Paul—Testing for homologies in echinoderms The appearance of uniserial pinnules is another major
innovation, but the structures need checking with thin sections to ensure that hidden sutures do not exist. Sprinkle’s (1973) description of the structure of the brachioles in Lichenoides provides a plausible mechanism by which uniserial pinnules might arise. Biserial flooring plates from which uniserial pin- nules arose are a logical ambulacral structure if the flooring plates are modified first pinnulars. Truly uniserial main ambu- lacral trunks occur in North American paracrinoids whose pin- nules only arise from one side of the ambulacrum (usually the
left side). This structure is still compatible with the idea that flooring plates are modified first pinnulars. So far as is known, all pinnules had biserial cover plates, but very few have been described in detail. It is interesting to note that Sprinkle and Parsley (1982, p. 228) described the unusual paracrinoid Bistomiacystis as having the most distal ambulacral plate in each ambulacrum being larger than usual and possibly being a terminal ambulacral plate analogous to those in starfish. Bistomiacystis has two oral openings; four ambulacra are curved counterclockwise and have facets on their right sides; and the associated appendages appear to have been biserial. It seems to have doubled everything associated with its axial skeleton. Uniserial plates that form part of the thecal wall and may be
extraxial in origin occur in gomphocystitids. The ambulacra spiral clockwise, and the appendage facets are all on the left- hand side. Unfortunately, the appendages are unknown, so it is not possible to argue that the ambulacral plates are modified first brachiolars or pinnulars. The unique triserial plating to the erect ambulacra of the diploporite Eumorphocystis remains puzzling. The biserial arm plates with facets for the pinnules make sense as modified first pinnulars, but the third, aboral, uniserial series of plates is apparently without homology within the Echinodermata. Finally, ambulacral cover plates show no evidence of
musculature by which they could have been opened. Their structure, shape, and orientation when closed resemble those of anal and gonal pyramids, except that they cover narrow elongate grooves rather than more-or-less circular holes. Both anal and gonal pyramid plates lack any evidence of musculature to open them. In life, they acted as one-way valves allowing egress from the theca (Paul, 1967a, p. 305–306). In some rhombiferans (e.g., Echinosphaerites, see Barrande, 1887, pl. 23, fig. 7), the anal plates have large lateral ligament or muscle pits on the interior surface, which closed the anal pyramid. Voiding of fecal pellets was by peristalsis. The similarity between the cover plates of food grooves and the pyramids over thecal orifices suggests that the only way to open cover plates was by pressure from the inside, and inflation of tube feet is the most obvious way to achieve this. Details of cover plates need further investigation. For
example, were cover plates of main food grooves permanently closed? Once food had been gathered in the brachioles, it needs to be transferred to the mouth without loss. Immovable cover plates would aid this process.
Discussion
Stephenson (1979) argued that a trimerous stage in echinoderm evolution was unnecessary to the origin of pentamery. At the
599
time, helicoplacoids were thought to have a single ambulacrum that branched once (Durham and Caster, 1963; Durham, 1967). Now we know that triradiate echinoderms actually existed. Smith and Zamora (2013) have added further Cambrian spiral echinoderms in Helicocystis, which is thought to be the latest common ancestor of all pentameral echinoderms. Smith and Zamora (2013, p. 4) stated that the helicoplacoids spiraled counterclockwise, whereas Helicocystis spiraled clockwise. This is an unexpected difference and weakens the argument that Helicocystis is directly intermediate between triradiate helico- placoids and pentameral echinoderms. However, if one orients both with the mouth toward the observer and with ambulacrum A toward the top of the diagram (the standard orientation for pentameral pelmatozoans), all the ambulacra in both forms spiral clockwise (Fig. 19). The ambulacra spiral to the right in relation to their direction of growth. Numerous echinoderms have spiral ambulacra, and in those
with erect appendages these always occur on the outside of the curve. Most have ambulacra that spiral clockwise and have facets on the left side. I presume this arrangement occurs for reasons of space; there is more room for brachioles or pinnules on the outside of curved ambulacra than on the inside. This would be particularly true of small individuals or juveniles. Further possible evidence for clockwise spirals in ‘cystoids’ includes the addition of ambulacral facets clockwise in multi- faceted sphaeronitids such as Archegocystis, Haplosphaeronis, and Tetreucystis (Paul, 1973; Bockelie, 1978). Bockelie (1982) also concluded that the addition of ambulacra in the Caryocystitoida was clockwise, first fromthe B and then fromthe D ambulacrum. It is interesting to note that Rozhnov (1994, p. 174) argued that the addition of ambulacral plates in the eocri- noids Cryptocrinites and Rhipidocystis Jaekel, 1900 occurred in a clockwise direction and in the order ambulacrum D, then B, then C or E, and last A. Bockelie (1981b, fig. 10a, b) also illustrated two examples of Cryptocrinites fromNorway,with only one facet in ambulacrum A, but two in all others. In one, the smallest and most recently added was in ambulacrum E, and in the other in ambulacrum C. This matches the order suggested by Rozhnov (1994). If Rozhnov’s order of addition of ambulacral plates is widespread among ‘cystoid’ groups, it might explain why all ‘cystoids’ with four ambulacra lack ambulacrum A. It was pre- sumably aborted before any ambulacral plates were developed. It might also be relevant to the ‘BD different’ patternofprimary brachioles in glyptocystitoids with five ambulacra. Zamora and Rahman (2015) have recently reviewed the
Cambrian echinoderms, including their oral plating. There is a surprising diversity of oral plating making detection of a ple- siomorphic state more difficult. Regrettably, the oral plating in helicoplacoids remains unknown despite recent attempts to elucidate it. Interestingly, the lower Cambrian imbricate ‘eocrinoid’ Lepidocystis has a circum-oral circlet (Sprinkle,
1973, pl. 3, fig. 3; reproduced in Kammer et al., 2013, p. 8, fig. 4m, and Zamora and Rahman, 2015, p. 1115, fig. 6e). By contrast, the middle Cambrian edrioasteroid Kailidicus Zhao et al., 2010 apparently has a typical peri-oral circlet (see Kammer et al. 2013, p. 8, fig. 4a, b), although in the latter case the ambulacra appear to form themouth frame. It is puzzling that only a single plate of the biserial ambulacra of many ‘cystoids’ was modified as a mouth frame plate The clockwise spiral ambulacra of Helicocystis
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