Introduction
Cyclocystoids (phylum Echinodermata, subphylum Echinozoa) are one of the most enigmatic clades of Paleozoic echinoderms. Despite an Ordovician–Mississippian range, cyclocystoids are extremely rare echinoderms (e.g., Smith and Paul, Reference Smith and Paul1982; Ewin et al., Reference Ewin, Reich, Graham and Cournoyer2019), and their paleoecology has been debated regularly (e.g., Nichols, Reference Nichols1969; Smith and Paul, Reference Smith and Paul1982; Glass et al., Reference Glass, Ausich and Copper2003; Sprinkle et al., Reference Sprinkle, Reich, Lefebvre, Zamora and Rábino2015; Reich et al., Reference Reich, Sprinkle, Lefebvre, Rössner and Zamora2017; Ewin et al., Reference Ewin, Reich, Graham and Cournoyer2019). Much of their stratigraphic range is known from only isolated marginal ossicles. Ordovician specimens with complete rings of marginal ossicles are well documented from Avalonia, Baltica, and Laurentia (Smith and Paul, Reference Smith and Paul1982), and this record has recently expanded both temporally and geographically (e.g., Reich et al., Reference Reich, Sprinkle, Lefebvre, Rössner and Zamora2017; Ewin et al., Reference Ewin, Reich, Graham and Cournoyer2019).
The cyclocystoid clade was clearly devastated by Late Ordovician extinctions (Smith and Paul, Reference Smith and Paul1982, fig. 14), but relatively little is known about their recovery during the Llandovery. Although reports of Llandovery isolated marginal ossicles exist (Frest et al., Reference Frest, Brett, Witzke, Boucot and Lawson1999; Hints et al., Reference Hints, Pärnaste, Männik, Reich and Rozhnov2022), only two Llandovery cyclocystoids with a complete ring of marginal ossicles have been described previously, including Polytryphocycloides davisii (Salter in Salter and Billings, Reference Salter and Billings1858) from the Mulloch Hill Sandstone in Scotland (Smith and Paul, Reference Smith and Paul1982) (upper Rhuddanian; Donovan et al., Reference Donovan, Lewis, Fearnhead and Widdison2012, p. 155) and Perforocycloides nathalieae Ewin et al., Reference Ewin, Reich, Graham and Cournoyer2019 from the Cybele Member, Jupiter Formation (Telychian) of Anticosti Island, Québec. In this report, a third cyclocystoid with a complete ring of marginal ossicles, Zygocycloides? foerstei n. sp., is reported from the Brassfield Formation (Aeronian) from Ohio. This new specimen is quite small and has a marginal ring with 15 ossicles and a few collapsed dorsal disc ossicles. This occurrence establishes the presence of cyclocystoids in Laurentian benthic faunas during the first major Silurian flooding of the North American continental interior after Late Ordovician extinctions. It also suggests promise for future discovery of Late Ordovician and early Silurian cyclocystoids in North America.
Location and stratigraphy
Zygocycloides? foerstei n. sp. is from Oakes Quarry Park, ~2 km (~1.8 miles) east of Fairborn, Greene County, Ohio (39°48′53″N, 84°59′06″W) north of Ohio Highway 235 (Fig. 1). It was collected in float from the Brassfield Formation (upper or red Brassfield of Sullivan et al., Reference Sullivan, McLaughlin, Brett, Cramer, Kleffner, Thomka and Emsbo2016; Kleffner, Reference Kleffner2020), which is Aeronian (Llandovery, Silurian) in age. The red Brassfield is within the Pranognathus tenuis Zone, which is middle Aeronian (M.A. Kleffner, personal communication, 2022).
Figure 1. Locality map of the Oakes Quarry Park in Greene County, Ohio. “X” marks the approximate position where the holotype of Zygocycloides? foerstei n. sp. was collected.
The Brassfield Formation in Oakes Park Quarry contained a small, coral–stromatoporoid reef and reef-associated facies (Sheehy, Reference Sheehy1979; Schneider and Ausich, Reference Schneider and Ausich2002; Schmidt et al., Reference Schmidt, Carney and Ausich2007). Quarrying since the 1990s removed the core and flank beds of the primary Oakes Park Quarry reef. Today, the remaining Brassfield at this site has one poorly exposed reef northeast of the northern quarry face (Schneider and Ausich, Reference Schneider and Ausich2002). Within the quarry, the exposed rocks are primarily encrinites with thin siliciclastic mudstone beds. The new cyclocystoid is preserved on the bedding surface of an encrinite bed. In addition to the echinoderms noted in the following (Table 1), the Brassfield Formation contains a diverse fauna that is largely unstudied, including, among others, corals, stromatoporoids, brachiopods, and bryozoans.
