Locality 1

A specimen of Bromidocycloides sprinklei n. gen. n. sp. (NPL 00070048) was collected from section 13B of Sprinkle (Reference Sprinkle1982); the Dunn Quarry, Criner Hills, Carter County, Oklahoma; 34°06'20.49"N, 97°10'52.97"W; Upper Ordovician (Katian), Simpson Group, Bromide Formation, Pooleville Member.

Locality 2

Seven specimens of Moroccodiscus smithi (INHS-P5120–P5126) recently collected by Hassane M. Hamdi in the Anti-Atlas Mountains of southern Morocco, 4.8 km SE of road N12 at Battou, southeastern Morocco; 30°57'49.88"N, 5°14'30.66"W; Middle Ordovician (Darriwilian), First Bani Group, Taddrist Formation. A single specimen of Moroccodiscus smithi (CASG 103597), newly acquired by legal purchase, was also available for study. Overall lithology, preservation, and information received with this specimen suggest that it was collected at approximately the same stratigraphic unit at or near Locality 2.

Terminology

Application of the EAT for the determination of echinoderm skeletal homologies and topologies revealed that cyclocystoids possess key anatomies homologous with those of other pentaradial echinoderms (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025). Accordingly, the epispire-bearing central disk is extraxial in origin, and the ring of stout ossicles surrounding the central disk are floor plates (axial in origin) bearing tube foot basins, a circumferential channel, and radial ducts, all housing specific elements of the water vascular system. A peripheral system of primary cover plates positioned adjacent to the tube foot basins and numerous small secondary cover plates occurred in a more distal position around the perimeter of the disk-shaped theca. These morphologies offer a model significantly different from that presented in the significant cyclocystoid monograph published by Smith and Paul (Reference Smith and Paul1982). A correlation table of morphologic terms was published by Kolata et al. (Reference Kolata, Mooi, Guensburg and Koniecki2025, table 1) as a guide to the necessary changes in nomenclature that reflect these newly revealed homologies with other Echinodermata.

Methods

ImageJ, an image processing software program (https://doi.org/10.1038/nmeth.2089), was used to measure and compare percentage surface area covered by epispires in several species of cyclocystoids, including Bromidocycloides n. gen., Brechincycloides Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025, and Moroccodiscus Reich et al., Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017. We also analyzed two Cambrian forms—Stromatocystites pentangularis Pompeckj, Reference Pompeckj1896 and Camptostroma roddyi Ruedemann, Reference Ruedemann1933—to determine the likely outgroup condition. Three quadrats were analyzed from specimens with exceptional preservation of the aboral surface, and of approximately equal diameters.

Repositories and institutional abbreviations

The single known specimen of Bromidocycloides sprinklei n. gen. n. sp. is housed in the nonvertebrate collection at the University of Texas Museum of Paleontology (NPL), Austin, Texas. The new specimens of Moroccodiscus smithi are housed in the nonvertebrate collection at the Prairie Research Institute Center for Paleontology, Urbana, Illinois, and the Invertebrate Zoology and Geology Department of the California Academy of Sciences (CASG), San Francisco, California. Other cited repositories are: The Natural History Museum, London (BMNH); Field Museum of Natural History, Chicago, Illinois (PE); Hunterian Museum, Glasgow, Scotland (HM); Institute of Geological Sciences, U.K. (Geological Survey Museum) (IGS GSM); Illinois Natural History Survey, Urbana-Champaign, Illinois (INHS); Národni (National) Museum of Prague, Czech Republic; Collections de Paléontologie, Laboratoire de Géologie, Lyon 1 Université, Villeurbanne, France (UCBL-FSL); University of Michigan, Museum of Paleontology, Ann Arbor, Michigan (UMMP).

Type and only genus

Bromidocycloides new genus.

Diagnosis

Proximal edge of each floor plate bearing wedge-shaped shelf with crescentic facets on aboral surface; thin, dome-shaped stereomic plate covering each epispire on aboral surface of central disk; tube foot basin side walls distinctively straight, intersecting at angles of 25–35° and forming unique zigzag pattern in lateral view; distal aboral surface of floor plate marked by prominent U-shaped depression with distal end curving down into tube foot basin; central aboral ramular plate pentaradial with periproct at center.

Age and occurrence

Late Ordovician, Oklahoma, USA

Genus Bromidocycloides new genus

Type species

Bromidocycloides sprinklei n. gen. n. sp.

