Sheffield and Sumrall—Generic revision of North American Holocystitidae
ambulacra lead to facetal scars that are partly positioned between the oral plate series and the facetal circlet (Fig. 6.1, 6.2). The positioning of the facets is looser than other taxa spread across the edges of more than one facetal. Holocystites species show wide morphological variability,
suggesting that more than one clade may be represented (Fig. 7.6, 7.7, 7.8). Proposed species show wide variation in the organization of the theca, ranging from moderately well- organized plate circlets to disorganized, irregularly plated thecae and species with large plates and relatively small plates. Holocystites species also encompass awide range of thecal body shapes, ranging from long and elongate to very globose. Further, ontogenetic sequences for most holocystitids are not under- stood; therefore, changes in plating patterns and thecal shape that occurred during ontogeny of species are likely adding to confusion concerning holocystitid systematics. Holdfasts can range from tapering to an almost stem-like projection to robust forms that lack noticeable narrowing. Unfortunately, many of the species currently assigned to Holocystites do not have oral areas preserved. Because the features of the theca and holdfast are so highly variable, they cannot be used to determine genus- level placement of species.
Genus Trematocystis Jaekel, 1899 Figures 6.7, 6.8, 7.2
Type species.—Holocystites subglobosus Miller, 1889 (H. globosus Miller, 1878)
Emended diagnosis.—Four ambulacra, B, C, D, and E, extend from peristome to facet scars that lie on top of facetal plates (Fig. 6.7, 6.8); the A ambulacrum not developed; B–E facets centered on facet-bearing facetal plates. Lateral facetals and A facetal plate depressed; O7 proportionally small. Thecal plates large and flat without sunken sutures (Fig. 7.2). Theca globular and squat, with relatively wide cementation disk.
Remarks.—Trematocystis bears the stereotypical holocystitid peristomial border plating (Fig. 6.7, 6.8). It is unusual because the A ambulacrum is not developed, likely resulting from pae- domorphic reduction, a phenomenon seen in other blastozoan groups (Sumrall and Wray, 2007). Among holocystitids, this reduction is also seen in Paulicystis, but there the appendages are recumbent, as evidenced by biserial scars extending a short way down the thecal plating near the summit. Large facets for the B–E ambulacra are centered on the facetal-bearing plates, and the presence of facets that do not cross the facetal plate boundaries serves to separate this taxon from Holocystites. The lateral, non-facet-bearing facetal plates and the A facetal plate are depressed with respect to others within the facetal plate series as seen in Pentacystis.
Genus Paulicystis Frest and Strimple, 2011 in Frest et al., 2011 Figures 6.9, 6.10, 7.3
Type species.—Paulicystis densus Frest and Strimple, 2011 in Frest et al., 2011
763
Emended diagnosis.—Four ambulacra, B, C, D, and E, extend from peristome to facet scars that lie on top of facetal plates. The A ambulacrum not developed. Ambulacra epithecally recum- bent upon theca (Fig. 6.9, 6.10). Lateral facetals and A facetal plate depressed. Periproct relatively large. Theca squat and globular (Fig. 7.3). Plates small, tumescent, with deeply depressed sutures. Relatively wide cementation disk.
Remarks.—Paulicystis bears the stereotypical plate arrange- ment for the peristomial border (Fig. 6.9, 6.10). As with Trematocystis, the A ambulacrum is absent, likely resulting from paedomorphic ambulacral reduction as seen commonly among blastozoans (Sumrall and Wray, 2007). The B–E ambulacra are not erect, as in the other genera, but instead lie recumbently on the thecal surface, beginning with the facet scars on the facetal plates. This shows the appendages to be biserial (Fig. 6.9) but adds little information about whether they are floor plates or brachiolar plates. Large facets for the B–E ambulacra are centered on facetal-bearing plates, and the presence of facets that do not cross plate boundaries serves to separate this taxon from Holocystites. The non-facet-bearing facetal plates and the A facetal plates are depressed with respect to others within the facetal plate series.
Genus Pentacystis Paul, 1971 Figures 6.2, 6.3, 7.4
Type species.—Pentacystis simplex Paul, 1971
Emended diagnosis.—Five ambulacra extend from peristome to very large facet scars that lie on top of facetal plates. Facetal plates large and elevated to form a low spout-like protuberant summit structure (Fig. 6.3, 6.4). Plates of the oral series narrow and confined to peristomial depression, except for relatively large O7. Theca elongate, narrows slightly toward the base without a constricted distal portion. Plates large and flat, without depressed sutures (Fig. 7.4).
Remarks.—Pentacystis bears the stereotypical holocystitid peristomial border plating. All five ambulacra extend to very large facets that bear erect feeding appendages. These facets are wholly supported on the facet-bearing plates and do not cross facetal plate boundaries. This serves to separate this taxon from Holocystites; the presence of a facet on the A facetal serves to separate Pentacystis from Trematocystis and Paulicystis. The oral plates within the oral plate circlet are proportionally narrow compared to O7. The facetals form a spout-like protuberant summit structure unlike the rounded summits of other holocys- titids. The non-facet-bearing lateral plates are not depressed with respect to other plates within the facetal series, adding to the spout-like appearance of the summit area. Humatipores are present on the thecal plates. Pentacystis was proposed as a separate genus within
Holocystitidae on the basis of the oral plates being greatly reduced or absent (Paul, 1971). Species within this genus were delineated by the presence or complete absence of oral plates; specimens with present or reduced (herein interpreted as partially disarticulated) oral plates were used to describe P. wykoffi (Miller, 1891), whereas specimens with absent oral
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