Table 1. Brassfield Formation (Aeronian) echinoderms from Greene and Clark counties, Ohio.
Llandovery echinoderm faunas
August Foerste (Reference Foerste1884, Reference Foerste1919, Reference Foerste1936) described the first echinoderm fossils from the Brassfield Formation of Ohio, which then included one glyptocystoid, five crinoids, and three asteroids (one of which was subsequently recognized as a crinoid holdfast) (Foerste, Reference Foerste1919, Reference Foerste1936; Ausich, Reference Ausich1986a) (Table 1). During the late 1970s, an exceedingly rich echinoderm fauna in reef-associated facies was discovered in what is now the Oakes Quarry Park near Fairborn, Ohio. Ausich and Schumacher (Reference Ausich and Schumacher1984) and Ausich (Reference Ausich1984a, Reference Ausichb, Reference Ausich1985, Reference Ausich1986a, Reference Ausichb, Reference Ausichc, Reference Ausich1987) described one new glyptocystoid and 31 crinoids (Table 1). This is the locality where the new cyclocystoid was collected, and with this new cyclocystoid, 33 echinoderm species assigned to four subclasses are known from the Brassfield Formation in Greene and Clark counties, Ohio. In addition, Broadhead and Ettensohn (Reference Broadhead and Ettensohn1981), Ausich and Dravage (Reference Ausich and Dravage1988), Blake and Ettensohn (Reference Blake and Ettensohn2009), Boyarko and Ausich (Reference Boyarko and Ausich2009), and Ausich et al. (Reference Ausich, Peter and Ettensohn2015) described additional crinoids and asteroids from the Brassfield Formation elsewhere in Ohio and Kentucky. Oakes Quarry Park has one of the richest Llandovery echinoderm faunas known, with other important faunas (with 10 or more taxa) from the Cabot Head Formation (Rhuddanian), New York and Ontario (Brett, Reference Brett1978; Eckert Reference Eckert1984); Farmer's Creek Member, Hopkinton Formation (Aeronian), Iowa (Witzke and Strimple, Reference Witzke and Strimple1981); Leijiatum Formation (Aeronian), China (Mao et al., Reference Mao, Ausich, Li, Lin and Lin2017); Chicotte Formation, Cybele Member of the Jupiter Formation, and Ferrum Member of the Jupiter Formation (Telychian), Québec (Ausich and Copper, Reference Ausich and Copper2010; Ausich and Cournoyer, Reference Ausich and Cournoyer2019); Osgood Formation (Telychian to Wenlock), Indiana (Frest et al., Reference Frest, Brett, Witzke, Boucot and Lawson1999); the Welton Member of the Scotch Grove Formation (Telychian–Wenlock), Iowa; Wolcott Limestone (Telychian), New York (Eckert and Brett, Reference Eckert and Brett2001); and the lower Visby Formation (Telychian), Sweden (Angelin, Reference Angelin1878). Of these diverse Llandovery echinoderm faunas, only the Cybele Member of the Jupiter Formation and now the Brassfield Formation have reported cyclocystoids with complete marginal rings. Other Llandovery occurrences with only isolated marginal ossicles have been reported, including those from the Blanding Formation, Iowa (lower Aeronian), the Brassfield Formation in Indiana (Aeronian) (see Frest et al., Reference Frest, Brett, Witzke, Boucot and Lawson1999), and the Telychian of eastern Estonia (Saaremaa) (Hints et al., Reference Hints, Pärnaste, Männik, Reich and Rozhnov2022).
Methods
Minimal preparation was possible due to the fragility of the specimen. Photographs were taken with an Apple iPhone SE attached to the eye piece of a Wild stereomicroscope. The new cyclocystoid specimen is preserved on a bedding surface of crinoidal grainstone, so there is no density contrast between the lower surface of the specimen and the matrix. Thus, implementation of X-ray computerized tomography (CT) scanning techniques would not reveal any morphological detail of the aboral surface of USNM PAL 777895.
Repositories and institutional abbreviations
The holotype of Z.? foerstei n. sp. is deposited in the Springer Room, Smithsonian National Museum of Natural History (USNM PAL).
