 View Fig , 20 View Fig ) is unique among holocystitid genera in having four ambulacra that extended radially over the theca ( Fig. 19A View Fig 1 View Fig ). The ambulacral channels are biserial ( Fig. 20 View Fig ), so it is assumed that the ambulacra were also. Holocystitid ambulacral appendages have never been preserved, so it is unknown if the recumbent ambulacral gave rise to erect brachioles. Nevertheless, this feature alone is sufficient to distinguish Paulicystis from other holocystitid genera.

Stratigraphic and geographic range.—Sheinwoodian (lower Silurian), Indiana, USA, where Paulicystis has only been re-

A1 A2 A 3

corded from the base of the Massie Formation as restricted by Brett et al. 2012. Thecal bases occur on a hardground surface capping the Lewisburg Formation ( Thomka and Brett 2014a, b, 2015), and detached specimens from the lower part of the overlying Massie Formation.