612


Journal of Paleontology 91(4):604–617


this clade bears the abradial floor plates comes largely from the nature of the expression of these plates. In Kailidiscus and Edriophus (Fig. 2.2, 2.7), the abradial floor plates are biserial and are widely expressed on the thecal exterior, perradially to the food groove. Ambulacral cover plates cover the food groove hiding the perradial suture from exterior view. A similar situa- tion exists among derived blastozoans. Here, widely expressed biserial, or in some cases double biserial, floor plates arise from the oral plates with a relatively narrow food groove and in most cases widely externally expressed floor plate bodies abradial to the food groove (Fig. 2.10–2.12). This floor plate body typically bears facets for brachioles in a number of different configura- tions with respect to the plate sutures (Sumrall, 1997).


In derived oral-plate-bearing blastozoans, evidence that


glyptocystitoids, the abradial floor plates are positioned as in Rhopalocystis, with biserial or double biserial floor plates forming the thecalwall. In the parablastoid Eurekablastus (Fig. 2.6), plating is similar, except that the side food groove lies along the floor plate sutures and distally extends onto brachiole facets that lie on the center of the abradial edge of the floor plates (Sprinkle and Sumrall, 2008). In this case, there is a 1:1 correspondence of floor plates to cover plates over the main food groove—a situation argued to be unique to crinoids and edrioasteroids (Guensburg et al., 2016). Taxa with this configuration can also house respiratory structures in the floor plate bodies as seen with the floor-plate-borne diplopores of Dactylocystis and Tristomiacystis (Sumrall et al., 2009).


Variation in floor plate expression.—Rhopalocystis (Fig. 2.12, 2.13) shows a typical configuration for the abradial floor plates of an oral-plate-bearing blastozoan. Abradial floor plates extend from the sutures of the oral plate series with the first plate on the left (Fig. 2.12), and these plates structurally form the thecal wall. The narrow primary food groove lies along the perradial suture between right and left side floor plates. Cover plates cover the food groove leaving the broad floor plate bodies exposed abradially to the food groove (Fig. 2.13). Side food grooves arise from the primary food grooves, leading to brachiole facets centered on each plate. Other abradial floor plate systems are variations on this general pattern with floor plates becoming erect, or positioned epithecally upon the theca, and brachioles arising from the floor plates in different positions. In some taxa, the floor plates are lost altogether. In many taxa, such as Tristomiacystis and early cheirocrinid


and Lipsanocystis (Fig. 2.10) and many paracrinoids, modified this pattern by having the oral plates sutured to the thecal plates and placing the abradial floor plates atop the thecal plates. These epithecal floor plates typically form scars on the surfaces of the thecal plates that are evident when the floor plates become disassociated taphonomically. In paracrinoids such as Canadocystis (Parsley and Mintz, 1975, pl. 11, fig. 14), the floor plates are heavily sutured to the thecal plates, and these plates remain intact taphonomically, covering the sutures of the oral plates and making them difficult to interpret. The abradial floor plates of blastoids are also positioned


Other clades, including the glyptocystitoids Glytocystites


epithecally upon the lancet plate of the theca (Sumrall and Waters, 2012). The ambulacral floor plates (called side plates in blastoid terminology) in most cases form the main food groove


along the parradial suture. However, in several taxa—notably Pentremeites—the abradial floor plates are divided along the midline, exposing the underlying lancet plate. In these taxa, the main food groove and proximal portion of the side food grooves lie upon the extraxial skeleton for a short distance before becoming axial again. This situation changes during ontogeny


of individual food grooves that form along sutures of adjacent floor plates at the growing tip of the ambulacrum and become overtaken by the distally expanding exposure of the lancet plate. Still other taxa, notably some diploporitans such as


Glyptosphaerites (Fig. 3.5) and Eucystis (Kesling, 1967, fig. 144.2e), extend the ambulacral food grooves without


underlying floor plates. In Glyptosphaerites, the oral plates bear food grooves across their surface rather than suturally (Fig. 3.5). The oral plates articulate to the thecal plates, and distally, the narrow food groove extends across the thecal surface without regard to the underlying thecal plate boundaries. At intervals, small side food grooves extend to small brachiole facets. Eucystis is similar except that numerous ambulacral branches occur on the oral plates, which lead to brachiole facets on


the oral plates and most proximal thecal plates (Kesling, 1967, fig. 144.2e).


Nonbiserial ambulacral floor plates.—There are two notable variants on the typical biserial plate arrangement of abradial floor plates in oral plate bearing Blastozoa. First, is the uniserial floor plates of paracrinoids. These plates arise only fromthe left side and bear unusual uniserial brachioles (Parsley andMintz, 1975). Floor plates typically form the left side of the food groove, and in many taxa the right side of the food groove is formed from enlarged oral plates such that the oral plates articulate to numerous floor plates (Fig. 3.2). In others, the floor plates become highly asymmetrical


and occupy both right and left sides of the food groove. These floor plates are often epithecalysutured to thetheca anddonot


structurally form the thecal wall. In a few notable cases, the floor plates are raised into erect ambulacra as in Implicaticystis (Fig. 3.10) (see the following). The second notable variant is the double biserial plating


found in blastoids, coronates, glyptocystitoids, hemicosmitoids, and perhaps trachelocrinids. In these taxa, brachioles arise along the sutures of paired primary and secondary abradial floor plates (Fig. 2.10). Typically, the primary plate is notably larger than the secondary plates and the side food groove lies along their common suture. These plates can be very large as in glyptocystitoids or very small as in blastoids, where they are called side plates. In coronates and hemicosmitoids, they are formed into erect appendages (see the following) but retain their differentiation in this state. In trachelocrinids, they are also erect but secondary plates occur only every third floor plate, alternating right and left down the length of the erect appendage (Sumrall et al., 1997). While it is possible that the primary and secondary plates of


these clades may correspond to the adradial and abradial plates of the plesiomorphic pentaradial condition, this seems unlikely. First, morphologically, primary and secondary plates in these taxa all have the characteristics of abradial plates with broad exposure along the edge of the food groove. Second, phylogenetic analyses place these taxa well derived within the


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