Systematic paleontology
Classification and terminology follow Smith and Paul (Reference Smith and Paul1982). All measurements are in millimeters.
Class Cyclocystoidea Miller and Gurley, Reference Miller and Gurley1895
Family Cyclocystoididae Miller, Reference Miller1882
Genus Zygocycloides Smith and Paul, Reference Smith and Paul1982
Type species
Zygocycloides variabilis Smith and Paul, Reference Smith and Paul1982.
Included species
Z. magnus (Miller and Dyer, Reference Miller and Dyer1878); Z. marstoni (Smith and Paul, Reference Smith and Paul1982); Z. variabilis Smith and Paul, Reference Smith and Paul1982; and Z?. foerstei n. sp.
Occurrence
Ordovician (Sandbian) to Silurian (Wenlock). North America, Scotland, England, Sweden (Smith and Paul, Reference Smith and Paul1982; Reich and Kutscher, Reference Reich, Kutscher, Harris, Böttger, Walker and Lesser2010).
Remarks
This single, partial cyclocystoid Brassfield specimen with a complete rim of marginal ossicles is a challenge to interpret (Figs. 2, 3). Is the preserved morphology that of an adult; and what, if any, details, including plate sculpturing, have been erased as a result of weathering? The total diameter of this specimen is 6.3 mm. The relatively large marginal rim of ossicles results in a radius of 1.8 mm for the central disc. Thus, the central disc is only ~23.9% of the area of the specimen, which is substantially smaller than any previously described taxon. The percentage of the disc area for Nicholsodiscus Glass et al., Reference Glass, Ausich and Copper2003 is ~47%, and other genera are 60% or more (Glass et al., Reference Glass, Ausich and Copper2003). Similarly, this Brassfield cyclocystoid has 15 marginal ossicles. Perforocycloides Ewin et al., Reference Ewin, Reich, Graham and Cournoyer2019 has 17–20, and Zygocycloides Smith and Paul, Reference Smith and Paul1982 has 18–20. All other cyclocystoids have more, with Polytryphocycloides having as many as 60 marginal ossicles. The ontogenetic stage of this new specimen remains a question. Regardless, this specimen needs to be interpreted.
Figure 2. Zygocycloides? foerstei n. sp., holotype, USNM PAL 777895. (1) Entire specimen. (2) Enlargement of marginal ring ossicles; note that the sutures between ossicles are obscured by calcite interpreted to be interseptal ossicles. Scale bars = 0.5 mm.
Figure 3. Camera lucida drawing of Zygocycloides? foerstei n. sp., holotype, USNM PAL 777895; compare with Figure 2.1. Shaded areas are depressions along plate sutures of adjoining marginal ossicles; arrow above the specimen designates the beginning of measured marginal ossicles in Figure 4. Scale bar = 1.0 mm.
Perforocycloides is distinguished from all other cyclocystoid genera by the presence of pores between marginal ossicles, and Zygocycloides is distinguished by the presence of interseptal ossicles on the dorsal surface. Pores between marginal ossicles are absent in the new Brassfield specimen, but interseptal ossicles are interpreted to be present. The depth of the sutural grooves between adjacent marginal ossicles is present from nearly the distal margin to the proximal margin of these ossicles. However, in a few examples, the distal margin is filled with calcite. This is not a filling of debris and has the surface texture of the remainder of the marginal plate. Indistinct plate sutures and a few plate edges of these presumed interseptal ossicles are visible but very small. The size and preservation make positive identification of these ossicles difficult to verify.
Due to the relatively small number of marginal ossicles and the likelihood that interseptal ossicles are present, this cyclocystoid is assigned to Zygocycloides? Furthermore, as will be discussed, it is very different from other described species of Zygocycloides, and it is considered the new species Z.? foerstei.