Diagnosis

As for the family.

Etymology

Named for the Upper Ordovician Bromide Formation in the state of Oklahoma where the holotype was found, combined with the Greek kykloeidēs = circular.

Remarks

As for species, see below.

Bromidocycloides sprinklei new genus new species

Figures 1–3, 4.3

Figure 1.

Bromidocycloides sprinklei n. gen. n. sp., holotype NPL 00070048, locality1, showing aboral surface of folded and partly disarticulated central disk with ring of ambulacral plates and prominent pentaradial ramular plate complex containing periproct at center. Ramular and interramular plates form reticulate framework with large epispires covered by thin stereomic domal plates. Growth zones marked by small (incipient stage) wedge-shaped floor plates (yellow). Notice gradational increase in size of floor plates on either side of incipient plate. Oldest floor plates (violet) have two tube foot basins.

Figure 1. Long description

The photo displays the aboral surface of a fossilized central disk.

* At the center, an arrow points to the pentaradial central aboral ramular plate with the periproct at its core, where a thin stereomic domal plate covers five flanking epispires.

* Extending from the center, a long white arrow identifies a channel on the aboral side of the ramular and interramular plates.

* The surrounding reticulate framework consists of plates with large epispires.

* Along the outer ring, small wedge-shaped floor plates in an incipient stage are highlighted in yellow. Yellow arrows point to these plates, with curved white arrows indicating a gradational increase in plate size on either side of the incipient stage.

* The oldest floor plates are highlighted in violet and labeled with the number 2. A text label notes that these violet plates are positioned where the growth arrows converge and contain two tube foot basins.

* A 1 mm scale bar is located in the bottom-right corner.

Figure 2.

Bromidocycloides sprinklei n. gen. n. sp., holotype NPL 00070048, locality 1: (1) edge view of six floor plates showing zigzag pattern of tube foot basin side walls, tubercles (red), nodule, and circumferential channel; (2) aboral surface of two floor plates showing ramular (green) and crescentic facets; (3) side view of floor plate showing extended shelf, circumferential channel, and lateral striae; (4) edge view of floor plates showing tube foot basin side walls, primary (blue) and secondary cover plates (yellow); (5) aboral view of floor plates, primary (blue) and secondary (yellow).

Figure 2. Long description

A multi-panel fossil analysis.

Panel 1 at the top shows an edge view of six floor plates. A zigzag pattern of tube foot basin side walls is visible. Red dots highlight tubercles at the center of each tube foot basin. A nodule is located at the top edge, and a circumferential channel runs horizontally along the right side. A 1 mm scale bar is provided.

Panel 2 at the middle-left shows the aboral surface of two floor plates. Crescentic facets are at the base, leading up to a green-shaded ramular facet. Dashed lines at the top indicate the reconstructed distal end of the floor plate. A 1 mm scale bar is at the bottom right.

Panel 3 at the middle-right shows a side view of a single floor plate. It features an extended shelf on the right with a green ramular facet on top. Below the shelf are lateral striae. A circumferential channel is notched into the left side. A 1 mm scale bar is present.

Panel 4 at the bottom-left shows an edge view of floor plates. Blue-shaded primary cover plates and yellow-shaded secondary cover plates are positioned above the tube foot basin side walls. A 1 mm scale bar is at the bottom.

Panel 5 at the bottom-right provides an aboral view. A large yellow-shaded area represents secondary cover plates, situated above smaller blue-shaded primary cover plates that sit atop the floor plate structure. A 1 mm scale bar is at the bottom right.

Figure 3.

Bromidocycloides sprinklei n. gen. n. sp., holotype NPL 00070048, locality 1: (1) magnified image of aboral surface showing network of ramular and interramular plates supporting epispires; (2) same surface showing thin stereomic domal plates covering epispires. Tips of yellow arrows show weathered edges of thin stereomic domal plates covering epispires.

Figure 3. Long description

A two-panel grayscale microscopic study of a fossil surface. A 1 mm scale bar is located at the bottom right.

Panel 1 (Top): A magnified view of the aboral surface. The surface consists of a dense, honeycomb-like network of raised ridges. Three white arrows on the left point to the network of ramular and interramular plates that form the structural borders around circular depressions called epispires.

Panel 2 (Bottom): A similar view of the surface with additional anatomical details.

