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Journal of Paleontology 91(4):582–603
show that having a facet confined to a single plate does not indicate the appendages that arose from the facet were uniserial. Equally, the coronate genera Stephanocrinus Conrad, 1842 and Cupulocorona Donovan and Paul, 1985 are known to have had isotomously branched erect biserial arms (Fay, 1962; Brett et al., 1983), but the first plate of each ambulacrum is uniserial and analogous to a crinoid axilliary plate (Donovan and Paul, 1985, p. 528). Thus, it seems that biserial brachioles can arise from one or a pair of flooring plates in ‘cystoids.’ Erect biserial arms are known in the eocrinoids Trachelo-
ambulacra and recumbent brachioles (Sprinkle, 1982b, p. 290, fig. 72i). Rhopalocystis and Bromidocystis alone are enough to
crinus Sprinkle, 1973 and Bockia Bockelie, 1981b. Trachelo- crinus is unique in having lateral brachioles that branch off every third trunk plate (Sprinkle, 1973, p. 125). Bockia has essentially the same structure as Rhopalocystis only with erect arms. Biserial brachioles arise alternately along the biserial main ambulacral trunk (Bockelie, 1981b, p. 129, fig. 2g, j). Both Bockia and Trachelocrinus have erect arms arising from an oral prominence, which Bockelie (1981b, fig. 3a, b) thought was composed of seven peri-orals in Bockia. Undoubted uniserial ambulacra (main trunk and pinnules)
occur in paracrinoids and may be erect (e.g., Comarocystites, see Parsley and Mintz, 1975, pl. 1, figs. 2, 3) or recumbent (e.g., Sinclairocystis, see Parsley, 1982b). Most paracrinoids have recumbent ambulacra in which the pinnules arise exclusively on one side of the ambulacrum, usually the left side, but the right in the unusual paracrinoid Bistomiacystis (Sprinkle and Parsley, 1982). Most commonly, paracrinoids have just two recumbent ambulacra, but sometimes one or the other branches. Both ambulacra are curved clockwise in Canadocystis, giving an S-shaped pattern (see Parsley and Mintz, 1975, pl. 10, fig. 9, pl. 11, figs. 14, 15). Clockwise ambulacra have their left sides on the outside of the curve, and ambulacral facets are confined to the outside of the curve. Thus, Bistomiacystis has two pairs of ambulacra curved in a counterclockwise manner (Sprinkle and Parsley, 1982, p. 225, fig. 60), and this probably explains why it
ambulacra of Achradocystites resemble those of the North American genus Comarocystites, but both North American genera have uniserial ambulacra bearing pinnules on one side only. Thus, it seems unlikely that either Heckerites or Achradocystites has close affinities with the North American paracrinoids. Diploporite ‘cystoids’ include yet other ambulacral struc-
tures that pose problems in recognizing homologies. First, sphaeronitid diploporites have radial circum-orals, rather than the interradial peri-orals of most other ‘cystoids.’ In many genera, the entire ambulacra are confined to these radial circum- orals and consist of one or more epithecal food grooves, which end in small (about 1mm or less) facets. The appendages that arose from them remain entirely unknown. Two significant variations in this basic pattern occur. First, in four sphaeronitid genera, Codiacystis Jaekel, 1899, Tholocystis Chauvel, 1941, Herpetocystis Termier and Termier, 1972, and Finitiporus Frest and Strimple (in Frest et al., 2011), two narrow food grooves leave the edges of the mouth and branch in opposite directions to form a palisade of ambulacral facets surrounding the mouth (see Frest et al., 2011, p. 32, fig. 19). Frest and Strimple described such ambulacra as ‘epipanniculate’ and assigned them to a new subfamily, Herpetocystinae (Frest et al., 2011, p. 59, table 19, p. 62), although Termier and Termier (1972) had previously erected a family Herpetocystidae. Second, in some species of Eucystis Angelin, 1878 and all
species of Glyptosphaerites Müller, 1854, some food grooves extend over the theca to plates beyond the circum-oral circlet. The arrangement of these longer food grooves was apparently
has its facets confined to the right side of each ambulacrum. The flattened eocrinoid Haimacystis Sumrall et al., 2001, has only two ambulacra curved in a rough semicircle, apparently com- posed of biserial trunk plates possibly with biserial brachioles arising from the outer edge of the curved ambulacra (Sumrall et al., 2001, p. 988, fig. 2.2, p. 990, fig. 4.3). Thus, one ambu- lacrum has brachioles exclusively to the left side and the other exclusively to the right. An oddity of paracrinoid ambulacra is that in some taxa, the first pinnule on each ambulacrum is dis- proportionately large (see, for example, Parsley and Mintz, 1975, pl. 3, fig. 5, pl. 7, figs. 14, 15). Finally, Malocystites murchisoni Billings, 1858, is unique among paracrinoids in having recumbent pinnules (Parsley and Mintz, 1975, p. 86). Rozhnov (2012, 2015) has recently summarized available
information on two possible paracrinoid genera from the Baltic region. Heckerites Rozhnov, 1987 has two recumbent biserial ambulacra, which are said to form part of the thecal wall, with biserial brachioles arising from only some of the flooring plates (Rozhnov, 2012, p. 310, 311, fig. 3). Similarly, Achradocystites has erect biserial arms with alternating biserial brachioles. Superficially, Heckerites resembles the North American para- crinoid Amygdalocystites Billings, 1854, and the erect
random, especially in Glyptosphaerites. Regnéll (1945, pl. 9, fig. 5) illustrated a specimen with two parallel ambulacral branches that produce two facets each on three plates. Similarly, Kesling (1968a, p. S235, fig. 135, 1a, 1b) illustrated ambulacra in which some thecal plates bear two ambulacral facets, whereas others bear the food groove but no facets. Apparently, the thecal plates existed before the ambulacral grooves extended over them and developed facets. If so, thecal plates bearing parts of the food grooves and facets cannot be considered to be ‘ambu- lacral plates’ or even part of the axial skeleton. Furthermore, the ambulacral grooves bear no trace of any cover plates. Other diploporites, such as Protocrinites Eichwald, 1840
and Dactylocystis Jaekel, 1899, have a thecal structure with a regular arrangement of ambulacral plates. In Protocrinites, the food grooves extend down the theca and give rise to lateral branches alternately, which end in a single facet confined to a single thecal plate (Bockelie, 1984, p. 28, fig. 16). Thus, the ambulacra appear to be composed of regularly arranged biserial thecal plates. Bockelie (1984, fig. 16) even showed that there was a growth zone where thecal plates were added above the
basal two circlets of plates, which coincided with the tips of the five ambulacra. The facets give rise to erect biserial brachioles despite being confined to a single thecal plate. Both main ambulacral grooves and brachioles are furnished with cover plates. Dactylocystis shows an even more regular arrangement in which narrow ambulacra are composed of regularly alter- nating plates that bear a single facet and all the diplopores. No interradial plates bear pores (Kesling, 1968a, fig. 146, 4a, b). The Gomphocystitidae bear spiral ambulacra that coil in a clockwise manner so appendages arise from the left side only.
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