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Journal of Paleontology 91(4):582–603
plates over its three ambulacra, of which only the central four appear to be oral cover plates (Fig. 9.1). However, Caryocrinites (Hemicosmitoida) is characterized by an extensive tegmen in which Frest (1975) recognized a central plate surrounded by eight pericentrals and various additional cover plates added as arms were added (Fig. 9.6, 9.7). Ambulacral cover plates are preserved in a number of
‘cystoid’ groups. In the eocrinoid Rhopalocystis, there is a simple set of biserial alternate cover plates along the ambulacra, which occasionally have one or two secondary cover plates near the central suture (Ubaghs, 1963, pl. 3, fig. 1). The ambulacral cover plates decrease in size as they approach the brachiole facets. By contrast, in the eocrinoid Ascocystites Barrande, 1887, Ubaghs (1968, p. S483) interpreted the ambulacra as covered by two sets of cover plates. However, he also thought a possible hydropore occurred in the outer cover plates of the CD interray. It would be extremely unusual for a stone canal to pass through an ambulacrum to open in a cover plate. Sprinkle (1973, p. 118–121) also described Ascocystites, including some of the same specimens seen by Ubaghs, and thought the supposed hydropore was merely some disturbed cover plates. Sprinkle (1973) followed Ubaghs (1968) in thinking the main ambulacra had two sets of paired cover plates, but only the central set continued up the brachioles. Double biserial cover plates are known in the main ambulacra of the diploporite Eumorphocystis (see Parsley, 1982a, p. 285). Brachiole and pinnule cover plates have been recorded in a
variety of genera, and Sprinkle (1973, p. 16, 17, fig. 5) illustrated several different arrangements. The simplest consist of a single biseries of cover plates, one extending from each side of the food groove and meeting in the middle. Even here, two slightly different arrangements are known. In several eocrinoid genera, each cover plate overlaps the next distal one and the two series interlock alternately. Sprinkle (1973, p. 15) called this type ‘distally imbricating’ cover plates. He recorded them in Kinzercystis Sprinkle, 1973 (fig. 5a), Lepidocystis Foerste, 1938 (p. 65), Lichenoides (p. 110), Eustypocystis Sprinkle, 1973 (p. 113), and Ascocystites (p. 119). Paul (1968, p. 589, fig. 8) illustrated the same arrangement in the glyptocystitoid rhombiferan Macrocystella Callaway, 1877. More commonly, the two series of cover plates meet at a zigzag suture line in the center of the food groove. This arrangement is seen in several species of Gogia Walcott, 1917 (Sprinkle, 1973) and so was probably established by the late lower Cambrian. It is also known in an unnamed rhipidocystid eocrinoid (Sprinkle, 1973, p. 16, fig. 5b), the hemicosmitoid Caryocrinites (Sprinkle, 1973, p. 17, fig. 5f), and the parablastoid Meristoschisma Sprinkle, 1973 (Sprinkle, 1973, p. 17, fig. 5g). The two series of cover plates are slightly unequal in size in the spiral brachioles of Gogia spiralis (Sprinkle, 1973, p. 16, fig. 5d). A simple biserial arrangement occurs in the glyptocystitoid rhombiferans Glyptocystella, Pirocystella, and Strabocystis (Sprinkle, 1982a, pp. 240, 245, and 266, respectively). It even occurs in the uniserial pinnules of the palaeocrinoid Bromidocystis (Sprinkle, 1982b, p. 294) and the paracrinoids Comarocystites, Amygdalocystites,and Malocystites (Parsley and Mintz, 1975, pp. 33, 47, and 89, respectively). Sprinkle (1973, pp. 16, 17, fig. 5) also illustrated a double biseries of cover plates in the eocrinoid Nolickuckia Sprinkle,
1973 where the two series were side by side on either side of the central suture line. The alternative arrangement for double biseries of cover plates where one series lines the edges of the food groove and the other fills the central region was illustrated for two species of blastoids sensu stricto (s.s.; Sprinkle, 1973, p. 17, fig. 5h, i). Finally, Sprinkle illustrated a complex arrangement of five sets of cover plates in the brachioles of glyptocystitoid rhombiferan Cheirocystis anatiformis (Hall, 1847) (Sprinkle, 1973, p. 16, fig. 5e). Kesling (1962) redescribed this species, but his description of the cover plates is limited. As far as I am aware, no other glyptocystitoid has such complex cover plates in its brachioles, but so few species have been described in sufficient detail that this is not necessarily significant. Parsley and Mintz (1975, p. 47) raised the possibility that
cover plates of main ambulacral grooves in the paracrinoid Amygdalocystites could not open in life, whereas those of the pinnules could. This idea is distinctly possible in other ‘cystoid’ groups. In particular, the cover plates in the wide main food groove of the ambulacra in aristocystitoid diploporites appear unlikely to have opened in life, whereas it seems most unlikely that cover plates of the brachioles and pinnules of any pelmatozoan echinoderm did not open for feeding.
Summary of ambulacral structure
The ambulacral structure found in glyptocystitoids with a dou- ble biseries of flooring plates and erect biserial brachioles with simple biserial cover plates appears to be least derived in the sense that all the elements are added in series of four columns of plates, but it is much simpler than the arrangement of cover plates in Cambrian echinoderms such as helicoplacoids, helico- cystoids, and stromatocystitids. The brachioles have biserial, alternate brachiolar plates and biserial cover plates. The main ambulacrum can be viewed as constructed of the first pair of brachiolar plates modified as flooring plates, and each brachiole arises from a facet shared by two ambulacral flooring plates. Different ratios of cover plates to brachiolars or to flooring plates are analogous to the different numbers of ambulacral and ‘interambulacral’ plates in sea urchins. Possibly the first major innovation in ambulacral design
was the ability to articulate a biserial brachiole on a single flooring plate, which allowed the development of main ambu-
lacral trunks with a single biseries of flooring or trunk plates. Trachelocrinus apparently represents a variation in which the brachioles still arose from a pair of trunk plates but with a single additional trunk plate that did not bear a brachiole in between each pair that did. Another innovation was the development of ambulacral grooves incised into extraxial plates and from which appendages arose as in the eocrinoid Lichenoides and sphaer- onitid diploporite
Glyptosphaerites.In Lichenoides, the appen- dages were brachioles, and more than one arose from the ‘radial’ plates. However, Sprinkle (1973, p. 110) described the bra- chioles as changing from biserial alternate over most of their length to biserial opposite near the base and even possibly to a uniserial first brachiolar. The appendages of sphaeronitids remain totally unknown. In both Lichenoides and sphaeronitids there is nothing comparable to a main ambulacral trunk or arm.
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