Paul—Testing for homologies in echinoderms
regarded as homologous. In effect, this is restating the EAT in a different way—all such plates are part of the axial skeleton and therefore homologous at that level of homology. However, some oral cover plates were added at intermediate positions during growth rather than always terminally (Fig. 3). For example, Sumrall (2010, p. 270, fig. 1) has identified five cover plates at the interrays between the ambulacra as ‘primary oral cover plates’ in the glytocystitoid rhombiferan Lepadocystis. I prefer to include a second in the CD interray, making six primary oral cover plates in all (Fig. 3), but either way, if these cover plates were the first to be secreted then the other oral cover plates between primaries 2 and 5 in Figure 3 were added later and could not have been added terminally as the ambulacra grew. Similar patterns occur in the oral cover plates of edrioasteroids (Bell, 1976a, b). This is not an anomaly under the EAT, as cover plates have not been considered to comply universally with the ocular plate rule of exclusively terminal addition (see, for example, Mooi et al., 2005, p. 546). Modern crinoid ambulacra differ from those of all other
living classes in lacking a tube foot/ampulla system and in having tube feet arranged in triplets (Nichols, 1960) (Fig. 18). They also lack calcified cover plates but have lappets alternately on either side of the food groove instead. When feeding, the long tube feet are extended horizontally, between the lappets (Fig. 18.3). The intermediate and short tube feet are attached to the lappets proximally and extend at different angles to the horizontal (Fig. 18.2, 18.3). When the latter two bend to deposit food in the food groove, they move the lappets. There is, thus, a 1:1 relationship between tube-feet triplets and lappets. It is possible that a similar 1:1 relationship occurred between ‘cystoid’ tube feet and cover plates or cover plate sets. It is equally possible that the structure of modern crinoid tube feet is a post-Paleozoic innovation and that such uniformitarian arguments do not apply to the Paleozoic. Sumrall (2008, 2010) used the ambulacra of glyptocysti-
toid rhombiferans as typical in developing his universal ele- mental homology (UEH) model, so it is appropriate to consider
595
glyptocystitoid ambulacral structure as a whole (Fig. 3). In glyptocystitoid rhombiferans, the first, horseshoe-shaped, bra- chiole facet is developed to the left (as viewed in the direction of growth) in all ambulacra (Fig. 3) and is shared by one of the peri-orals (PO1-PO5) and the plates labeled L (“first left ambulacral floor plates” of Sumrall and Waters, 2012). There- after, each brachiole facet is supported by a smaller adoral ambulacral flooring plate and a larger distal flooring plate. Here, I suggest these pairings are homologous and represent the first pair of brachiolar plates modified to form a peri-oral plus an L plate or pairs of ambulacral flooring plates. Glyptocystitoids with all five ambulacra have an unusual arrangement in which ambulacra B and D have the first two brachioles to the left, and thereafter brachioles alternate right and left to the tip of the ambulacrum. In ambulacra A, C, and E, only the first brachiole lies to the left of the ambulacrum, and brachioles alternate regularly throughout these ambulacra (Fig. 3). Irrespective of this unexpected ‘B D different’ pattern of primary brachioles in glyptocystitoids, in early forms the ambulacral flooring plates form part of the thecal wall. When extensive ambulacra are developed, as in the cheirocrinid Coronocystis Paul, 1972, clefts develop in the extended radial plates to accommodate the ambulacra. Furthermore, the oral frame is composed of the first ambulacral plates secreted on the left side of all five ambulacra plus a sixth (PO6) in the CD interray. Brachioles are biserial, and in extensive ambulacra the main trunk is composed of a double biseries of flooring plates. Here, this structure is inter- preted as resulting from the first pair of brachiolar plates on either side being modified as flooring plates. A similar structure occurs in the main trunk of the erect arms of the hemicosmitoid rhombiferan Caryocrinites Say, 1825 (see Sprinkle, 1975; Lanc et al., 2015) and the recumbent ambulacra of blastoids (Beaver, 1968, p. S322, fig. 190). Main ambulacral trunks occasionally branch in the glyp-
tocystitoid families Callocystitidae and Glyptocystitidae. In Callocystites and Coelocystis Schuchert, 1903 (Callocystitidae), the branching is equal (Paul, 2015), whereas in Sphaerocystites Hall, 1859, Strobilocystites White, 1876 (Callocystitidae), and Hesperocystis Sinclair, 1945 (Glyptocystitidae) the branching is unequal (Sprinkle, 1982a; Paul, 2015). The suggestion that peri-orals and L plates are basically
modified first brachiolar plates can be tested by seeing whether a similar pattern can be found in other groups of ‘cystoids’ besides glyptocystitoids and the hemicosmitoid Caryocrinites. The eocrinoid Rhopalocystis (Fig. 8) has peri-orals that do not bear or share brachiole facets. Thereafter, a simple biserial set of mural ambulacral flooring plates bear brachiole facets contained entirely within the individual plates. Such an ambulacral struc- ture could be derived in an analogous way to the ambulacral structure of glyptocystitoids if the appendages were pinnules and the first pinnular plates were modified as ambulacral floor- ing plates. Unfortunately, Rhopalocystis has biserial brachioles (Ubaghs, 1963, p. 36, pl. 1, fig. 4). Recumbent biserial ambulacra occur in the palaeocystitids
Figure 18. Water vascular system in a pinnule of the Recent comatilid crinoid Antedon.(1) Ventral view showing the triplets of tube feet (black) and their relationship to the lappets. C – C = position of the cross section in (3). (2) Oblique ventral view. (3) Cross section to show orientation of tube feet with respect to the long axis of a pinnule. Redrawn from Nichols (1960, fig. 1).
Ulrichocystis Bassler, 1950, and Bromidocystis Sprinkle, 1982b, with each ambulacral facet confined to a single flooring plate (Paul, 1988; Sprinkle, 1982b). In Bromidocystis, the peri- orals lack ambulacral facets. Ulrichocystis had three ambulacra with unknown appendages, whereas Bromidocystis had five
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