Paul—Testing for homologies in echinoderms
The ambulacra are effectively uniserial, being composed of a single series of plates with one short lateral food groove and facet confined to each plate. Bockelie (1979a, p. 160, fig. 5a) showed an unexpected arrangement in Celticystis Bockelie, 1979a, in which the first ambulacral facet was shared between a peri-oral and the first ambulacral plate in ambulacra A, B, D, and E, but the first facet was confined to the first ambulacral plate in ambulacrum C. The figure is a partial reconstruction from two specimens, so perhaps one should not read too much into the anomalous ambulacrum C. Another interesting feature of the ambulacra of Celticystis is that over most of the theca the ambulacral plates form part of the thecal wall, but on reaching the basal circlet of plates by which the diploporites were attached to the substrate in life, the ambulacrals lie on top of the ‘basals.’ The tips of the ambulacra are recumbent (Bockelie, 1979a, p. 161, fig. 6). Uniserial, mural ambulacra also occur in Gomphocystites Hall, 1864 (Bockelie, 1979a, p. 165, fig. 11). No ambulacral appendages of gomphocystitid diploporites have been found. Finally, among diploporites with arms as defined here,
Eumorphocystis has the most complex ambulacral structure (Parsley, 1982a; Paul and Fone, 1997). The mouth is surrounded by five interradial peri-orals of which the one in the CD interray is the largest but shows no signs of being double. A structure interpreted as the hydropore by Parsley (1982a, p. 283) occurs adjacent to the CD peri-oral. Five ambulacra radiate from the mouth with alternate biserial flooring plates that bear undoubted brachiole facets. The flooring plates form part of the thecal wall. At the edge of the oral surface, the ambulacra become exothecal with an erect, triserial main trunk, which gives rise laterally to erect pinnules. Paul and Fone (1997, p. 159) questioned whether the lateral structures on the free arms were truly pinnules because no illustration showed the critical aboral surfaces of the pinnular plates where a median suture might be expected if the appendages were brachioles. However, Parsley (1982a, p. 284, pl. 36, fig. 15) does show one pinnule in adoral view that clearly has aligned sutures on opposite sides of the food groove. This is compatible with pinnular structure, and so I now accept Parsley’s description in full. Thus, Eumorphocystis combines almost all possible structures in its ambulacra. It has simple biserial floor plates that form part of the thecal wall, erect biserial brachioles that arise from a facet within a single flooring plate, and erect triserial arm trunks from which uniserial pinnules arise alternately. The facets from which the erect arms arise are each composed of five thecal and ambulacral plates. In the British species Eumorphocystis coxi Paul and Fone,
1997, the ratio of aboral to lateral arm plates was exactly 2:1 (Paul and Fone, 1997, p. 157). Parsley (1982a, p. 286) stated that this ratio varied from 2.5:1 proximally to 1.5:1 distally in the type species E. multiporata Branson and Peck, 1940. However, Parsley’s illustrations (1982a, p. 284, pl. 36, figs. 15, 18) show a regular arrangement, and in the text (p. 286) he stated ‘every other bracing plate [my aboral arm plate] inserting slightly into am series [my lateral arm plates].’ Thus, it seems a fairly regular arrangement of two aboral arm plates per lateral arm plate occurred in E. multiporata too. If one seeks a possible homologue of the erect arm structure in Eumorpholocystis, the biserial, alternate, lateral arm plates make sense as first pinnulars modified to become flooring plates in an analogous manner to
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the structure of Caryocrinites arms, in which the trunk plates can be considered as the first pair of brachiolars modified as trunk plates. However, suggesting a homologue of the aboral series of arm plates in the triserial arm of Eumorphocystis is more difficult. Finally, care is required when interpreting the structure of
entire appendages from only the basal parts. Sprinkle (1973, p. 110) described the brachioles of the eocrinoid Lichenoides as possibly starting with a single plate, then having a few biserial opposite plates, and finally, through most of the length of the brachioles, having normal, biserial, alternate plating. He also illustrated brachioles in the flattened eocrinoid Petalocystites Sprinkle, 1973, with the first five to 10 proximal brachiolar plates biserial opposite but more distal ones biserial alternate (Sprinkle, 1973, p. 133, fig. 31b). Mention has already been made of the biserial branched arms of coronoids that start with a single uniserial plate. Thus, although Frest and Strimple (in Frest et al., 2011, pl. 4, figs. 5, 7) illustrated uniserial plates still attached to the ambulacral facets in holocystitid diploporites, it is uncertain that the entire structures were uniserial. In Paulicystis Frest and Strimple, 2011 (in Frest et al., 2011), the ambulacra were recumbent on the theca and appear to have been biserial (Frest et al., 2011, pl. 4, fig. 2).
Cover plates.—Ambulacral cover plates are also poorly known, partly due to preservational deficiencies but also due to less taxonomic significance being placed on them compared with, for example, cover plate arrangements in edrioasteroids (Bell, 1976a, b). It is convenient to discuss cover plates in the three parts of the ambulacral system separately. Cover plates occur over the oral area, along main trunks of arms (as defined here) or ambulacra, and along erect appendages. Clearly, where ambu- lacral appendages remain unknown (e.g., in the diploporite family Sphaeronitidae), their cover plates must also be unknown. Cover plates in the other two parts of the ambulacral system are better known. The simplest arrangement of oral cover plates occurs in the
diploporite families Sphaeronitidae and Holocystitidae, where the large peristome is covered by six interradial plates, which can be considered as primary oral cover plates (Fig. 5.2, 5.3, 5.6). Paul (1971, p. 7) called these plates palatals, and they were almost certainly immovable in life. In other ‘cystoids’ in which the mouth is smaller and within a narrow food groove, I think some or all of these palatals can still be recognized. For example, glyptocystitoid rhombiferans have a simple biseries of oral cover plates (e.g., Fig. 3) in which all the palatals (primary oral cover plates of Sumrall, 2008) appear to be recognizable. However, in the eocrinoid Rhopalocystis (see Ubaghs, 1963, pl. 3, figs. 1, 2) and in the caryocystitid rhombiferan genera Echinosphaerites (Fig. 6.1–6.3; Bockelie, 1982, p. 493, fig. 2)
and Stichocystis Jaekel, 1899 (Bockelie, 1981c, pp. 54, 55, figs. 2a, 3a), only the pair of palatals in theCDinterray are easily recognized. In aristocystitoid diploporites, the broad main food groove
is covered by a double biseries of cover plates (Fig. 7.2), and it is not possible to distinguish oral from ambulacral cover plates. This is equally true of the flattened eocrinoid Lingulocystis Thoral, 1935 (Ubaghs, 1968, p. S464, fig. 299.3a). The hemicosmitoid rhombiferan Hemicosmites has biserial cover
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