- On the far left, two white arrows point to floor plates which appear as porous, deeper structural elements.

- Two white arrows in the upper-left quadrant point to thin stereomic domal plates that are partially covering the circular epispires.

- Eight small yellow arrows point to the specific weathered edges of these thin stereomic domal plates, highlighting where the covering has been eroded to reveal the underlying epispire cavity.

Figure 4.

Three types of epispire morphology in cyclocystoids: (1) Scotiadiscus wrighti Begg, Reference Begg1939, holotype HM E5073, latex peel of annular plates resting on and concealing ramular and interramular plates; (2) Brechincycloides stanhynei Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025, paratype UMMP 833167, epispires surrounded by raised rims; (3) Bromidocycloides sprinklei n. gen. n. sp., holotype NPL 00070048, each epispire covered by a thin stereomic domal plate.

Figure 4. Long description

Panel 1, at the top center, shows Scotiadiscus wrighti. It features a dense honeycomb-like texture. Arrows point to the opening of an epispire and the annular plates that surround the epispires, concealing the ramular and interramular plates of the central disk.

Panel 2, at the bottom left, shows Brechincycloides stanhynei. The surface is more granular and pustulose. Arrows identify the opening of an epispire surrounded by a raised rim and the pustulose aboral ramular and interramular plates of the central disk.

Panel 3, at the bottom right, shows Bromidocycloides sprinklei. This specimen features a reticular network of ramular and interramular plates of the central disk. Arrows point to thin stereomic domal plates that cover the epispires.

A 1 mm scale bar is located at the bottom center between panels 2 and 3.

Holotype

University of Texas Museum of Paleontology, NPL 00070048.

Diagnosis

As for the family.

Description

New species based on single, partially preserved specimen exposing aboral surface of floor plates, primary and secondary cover plates, tube foot basins, tubercles, and central disk with epispires, each covered by thin, domal stereomic plate (Figs. 1–3, 4.3). Thecal diameter (including cover plates) 16.2 mm. In aboral view, center of disk occupied by prominent pentagonal ramular plate 3 mm in diameter. Plate possesses central pore ~0.8 mm in diameter, appearing to be single ossicle. Five series of ramules radiating from central ramular plate, bifurcating three times; distalmost ramules imbricating with aboral side of extended proximal shelf on floor plates. Central disk and floor plates constituting ~80% of thecal diameter. Ramular-interramular plates forming reticulate network; in aboral view, plates imbricating away from center. Aboral surface of more proximal plates marked by prominent channel diminishing distally to slightly beyond first bifurcation. Epispires relatively large, each covered by thin, circular to domal stereomic plate.

Floor plates estimated to be 55 in number; in aboral view, plates rectangular to trapezoidal in outline. Width of each plate 50% of its length. Floor plates typically with one or two tube foot basins organized in pattern of ‘21111211112’ where central 2 represents one of five growth zones occupied by incipient plate bearing two tube foot basins when fully grown. Aboral surface of each floor plate bearing relatively large pits and depressed lateral edges, and distal margin of aboral surface with distinctive U-shaped depression bearing small node positioned in line with, but not connected to, tubercle (Figs. 1, 2.1, 2.2, 2.5). Tube foot basin side walls distinctively straight, intersecting at angles of 25–35°, and forming conspicuous zigzag pattern in lateral view (Fig. 2.1, 2.4). Proximal shelf of floor plate one-third of length of plate. Proximal margin commonly cleft and surrounding single epispire (Fig. 2.2). Crescentic facets curved. Oral surfaces of floor plates with subtle saddle, plates broadly curved, smooth, and lacking tubercles. Approximately 12 striae occupying oral edge of articulation surface between floor plates. Circumferential channel well developed.

Cover plate system consisting of relatively large, linguiform primary plate, the latter notably concave on proximal, oral portion (Fig. 2.4). Secondary cover plates relatively small, linguiform to arrow-shaped, arranged in approximately five rows, diminishing in size distally.

Etymology

Named in honor of James Sprinkle for his landmark contributions to understanding the diverse echinoderm faunas of the Late Ordovician Bromide Formation of Oklahoma, USA.

Preservation

The single available specimen is a well-preserved calcitic skeleton positioned in aboral aspect on a bedding plane of argillaceous, diversely fossiliferous wackestone. Surface detail is very good except for a few regions that have been damaged by aggressive preparation with an air abrasive tool. Part of the specimen is draped over a well-preserved theca of the rhombiferan Tanaocystis watkinsi Sprinkle, Reference Sprinkle1982.

