758 1 A E L D O7 P O3 O2 O1 M O4 O5 O6 C D B L L E M
O1 O6 O7
P C
Journal of Paleontology 91(4):755–766 2 A B L L D O7 P
Figure 3. Common disarticulation patterns of holocystitid oral plating. (1) All seven oral plates associated with the theca, bordering the peristome. Schematic line drawing of Holocystites scutellatus (SUI 48183). (2)O2–O5 become disarticulated from the peristome, giving the appearance of an enlarged peristome being bordered by the facetal circlet and O6 and O7. Schematic line drawing representative of the oral area of Holocystites spangleri Miller, 1891 (SUI 48197). (3)O1–O6 have been disarticulated, giving the appearance of a greatly enlarged peristomial opening being bordered by the facetal plate circlet. This plating pattern has been used to separate Pentacystis Paul, 1971 (those without most oral plates) and Osgoodicystis Frest and Strimple, 2011 in Frest et al., 2011 (those with oral plates); because the only significant difference between these two proposed genera is taphonomic, Osgoodicystis is rejected as a junior synonym of Pentacystis. Schematic line drawing of Pentacystis gibsoni Frest and Strimple, 2011 (SUI 46316).M = mouth; P = periproct; L = lateral facet.
of the oral area. Regardless, the plating of the oral area and thecal morphologies are much more readily identifiable in fossils preserved as original calcite than in those with moldic preserva- tion. Unfortunately, associated free feeding appendages are not preserved with specimens of either preservation type in current collections. External morphologies of specimens preserved as original calcite are often difficult to reconcile with internal morphologies of those preserved as molds because of the inability to identify thecal morphologies and oral plating in the latter. While holocystitid thecae are thick and heavily plated, the
oral plates seem to be easily disarticulated and often become disassociated from otherwise well-preserved thecae, presum- ably due to some amounts of reworking after initial burial. This has taphonomically produced a variety of oral–facetal plate configurations described among specimens (Paul, 1971; Frest et al., 2011). When fully articulated, the peristomial opening is bordered by six oral plates, O1–O6. Oral 7 and the seven facetal plates are in contact with this proximal oral circlet and are not in contact with the peristomial opening (Fig. 3.1). In some specimens, O2–O5 have become disarticulated from the peristome, making it appear as if the seven facetals, O1, and O6 (but not O7) border the taphonomically enlarged peristomial opening (Fig. 3.2). In other cases, O1–O6 have become disassociated with the theca, leaving a taphonomically enlarged peristomial border bordered by seven facetals and O7 (Fig. 3.3). Taphonomic effects are often exacerbated by aggressive use of air abrasion during specimen preparation that has worn away oral plates either in part or in whole. This situation formed the basis for the identification of Osgoodicystis, which is only distinguishable from Pentacystis by the lack of oral plates.
Non-holocystitid Silurian diploporitans from North America.— Holocystitids are typified by a number of features that appear to be unique to this clade. The thecae are typically large (compared to other diploporitans), with some specimens reaching 15 cm in thecal height. With the possible exception of diplopore-bearing Triamara (which may or may not be a holocystitid; see
discussion in the following), holocystitids bear humatipores that lie solely within individual thecal plates and have numerous coelomic canals covered by a bulbous, lightly skeletonized covering. Other diploporitan groups typically have simple diplopores composed of a single, uncalcified thecal canal that is rarely preserved. In fossils, these structures are expressed as two pores contained within a depression on the external portion of the thecal plate (for further information regarding diploporitan respiratory structures, see Paul, 1972). The ambulacra of holocystitids are also morphologically
different from other diploporitans. They have highly reduced proximally recumbent ambulacral systems that are restricted to the summit on the orals and facetals, and erect appendages (recumbent and epithecally positioned in Paulicystis)of
E M C 3 A B L
unknown affinities; the oral areas of holocystitids do not bear floor plates incorporated into the theca as do most blastozoans (Sumrall, 2010, 2015). Erect appendages, either erect ambula- cral floor plates presumably bearing brachioles or more likely greatly enlarged terminal brachioles borne on facets that are positioned on the facetal circlet, are unique to holocystitids. Judging from the size of the facets and plating scars on Paulicystis, these appendages are biserial and proportionately exceptionally large for a blastozoan. Other occurrences of diploporitans from the Silurian of
North American are morphologically highly dissimilar to holocystidids, and until recently, the only described taxon was Gomphocystites Hall, 1864 (Fig. 4.1). Gomphocystites occurs slightly earlier than the holocystitids, with the earliest undoubted occurrence in the Llandovery-age Hopkinton dolomite of Iowa (Witzke, 1976) and persists as a common faunal component in strata containing the Holocystites Fauna (which are largely restricted to Wenlock age). Gomphocystites also has a greater biogeographic range than holocystitids, with fossils known from New York, USA (Brett, 1985a), and the Baltic Celtacystis (Gomphocystites) gotlandicus (Angelin, 1878), which has been proposed to be very closely related to Gomphocystites (Bockelie, 1979, 1984). The morphology of Gomphocystites deviates
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