Journal of Paleontology, 91(4), 2017, p. 755–766 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2016.159
Generic revision of the Holocystitidae of North America (Diploporita, Echinodermata) based on universal elemental homology
Sarah L. Sheffield, and Colin D. Sumrall
Department of Earth and Planetary Sciences, 1412 Circle Dr., 306 EPS, University of Tennessee, Knoxville, Tennessee, 37996-1410, USA ⟨
ssheffi2@vols.utk.edu⟩, ⟨
csumrall@utk.edu⟩
Abstract.—The Holocystites Fauna is an enigmatic group of North American diploporitans that presents a rare window into unusual middle Silurian echinoderm communities. Multiple systematic revisions have subdivided holocystitids on the basis of presumed differences in oral area plating and respiratory structures. However, these differences were based on a fundamental misunderstanding of the homologous elements of the oral area and the taphonomic process; taphonomic disarticulation of the oral area formed the basis for the erection of Pentacystis and Osgoodicystis as separate genera, and Osgoodicystis is interpreted as the junior synonym of Pentacystis. Holocystitids show a conservative peristomial bordering plate pattern that is shared among all described genera. The peristome is bordered by seven interradially positioned oral plates as is typical for oral plate–bearing blastozoans. A second open circlet of facetal plates lies distal to the oral plates; five of these facetal plates bear facets for feeding appendages (lost on the A ambulacrum in some taxa), while two lateral facets (present in all taxa except Pustulocystis) do not. Holocystitid taxa show minor modifications to this basic peristomial bordering plate pattern. As thecal morphologies are highly variable within populations, taxonomic revision of holocystitids is based on modifications of the plating of the oral area.
Introduction
Silurian-age diploporitan echinoderms are relatively scarce in the fossil record (Witzke et al., 1979; Thomka and Brett, 2014). The Holocystites Fauna, however, is an important exception to this paucity of fossil material and presents a rare window into unusual middle Silurian echinoderm communities. This fauna is an abundant and diverse collection of middle Silurian diplo- poritan taxa from the midcontinental region of North America (i.e., Wisconsin, Illinois, Indiana, Kentucky, Ohio, Tennessee) (Frest et al., 2011) and possibly Australia (Jell, 2011) that pro- vides a unique opportunity to study paleoecology, taphonomy, and phylogenetics of this enigmatic clade. This study focuses on the generic classification of holocystitids, the dominant component of this fauna. Understanding the systematics of Holocystitidae has been
complicated by a number of issues. First, holocystitids have a plastic thecal morphology that shows wide variation within populations, resulting from irregular plating of the theca, allo- metric changes, and ecophenotypic variation, making species identification based on thecal morphologies unreliable (Sheffield and Sumrall, 2015a). Some work has been done to identify holocystitids from preserved holdfasts attached to hardgrounds (e.g., Thomka et al., 2016). However, thecae are disassociated from these holdfasts, so the taxonomic affinity is based on pre- served aboral plating and general size of the holdfast. However, when found attached to large bioclasts, holocystitid holdfasts in softground settings can be extremely plastic, taking on the size and morphology of the underlying attachment surfaces, casting
some doubt on the reliability of holdfast morphology for dis- criminating taxa across holocystitid-bearing localities. Second, many holocystitid species are based on extremely poorly pre- served internal molds in sugary dolomite that are difficult to reconcile with more pristine specimens preserving external morphologies as original calcite. Third, species and genera were describedwith a poor understanding of the plating of the oral and summit structures, emphasizing presumed differences (often preservational) while overlooking fundamental similarities. Fossils from the Holocystites Fauna were first published
over a century and a half ago (e.g., Hall, 1861, 1864, 1870). At that time, Holocystites Hall, 1861 was the only proposed genus within the fauna, encompassing a wide variety of morphologies; a multitude of later studies (e.g., Miller, 1878, 1879; Miller and Gurley, 1888, 1894, 1895) proposed over 50 species assigned to this genus alone. Frest et al. (2011) noted that the number of species proposed by Miller correlated closely with the number of specimens found within the formations being studied. These initial papers sought to document the wide disparity of morphologies present within holocystitids primarily via describing the differences noted across the thecae. Detailed descriptions of a large majority of these specimens were com- plicated by poor preservation that erased important information concerning thecal ornamentation and the oral area morphology. More recent studies (Tillman, 1967; Paul, 1971; Frest et al.,
1977, 2011) recognized the high morphological disparity within the numerous species of Holocystites and divided known taxa among multiple genera, including Holocystites Hall, 1861; Trematocystis Jaekel, 1899; Triamara Tillman, 1967; Pentacystis
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