Sumrall—Echinoderm homology Oral-plate-/abradial-floor-plate-bearing taxa


Glyptocystitoid rhombiferans as a model.—A common expres- sion of the UEH model is found in glyptocystitoid rhombiferans and forms the archetype for oral-plate-bearing taxa (Fig. 1.1). In glyptocystitoids, seven oral plates are positioned interradially around the peristome with O1, O6, and O7 lying in the CD interray and O2–O5 positioned around the peristomial opening clockwise from O1. The 2-1-2 ambulacral symmetry of Sprinkle (1973) iswell developedwith the shared ambulacra positioned on the O4/O6 and O3/O1 sutures (Fig. 3.1). This results in O2 and O5 being precluded from the peristomial opening by these sutures. The PPCPs are small and separated, with plates 1, 3, and 4 marking the insertion point of the A ambulacrum (when present) and plates 2 and 5 marking the bifurcation points of the lateral B, C, D, and E ambulacra (Fig. 1.1). Double biserial abradial floor plates extend from the oral plates beginning on the left (Fig. 2.10). The ambulacral grooves are covered by undifferentiated cover plates.


Remarks.—The oral system of glyptocystitoids is straightforward. The floor plates are interpreted as the abradial floor plate series because they are broadly exposed along the margins of the ambulacral system and bear brachiole facets along the primary/ secondary sutures of the floor plate bodies. Similar morphologies are seen in the other derived blastozoans and can be interpreted as variations on this theme and easily reconciled with the glyptocystitoid model (Fig. 3). Following are discussions of variation in expression of the oral plate and floor plate systems seen throughout this large echinoderm clade. There is considerable variation among derived blastozoans


with respect to the fate of the lateral oral plates O2 and O5. Some taxa, such as Rhopalocystis, show a simplification of the 2-1-2 symmetry by bearing shorter shared ambulacra (Sumrall and Wray, 2007; Kammer et al., 2013) (Fig. 2.12, 2.13). Such taxa do not bear sutures between O4/O6 and O3/O1 that preclude the lateral oral plates from the peristomial border as in Kammer et al.’s (2013) peristomial border A2. Instead, they have O2 and O5 contact with the peristomial border forming Kammer et al.’s (2013) peristomial border A3. This type of shared ambulacral reduction (pseudo-five-fold symmetry of Sumrall and Wray, 2007) is common in derived stemmed echinoderms, and variations are seen in coronates (Fig. 3.7), blastoids (Fig. 3.8), plesiomorphic crinoids (Fig. 3.9), and several of the diploporitan clades (Figs. 2.11, 3.11).


Blastoids and coronates.—The oral plate systems of blastoids (Fig. 3.8) and coronates (Fig. 3.7) are similar owing to their close phylogenetic relationship (Brett et al., 1983; Donovan and Paul, 1985). In these taxa, oral plates are the deltoids whose adjacent sutures form the proximal-most food grooves (Sumrall and Waters, 2012). In many blastoids, the orals bear large del- toid bodies that extend distally down the theca, becoming important constructional elements of the thecal wall. The ambulacra share the peristome more or less evenly, and conse- quently, these taxa show almost no hint of 2-1-2 symmetry except in the configuration of the primary peristomial cover plates (Fig. 3.7) and a slightly wider than high peristomial


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opening in most blastoids (Fig. 3.8). The CD interray bears two dominate plates, likely O1 and O7, and in blastoids may include accessory plates (cryptodeltoids, etc.). These plates show a variety of relationships to the periproct and spiracles (exit pores of the respiratory system in many blastoid groups), forming the basis for present classifications of blastoids (but see Bauer et al., 2017).


Hemicosmitoids.—In the hemicosmitoid Hemicosmites, the oral area is greatly simplified by paedomorphic ambulacral reduction to include only the A, shared BC, and shared DE ambulacra (Sumrall and Wray, 2007; Sumrall, 2008). The lateral oral plates O2 and O5 do not develop in this taxon, associated with the failure of the shared ambulacra to bifurcate (Fig. 3.6). Enlarged oral plates O7, O3, and O4 occupy the interradial areas of the remaining three ambulacra. Plates O1 and O6 (wedge plates of Bockelie, 1979, in part) are extremely small and lie between O7 and the oral area (Fig. 3.6). The first left brachiole facets lie on the sutures between the first secondary floor plates (wedge plates of Bockelie, 1979, in part) and the remaining oral plates (Sumrall, 2008). Paired, likely fused, floor plates positioned between the oral plates form facets for erect, brachiole-bearing ambulacra of the distal ambulacra (Sprinkle, 1975) (see the following).


Diploporitans.—The diploporitan Protocrinites (Fig. 3.3) shows an oral plate arrangement that is consistent with the glyptocystitoid model. Seven interradial oral plates are present, with O2 and O5 being precluded from the peristome by the BC and DE shared ambulacral grooves. As the result, only O1, O3, O4, and O6 form the peristomial opening. The hydropore and gonopore are positioned in the CD oral plate complex. Some diploporitan clades, such as Glyptosphaerites and


Eucystis,show a 36º clockwise rotation of the oral plates with respect to the ambulacra (Fig. 3.5). Here, the main food grooves lie atop the oral plates (repeatedly branching in Eucystis), rather than along the adjacent sutures, and extend distally irregularly down the theca. The oral plates are diagnosable despite their radial position because they retain the plesiomorphic seven-plate oral configuration condition (Fig. 3.5). Plates O1, O6, and O7 are positioned in the CD interradius as evidenced by 2-1-2 ambulacral symmetry, the primary peristomial cover plate configuration, and the positions of the hydropore, gonopore, and periproct (Fig. 3.5). The PPCPs, however, retain the plesiomorphic interradial position. Holocystitid diploporitans are modified from the plesio-


morphic condition by the addition of a differentiated, open facetal circlet positioned distally to the oral circlet (Fig. 3.11). This has led to considerable confusion of homology in the clade, but all taxa seem to retain the plesiomorphic seven oral plate condition (see Sheffield and Sumrall, 2015; 2017). Generally, extremely short food grooves extend along oral plate sutures epithecally to large appendage facets mounted on differentiated thecal plates (facetals). Here, the oral plates tend to be rather small, and food grooves are confined to their sutures or adjacent plate surfaces. This same pattern is seen in nearly all taxa, but the facetal count may be reduced in Pustulocystis and there may be additional oral plates in Brightonicystis (Paul 1971;


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