Remarks

The only other evidence of a cyclocystoid from the Bromide Formation of Oklahoma is a single floor plate of Cyclocystoides (Kolata, Reference Kolata1982, p. 309).

Bromidocycloides n. gen. resembles Moroccodiscus smithi in having an ambulacral ring of ~55 floor plates in growth sectors exhibiting the arrangement of ‘21111211112’, where 2 marks a growth zone with a nascent floor plate.

Family Brechincycloididae Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025

Genus Moroccodiscus Reich et al., Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017

Type and only species

Moroccodiscus smithi Reich et al., Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017 from the Middle Ordovician (Darriwilian), First Bani Group, Taddrist Formation, southeastern Morocco, by original designation.

Occurrence

Seven newly discovered specimens from the Middle Ordovician (Darriwilian), First Bani Group, Taddrist Formation, southeastern Morocco, were examined. These specimens are preserved as molds in siderite-encrusted concretions like those described by Reich et al. (Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017).

Description of Moroccodiscus smithi

The original description of Moroccodiscus smithi (see Reich et al., Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017) provided important morphological details, many of which we do not repeat here. The following description focuses on information gained from newly discovered specimens, with emphasis on the central disk, ambulacral growth zones, and tube foot basins.

Central disk composed of single layer of subpolygonal ramular and interramular plates bifurcating distally at least three times. Two types of epispires: round pores ~0.1 mm in diameter occurring within plate; and relatively large, oval, or hourglass-shaped pores up to 0.8 mm in length positioned between plates (Figs. 5, 6). Central ramular plates bearing medial pore 1.0 mm in diameter (Fig. 6). Oral surfaces of ambulacral floor plates possessing well-defined tube foot basins (“cupules” of Smith and Paul, Reference Smith and Paul1982 and referenced as such by Reich et al., Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017) not apparent in oral view because overhanging distal edge of floor plate crests covers them (Figs. 5, 6). Floor plate side walls curved; facet canal openings, radial ducts, circumferential channel, and prominent proximal shelf well exposed. Floor plates twice as long as wide. Primary cover plates twice as long as wide, terminating distally in one to three blunt spines. Oral side of each floor plate marked by proximal, triangular depression corresponding with tube foot basin of adjacent floor plate. Secondary cover plates elongate, forming wide peripheral skirt. Two radial ducts (Figs. 5, 6). Tube foot basins visible and well defined when viewed along edge of disk-shaped body. Basins oval, side walls funnel-shaped, gradually descending into radial ducts confluent with circumferential channel (Fig. 5.1). Water vascular system connected to internal ampullae via radial ducts within and between floor plates (Fig. 6.2).

Figure 5.

Moroccodiscus smithi Reich et al., Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017, topotype INHS-P5120, latex peels: (1) edge view of floor plates showing tube foot basins and side walls, radial ducts, circumferential channel, primary and secondary cover plates (incipient (youngest) plate in growth zone shown in yellow; radial ducts positioned to far right and left margins of plate; oldest floor plates shown in violet); (2) plan view of oral surface of same specimen showing growth zone, growth sector positioned between red lines, internal opening to radial duct, large and small epispires, and facet canal openings (colors as in Fig. 5.1).

Figure 5. Long description

A two-panel set of grayscale microscopic images with color-coded highlights and a 5 mm scale bar.

Panel 1 shows an edge view of floor plates. From the lower left to the upper right, the structure reveals:

* Violet-colored regions at the far left and right margins representing the oldest floor plates.

* A central yellow-highlighted region indicating an incipient youngest plate in the growth zone.

* Anatomical labels point to the tube foot basin, the radial duct connecting the tube foot to the internal ampulla, and the circumferential channel.

* The upper margin features the side wall of the tube foot basin, concavity in the cover plate, the primary cover plate, and secondary cover plates.

Panel 2 shows a plan view of the oral surface.

* Two red lines define a growth sector.

* A yellow arrow points to the growth zone at the top edge.

* Ambulacral floor plates are arranged in a radial pattern.

* Violet highlights mark the oldest plates at the left and bottom right, while a yellow highlight marks a younger plate near the center-right.

* Internal openings of radial ducts are visible as small dark pores.

* The lower section identifies large and small epispires, facet canal openings, and the proximal extended shelf of the floor plate.

