Recovery of Laurentian cyclocystoids following Late Ordovician extinctions (Brassfield Formation, Llandovery; southwestern Ohio)

Abstract. Zygocycloides? foerstei n. sp. is described from the Llandovery (Aeronian) Brassfield Formation of southwestern Ohio. This is among the oldest reported Silurian cyclocystoids from North America and is the only North American Llandovery cyclocystoid that is preserved with a complete rim of marginal ossicles. Zygocycloides Smith and Paul, 1982 is most similar to Nicholsodiscus Glass et al., 2003 (Katian) and Perforocycloides Ewin et al., 2019 (Llandovery, Telychian), both from Anticosti Island, Québec. Cyclocystoids (Ordovician to Mississippian) survived Late Ordovician extinctions, and this discovery documents that this echinoderm clade was part of shallow-water, marine paleocommunities during the initial post-extinction transgression onto the Laurentian platform.

The cyclocystoid clade was clearly devastated by Late Ordovician extinctions (Smith and Paul, 1982, fig. 14), but relatively little is known about their recovery during the Llandovery. Although reports of Llandovery isolated marginal ossicles exist (Frest et al., 1999;Hints et al., 2022), only two Llandovery cyclocystoids with a complete ring of marginal ossicles have been described previously, including Polytryphocycloides davisii (Salter in Salter and Billings, 1858) from the Mulloch Hill Sandstone in Scotland (Smith and Paul, 1982) (upper Rhuddanian; Donovan et al., 2012, p. 155) and Perforocycloides nathalieae Ewin et al., 2019 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.
The Brassfield Formation in Oakes Park Quarry contained a small, coral-stromatoporoid reef and reef-associated facies (Sheehy, 1979;Schneider and Ausich, 2002;Schmidt et al., 2007). 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, 2002). 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 *Corresponding author. unstudied, including, among others, corals, stromatoporoids, brachiopods, and bryozoans.

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 (1982). All measurements are in millimeters.
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., 2003 is ∼47%, and other genera are 60% or more (Glass et al., 2003). Similarly, this Brassfield cyclocystoid has 15 marginal ossicles. Perforocycloides Ewin et al., 2019 has 17-20, andZygocycloides Smith andPaul, 1982 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.
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 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, 1975); Z.? sp. has been reported from the Telychian of Estonia (Hints et al., 2022), Zygocycloides n. sp. 1 has been reported from the lower Visby Formation (Wenlock, Sheinwoodian) on Gotland, Sweden (Reich and Kutscher, 2010), and Z. sp. was reported from the Wenlock (Homerian) of Gotland, Sweden (Reich and Kutscher, 2010).
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.
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 (1862Foerste ( -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.

Ausich and Zehler-Early Silurian cyclocystoid
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., 1994;Baumiller, 1994;Ausich and Deline, 2012;Deline et al., 2012). Global recovery of crinoid faunas during the early Silurian was dominated by clades previously established in Laurentia .
As argued by Sheffield et al. (2022), 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 welldefined clade, and their paleobiogeographic history was explored by Sheffield et al. (2022). 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 (1982) 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 (2010) 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 (1982) recognized two cyclocystoid genera that survived through the Late Ordovician mass extinction, Reich and Kutscher (2010) 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. (2019) 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).