Paul—Testing for homologies in echinoderms
593
Figure 15. Ambulacra and oral plating in some glyptocystitoid rhombiferan ‘cystoids’ with two ambulacra. (1, 2, 5) Pleurocystites Billings, 1854. (3, 4, 6) Praepleurocystis Paul, 1967b. (7) Schizocystis Jaekel, 1895. B1–B4 = basal plates; C and E = ambulacra; G = gonopore; H = hydropore; IL1–IL5 = infra- lateral plates; L1–L5 = lateral plates; Pe = enlarged periproct of pleurocystitids; R1–R6 = radial plates; 1–7 = peri-oral plates. Although only two ambulacra occur, the full complement of seven peri-oral plates remains. See text for further explanation of thecal plating in the pleurocystitids. (1–4) Redrawn from Paul (1984, p. 119, fig. 76); (5, 6) from Paul (1967b, p. 113, figs. 8, 9), respectively; (7) from Kesling (1968a, p. S185, fig. 89, 1b). The ambulacral facets in Schizocystis (7) are largely to the left in both ambulacra.
how these plates could have become detached from the oral frame yet remained only slightly displaced. The theca would have had to be almost completely filled with sediment for this to happen, otherwise the plates would have fallen into the empty theca or been carried away by currents. In addition, I know of no other diploporite (or ‘cystoid’) taxon that has completely lost one circlet of plates but is otherwise undamaged. The mouth of P. simplex is rounded, not angular as one would expect if a circlet of angular plates were missing. In addition, Frest and Strimple (2015) cited multiple specimens of the new species they erected, so this loss of peri-orals apparently occurred repeatedly. I have not examined any of the thousands of additional holocystitids that have been found at Napoleon, Indiana, since the expansion of the quarry there (see Frest et al., 2011, p. 2). I think Sheffield and Sumrall’s (2015) suggestion needs further examination. One thing can be said: the oral frame was extremely small in Pentacystis and Osgoodicystis. In both genera, if the oral cover plates were preserved, the oral frame plates would be almost completely hidden.
Mouth frames with five orals.—In contrast to the ‘cystoids’ described in the preceding section, others appear consistently to have had five peri-orals. This is true of caryocystitid rhombiferans (Fig. 6) and the unique hemicosmitoid rhombi- feran Thomacystis Paul, 1969 (Fig. 9.5). Five peri-orals also occur in the diploporite genus Eumorphocystis, which has triserial erect ambulacra among other unique features. In these cases, five peri-orals arose independently. Even in examples of
ferans may have from one to five ambulacra, and in some genera, e.g., Callocystites Hall, 1852, the ambulacra may branch (Paul, 2015), yet all seven peri-orals are always present as far as is known. By contrast, in ‘cystoid’ genera with five peri-orals, it seems possible to distinguish which of the peri-orals is missing. Thus, if peri-orals are primary ambulacral plates, then at least in glyptocystitoids all the peri-orals were consistently developed before the growth of an ambulacrum was ‘turned off.’ However, in caryocystitids and Thomacystis, PO3 was lost along with the associated ambulacrum A, whereas in other ‘cystoids’ with five peri-oral plates, it was the ‘extra’ peri-oral in the CD interray (PO6) that was lost. As far as is known, paracrinoids commonly have five plates
caryocystitids with four ambulacra (e.g., Fig. 6.3, 6.6) there are two peri-oral plates in the CD interradius. Ambulacrum A is never developed, although all four other ambulacra may divide to produce two appendages per ambulacrum in Echinosphaerites (e.g., Bockelie, 1981a, p. 199, fig. 10; Paul, 1997, p. 169, fig. 102). Thus, in caryocystitids (Fig. 6) and Thomacystis (Fig. 9.5), the five peri-orals present are interpreted as PO1, PO2, and PO4–PO6 because PO3 is associated with the missing ambulacrum A. However, in Eumorphocystis, there is apparently a single oral in the CD interradius, and the five orals are here interpreted as PO1–PO5. This last point may be significant. Glyptocystitoid rhombi-
associated with the mouth (Fig. 16), although Parsley (1978, p. 474, fig. 1) illustrated a specimen of Comarocystites Billings, 1854, with a very small mouth surrounded by just three ‘orals.’
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