Named species of Zygocycloides range in age from Sandbian to Wenlock. Zygocycloides magnus is reported from Katian strata in the Fairmont and Arnheim formations of southwestern Ohio and southeastern Indiana; Z. marstoni is reported from the Acton Scott Beds of Cheney Longville Beds of Shropshire, U.K., Sandbian, and the Katian of Ottawa, Canada, and Cincinnati, Ohio; and Z. variabilis is from the Katian Drummock Group of Threave Glenn and Quarrel Hill, Ayrshire, Scotland, and the Fauquez Shales, Katian?, near Fauquez Castle, Belgium. In addition to the named species, Zygocycloides sp. has been reported from the base of the Eagle Point Member, Dunleith Formation, Galena Group (Katian) of Illinois (Kolata, Reference Kolata1975); Z.? sp. has been reported from the Telychian of Estonia (Hints et al., Reference Hints, Pärnaste, Männik, Reich and Rozhnov2022), Zygocycloides n. sp. 1 has been reported from the lower Visby Formation (Wenlock, Sheinwoodian) on Gotland, Sweden (Reich and Kutscher, Reference Reich, Kutscher, Harris, Böttger, Walker and Lesser2010), and Z. sp. was reported from the Wenlock (Homerian) of Gotland, Sweden (Reich and Kutscher, Reference Reich, Kutscher, Harris, Böttger, Walker and Lesser2010).
Zygocycloides? foerstei new species
Figures 2–4
Figure 4. Outer and inner widths of marginal ossicles around the ring of marginal ossicles (holotype, USNM PAL 777895). The arrow above the specimen in Figure 3 is the first (left) ossicle for which measurements are recorded; subsequent measurements are for marginal ossicles in a clockwise direction from the marginal ossicle indicated by an arrow.
Type
Holotype: USNM PAL 777895.
Diagnosis
Based on characters of the dorsal surface: circular to elliptical test shape, 15 marginal ossicles, dorsal surface of marginal ossicles smooth and slightly convex, dorsal outline of marginal ossicles either modestly tapering trapezoid or rectangular, proximal margin of two adjacent marginal ossicles with slight indentation, and narrow and relatively shallow sutural grooves between adjacent marginal ossicles.
Occurrence
Brassfield Formation (red Brassfield of Sullivan et al., Reference Sullivan, McLaughlin, Brett, Cramer, Kleffner, Thomka and Emsbo2016; Kleffner, Reference Kleffner2020), Oakes Quarry Park, ~2 km east of Fairborn, Greene County, Ohio (39°48′53″N, 84°59′06″W) (Fig. 1); Silurian (Llandovery, middle Aeronian).
Description
Circular test shape (Figs. 2.1, 3), diameter of specimen 63 mm, marginal rim with 15 ossicles (Figs. 2.1, 3). Marginal ossicles relatively large and robust; only dorsal surface exposed; subequal in size but no discernable pattern (Fig. 4); modestly tapering trapezoid or rectangle in dorsal outline (Fig. 4); and surface smooth and slightly convex. Distal margin of marginal ossicles broadly convex with smooth sculpturing; proximal margin between adjacent marginal ossicles sharply convex into an indentation along sutures. Contact between adjacent ossicles along a straight suture in a relatively narrow and shallow radial groove that extends from the outer margin to the inner margin, groove widens slightly proximally. Interseptal ossicles interpreted to be present (but most are not preserved) (Fig. 2b). Pores between marginal ossicles absent.
Disk diameter 3.6 mm, 32.5% of total area. A few distalmost annular ossicles are preserved but collapsed into central cavity; no distinction between radial and interradial annular ossicles. The distal edges of a few peripheral skirt ossicles may be preserved under a small portion of the marginal ossicles. Other details not preserved.
Etymology
The species name foerstei recognizes August Foerste (1862–1936), who was a very influential paleontologist from Dayton, Ohio, and the first person to document many echinoderms from the Brassfield Formation in the Dayton area.
Measurements
Total diameter, 6.3 mm; disk diameter, 3.6 mm.
Remarks
Zygocycloides? foerstei can be distinguished on the basis of dorsal surface characters. Zygocycloides variabilis has a circular to elliptical test shape, 18–20 marginal ossicles, dorsal surface of marginal ossicles smooth and slightly convex, dorsal outline of marginal ossicles modestly tapering trapezoid, proximal margin of two adjacent marginal ossicles with modest indentation, and broad and relatively deep sutural grooves between adjacent marginal ossicles. Zygocycloides magnus has a circular test shape, 20 marginal ossicles, the dorsal surface of marginal ossicles smooth and flat, dorsal outline of marginal ossicles modestly tapering trapezoid, proximal margin of two adjacent marginal ossicles with modest indentation, and broad and relatively deep sutural grooves between adjacent marginal ossicles. Zygocycloides marstoni has a subcircular test shape, 18–20 marginal ossicles, dorsal surface of marginal ossicles smooth or very weakly pitted and flat, dorsal outline of marginal ossicles strongly tapering trapezoid, proximal margin of two adjacent marginal ossicles with deep indentation, and broad and relatively deep sutural grooves between adjacent marginal ossicles. Zygocycloides? foerstei has a circular to elliptical test shape, 15 marginal ossicles, dorsal surface of marginal ossicles smooth and broadly convex, dorsal outline of marginal ossicles either modestly tapering trapezoid or rectangular, proximal margin of two adjacent marginal ossicles with slight indentation, and narrow and relatively shallow sutural grooves between adjacent marginal ossicles.