Figure 6.

Moroccodiscus smithi Reich et al., Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017 topotype INHS-P5125, latex peels: (1) aboral surface showing central ramular plates with periproct; incipient floor plate (yellow) at center of growth zone; (2) oral surface of same specimen showing growth sectors positioned between red lines; floor plates with two tube foot basins (violet) occuring in pairs, each plate on either side of boundary between two adjacent growth zones. Note (white arrows): on proximal side of floor plate showing radial ducts shared with adjacent floor plate.

Figure 6. Long description

A two-panel scientific photo of fossil latex peels with a 5 millimeter scale bar at the bottom right.

Panel 1 shows the aboral surface. At the proximal center is the central aboral ramular plate with a periproct in a large central pore. Moving outward, red lines define growth sectors. White arrows point to epispires located between ramular and interramular plates. Along the curved distal margin, primary and secondary cover plates are visible. A single yellow-shaded region at the top center identifies an incipient floor plate marking the growth zone. Two violet-shaded regions on the left and right margins identify the oldest floor plates.

Panel 2 shows the oral surface of the same specimen. Red lines originate from the proximal center to define growth sectors. Along the distal arc, floor plates are visible. Two violet-shaded sections highlight pairs of floor plates, each containing two tube foot basins. White arrows at the top center point to the proximal side of a floor plate, indicating radial ducts shared with the adjacent plate. Two yellow-shaded regions highlight incipient floor plates at the center of growth zones.

Ontogeny

Ambulacral growth zones are preserved in various stages of development depending on the thecal diameter at time of death. The holotype of Moroccodiscus smithi (UCBL-FSL 712001) remains the most complete and mature specimen available. It is a relatively large specimen possessing a ring of 54 floor plates divided into five sectors, four with a pair of large floor plates, each with two radial ducts, and single large floor plate with only two radial ducts (Reich et al., Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017, fig. 8.1). The overall arrangement of floor plates reveals a pentaradial body plan with five growth zones marked by the presence of small, incipient floor plates (Fig. 6.1). Arranged between each of these five sectors is series of eight or nine floor plates, each of which possesses a single radial duct except for a central, youngest nascent plate.

Cyclocystoid skeletal homologies

As described by Kolata et al. (Reference Kolata, Mooi, Guensburg and Koniecki2025) and supported with new specimens, there is significant evidence to conclude that the central disk in cyclocystoids is composed of perforate extraxial plates. This is strongly indicated by abundant and morphologically diverse epispires. The central disks in Bromidocycloides n. gen. and Moroccodiscus are composed of ramular and interramular plates arranged in a pentaradial pattern and bearing epispires. The periproct is positioned at the center of the disk (Figs. 1, 6.1). The gonopore and/or hydropore have not been observed in Bromidocycloides n. gen. but are assumed to have been positioned near the margin of the disk adjacent to the floor plates, as in Brechincycloides (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025).

Central disk and epispires

In all known cyclocystoids, the central disk is composed of perforate extraxial plates (ramulars and interramulars) housing epispires (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025). At least three morphologically distinct types of epispires are known to occur in cyclocystoids. These include Type 1, found in cyclocystoids like Cyclocystoides Salter and Billings, Reference Salter and Billings1858 and Scotiadiscus Reich and Smith, Reference Reich and Smith2008, and possess a unique annular ossicle surrounding each epispire on the aboral surface (Figs. 4.1). In many cyclocystoids possessing this type, the annular ossicles are irregular in shape and pulled apart, suggesting that they were flexible and could be repositioned during life (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025). Type 2, typical of Brechincycloides, are epispires with a thin, rigid, elevated rim (Fig. 4.2), similar to| those in eocrinoids like Akadocrinus jani Prokop, Reference Prokop1962. Other cyclocystoids with Type 2 epispires include Moroccodiscus, in which epispires lack rims, or have low profile rims surrounding the epispires. Type 3 epispires are exemplified by those of Bromidocycloides n. gen. and are distinguished by having relatively large epispires, each of which is covered by a thin domal plate made of stereom (Figs. 3, 4.3). Epispire Types 1–3 are all sutural pores. Cyclocystoids like Brechincycloides and Moroccodiscus possess epispires (sutural pores) and equally sized or slightly smaller pores that occur within the thecal plates (e.g., Kolata et al. Reference Kolata, Mooi, Guensburg and Koniecki2025, fig. 12).