Paleobiogeography
A major shift among dominant crinoid clades occurred following the Late Ordovician mass extinction from the Ordovician early Paleozoic crinoid fauna to the Silurian to middle Mississippian middle Paleozoic crinoid fauna (Ausich et al., Reference Ausich, Kammer and Baumiller1994; Baumiller, Reference Baumiller, David, Guille, Feral and Roux1994; Ausich and Deline, Reference Ausich and Deline2012; Deline et al., Reference Deline, Ausich and Brett2012). Global recovery of crinoid faunas during the early Silurian was dominated by clades previously established in Laurentia (Ausich and Deline, Reference Ausich and Deline2012).
As argued by Sheffield et al. (Reference Sheffield, Lam, Phillips and Deline2022), understanding biogeographic aspects of the Silurian recovery among other Paleozoic echinoderms has not been accomplished because of a general lack of understanding of clades based on modern phylogenetic methods. However, the sphaeronitid diploporans form a well-defined clade, and their paleobiogeographic history was explored by Sheffield et al. (Reference Sheffield, Lam, Phillips and Deline2022). They concluded that sphaeronitid diploporans dispersed from Baltica to Laurentia during the Late Ordovician. Silurian Laurentian dispersal was confined to Laurentia.
Cyclocystoids are a well-defined clade, but their fossil record is poor, and determination of phylogenetic relationships within the clade is needed. The new Brassfield Formation cyclocystoid is assigned to Zygocycloides?. As noted, Smith and Paul (Reference Smith and Paul1982) recognized three species in Zygocycloides: Z. variabilis from the Katian of Scotland and Belgium, Z. marstoni from the Sandbian of England and Katian of North America, and Z. magnus from the Katian of North America. Reich and Kutscher (Reference Reich, Kutscher, Harris, Böttger, Walker and Lesser2010) also reported Zygocycloides from the Wenlock of Sweden on the basis of isolated marginal ossicles. Zygocycloides? sp. is also reported from the Telychian of Baltica (Estonia). Thus, as known, Zygocycloides was present on both Laurentia and Avalonia during the Ordovician. Zygocycloides questionably remained on Laurentia into the Llandovery, and perhaps by the Telychian, this genus migrated to Baltica and persisted through to at least the Wenlock.
Smith and Paul (Reference Smith and Paul1982) recognized two cyclocystoid genera that survived through the Late Ordovician mass extinction, Reich and Kutscher (Reference Reich, Kutscher, Harris, Böttger, Walker and Lesser2010) extended the range of Cyclocystoides from the Ordovician to the Silurian, and Zygocycloides also has an Ordovician through Silurian range. All four of these genera were present in the Ordovician of Laurentia with Zygocycloides known from both Laurentia and Avalonia. Silurian occurrences of these genera include Polytryphocycloides on both Baltica and Avalonia, Apycnodiscus and Cyclocystoides on Baltica, and Zygocycloides on both Laurentia and Baltica. Judging from these occurrences, it is possible that the four Ordovician–Silurian boundary-crossing cyclocystoids had Laurentia as their center for dispersal to Avalonia and Baltica.
Further, Ewin et al. (Reference Ewin, Reich, Graham and Cournoyer2019) suggested that both cyclocystoids reported from Anticosti Island, Québec (Nicholodiscus and Perforocycloides), were closely related to Zygocycloides, making it possible that Zygocycloides, Nicholodiscus, and Perforocycloides may represent a Laurentian clade. The biogeographic and evolutionary hypotheses suggested in the preceding must be verified with an increased number of fossil occurrences and modern phylogenetic analyses, as discussed by Sheffield et al. (2002).
Acknowledgments
We thank D.B. Blake and M.A. Kleffner for their recommendations on Brassfield asteroids and chronostratigraphy, respectively. We also thank B. Lefebvre, M. Reich, J.R. Thompson, and S. Zamora for their thoughtful reviews that greatly improved this manuscript.
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