The functional significance of the relatively large epispires and associated domal plates in Bromidocycloides n. gen. was probed through an analysis of the percent coverage by the epispires on the extraxial part of the aboral surface (Fig. 4.2, 4.3). The results show Bromidocycloides n. gen. devotes nearly 10 times the area of the extraxial region on the aboral surface to epispires than Brechincycloides (Table 1). Such enlarged epispires also far exceed the dimensions of epispires in other cyclocystoid taxa measured, as well as in any of the other noncyclocystoids (Table 1). This strongly suggests that the epispires in Bromidocycloides n. gen. are highly derived. The physiological significance of this is difficult to estimate without knowing diffusion rates through the thin plates covering the epispires. However, these preliminary results suggest that the increase in area can be interpreted as an adaptation by Bromidocycloides n. gen. to increase the area over which gaseous exchange can occur, thereby compensating for any decrease in diffusion rates due to the presence of the domal plates. This, in turn, raises the question of the purpose of the stereomic plates covering the epispires in Bromidocycloides n. gen. Epispires represent weak points for entry into the coelom that could be used by parasites and other harmful organisms. Bromidocycloides n. gen. might be trying to deter such entry in a way that does not necessitate complete elimination of epispires.

Table 1.

Comparison of percentage areas occupied by epispires in the extraxial regions of six cyclocystoid species and two early pentaradiate echinoderms

Table 1. Long description

The table consists of three columns: Taxon and samples, Area (150 x 150 mu m quadrat) occupied by epispires (mu m super 2), and Area occupied by epispires (percent).

* Brechincycloides stanhynei (U M M P 83167): Mean area 1668.2 mu m super 2 (7.4 percent).

* Bromidocycloides sprinklei n. gen. n. sp. (N P L 00070048): Mean area 11136.9 mu m super 2 (49.5 percent).

* Moroccodiscus smithi (C A S G 103597): Mean area 3587.5 mu m super 2 (15.9 percent).

* Scotiadiscus wrighti (holotype H M E5073): Area 28.1 percent (external) and 35.5 percent (internal).

* Zygocycloides variabilis (paratype B M N H E29050): Area 21.3 percent.

* Zygocycloides marstoni (holotype I G S G S M 60303): Area 19.3 percent.

* Stromatocystites pentangularis (C A S G 103662): Mean area 3231.4 mu m super 2 (14.4 percent).

* Stromatocystites pentangularis (Narodni Museum 36675): Mean area 3883.4 mu m super 2 (17.3 percent).

* Camptostroma roddyi (P E 93326): Mean area 4909.4 mu m super 2 (21.8 percent).

The epispires in the central disk are evidence that the reticulate framework of ramulars and interramulars are integral to the perforate extraxial region. Although the central disk in cyclocystoids can have a pentaradial framework, e.g., in Zygocycloides Smith and Paul, Reference Smith and Paul1982 and Sievertsia Smith and Paul, Reference Smith and Paul1982, it can also display fourfold (Apycnodiscus Smith and Paul, Reference Smith and Paul1982 and Polytryphocycloides Smith and Paul, Reference Smith and Paul1982) or sixfold (Cyclocystoides) symmetries. The fourfold and sixfold conditions are not consistent with the nearly universal expression of pentaradiality in the axial system that characterizes the phylum. This provides additional evidence supporting the conclusion that the cyclocystoid water vascular system was not positioned in the central disk but rather in the outer ring of morphologically complex (axial) ossicles (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025).

Cyclocystoid growth zones and the ocular plate rule (OPR)

Echinoderms have serial plates along their radii that form according to the ocular plate rule (OPR), a type of terminal growth in ambulacral plates that characterizes the rays as growth zones in all echinoderms. The growth zone of a given ray in an echinoderm is defined as all of the plates in the series that are first added at the blastema of that zone (Mooi and David, Reference Mooi and David1997; Mooi et al., Reference Mooi, David and Wray2005).

Pentaradial symmetry in cyclocystoids is clearly expressed in the growth pattern of the marginally positioned ring of ambulacral floor plates (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025). Specimens fossilized during early to midlife commonly possess one or more relatively small incipient floor plates. These were called “stunted” ossicles by Reich et al. (Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017). However, each of these plates is not stunted so much as nascent, being most recently added elements that appear near the center of one of the five growth zones within the ring of ambulacral plates (Fig. 1). For example, as shown in Bromidocycloides n. gen. and Moroccodiscus, the youngest floor plate in the five sectors is adjacent to the center of a growth zone, supporting the idea that this plate is also directly adjacent to the site of a blastema at which plate addition occurs according to the OPR (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025). It should be noted that cyclocystoids lack the calcified terminal (= ocular) plate that marks the position of the blastema in all members of the echinoderm clade that includes asterozoans and echinozoans (in fact, the terminal, or ocular, plate is a synapomorphy for that clade). By definition, the incipient (or nascent) plates are the youngest floor plates in the growth zone, and do not represent the terminal (or ocular) plate. A nascent plate in mature Bromidocycloides n. gen. and Moroccodiscus bears two tube foot basins and is commonly flanked on right and left sides by four floor plates, each bearing a single tube foot basin followed by one floor plate (oldest in sector) bearing two tube foot basins (Figs. 5, 6). This arrangement of floor plates comprises one of five ‘sectors’ of growth that form the complete ring of floor plates. In some cyclocystoids, like the type specimens of Cyclocystoides scammaphoris (Smith and Paul, Reference Smith and Paul1982), the individual growth sectors can be difficult to identify because all the floor plates have two tube foot basins. However, even in this case, the presence of a single, relatively small incipient growth stage floor plate can be helpful in identifying the five growth zones.

Cyclocystoid water vascular system

The ring canal of the water vascular system most likely encircled the distal side of the floor plates within the circumferential channel (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025, fig. 15). Within the five growth zones, new tube feet, basins, and associated ampullae grew in the space between ambulacral floor plates (Fig. 6.2). The water vascular tissue was progressively overgrown by the stereom of adjacent floor plates. One to three tube feet are often incorporated within a single floor plate via the radial ducts. In some cyclocystoids like Cyclocystoides scammaphoris, as observed by Kolata et al. (Reference Kolata, Frank, Kaplan and Guensburg2023, p. 640) and Minicycloides carbonicus Haude and Thomas, Reference Haude and Thomas1994 (fig. A, table 1), there can be as many as five tube foot basins and associated radial ducts in a single floor plate.

If cyclocystoids evolved from a pentaradial ancestor like Stromatocystites Pompeckj, Reference Pompeckj1896 by unzipping along the perradial suture of each ambulacrum (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025, fig. 19), this would result in the five growth zones of ambulacral plates being evenly repositioned in a monoserial ring of floor plates along the outer margin of the disk-shaped theca. Furthermore, the ambulacral plates in all five rays nearest to the mouth in the pentaradial ancestor would be repositioned in the monoserial ring as five separate plates, or pairs of plates, evenly spaced between the five growth zones. This topology appears to characterize all known cyclocystoids. As discussed by Kolata et al. (Reference Kolata, Mooi, Guensburg and Koniecki2025), although the occurrence of multiple tube feet within a single floor plate is not the norm for pentaradial echinoderms, this condition in cyclocystoids likely benefitted the animal by increasing the number of tube feet without compromising the robustness of the disk-shaped body.

Cover plates

Bromidocycloides n. gen. and Moroccodiscus, like all known cyclocystoids, possess a cover plate system consisting of relatively large, proximally positioned primaries and small secondary plates that diminish in size distally. The primary cover plates cover most of the tube foot basin. Moroccodiscus is unique in possessing primaries that are twice as long as wide and deeply excavated on the oral surface at the proximal end adjacent to the tube foot basin.

Remarks

The original description by Reich et al. (Reference Reich, Sprinkle, Lefebvre, Rössner, Zamora and Rahman2017) stated that no ‘cupules’ (tube foot basins) are present. This was refuted by Kolata et al. (Reference Kolata, Mooi, Guensburg and Koniecki2025) and their presence confirmed by newly collected specimens figured and discussed herein (Fig. 5). Five floor plates that have two radial ducts each are arranged between the five growth sectors. All other floor plates have a single radial duct. A similar pentaradial arrangement of floor plates is present in Bromidocycloides n. gen. (Fig. 1), Brechincycloides (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025, figs. 3.3, 18), Zygocycloides (Kolata et al., Reference Kolata, Mooi, Guensburg and Koniecki2025, fig. 5.2), and Cyclocystoides (Kolata et al., Reference Kolata, Frank, Kaplan and Guensburg2023, figs. 2.1, 3.1).