Homology versus analogy

‘Homology’ may be defined as ‘the same structure modified for different functions,’ such as the forelimbs of ichthyosaurs, dinosaurs, and pterosaurs. Analogy involves different structures that perform the same function, such as the wings of birds and insects. Homology can be postulated initially using positional and developmental arguments. Such hypotheses of homology can be tested by comparison with the distribution of other postulated homologies (see Patterson, Reference Patterson1982, Reference Patterson1988 for a full discussion). In effect, homologous characters are synapomorphies, and the more postulated homologies involved in the analysis the stronger the test. A relevant example of positional evidence is the frequent development of five ambulacra in primitive echinoderms arranged in a ‘2-1-2’ pattern (Bather, Reference Bather1900, p. 11, fig. 9; Sprinkle, Reference Sprinkle1973, p. 43, fig. 16A) (Fig. 1). Three ambulacra leave the mouth, and the two opposite each other divide once to give a total of five. This pattern is particularly obvious in echinoderms with oral frames composed of just four plates (the peristomial border system type A2 of Kammer et al., Reference Kammer, Sumrall, Zamora, Ausich and Deline2013) and enables identification of individual ambulacra. The undivided ambulacrum is A under Carpenter’s system (Carpenter, Reference Carpenter1884, Reference Carpenter1891), and clockwise in oral view the others are B, C, D, and E. The idea that these ambulacra are homologous in all echinoderms is strengthened by the fact that the hydropore always lies in the CD interradius. When only a single gonopore is present, this too is always in the CD interradius. However, the position of the anus is less reliable. In some early echinoderms, it also lies in the CD interradius, but it occurs in the BC interradius in glyptocystitoid rhombiferans and paracrinoids. See Sumrall (Reference Sumrall1997, p. 270, fig. 1) for another example of the positional argument applied to echinoderms.

Figure 1 Illustration of extraxial-axial theory (EAT) using lower Cambrian Camptostroma Ruedemann, Reference Ruedemann1933 as an example. A–E=ambulacra in the Carpenter system (Carpenter, Reference Carpenter1884, Reference Carpenter1891); AR=axial region; IER=imperforate extraxial region; PER=perforate extraxial region. Redrawn from Paul and Smith (Reference Paul and Smith1984, fig. 19).

Evolutionary tests include the distribution of characters in different major groups. For example, in glyptocystitoid rhombiferans, recumbent ambulacra (restricted here to ambulacra growing on top of thecal plates) have developed in the families Callocystitidae (virtually all genera) and Glyptocystitidae (only Glyptocystites Billings, 1954), but they have also developed in the diploporite family Holocystitidae (Paulicystis Frest and Strimple, 2011 in Frest et al., Reference Frest, Strimple and Paul2011) and in paracrinoids (Malocystites Billings, Reference Billings1858). Rhombiferans, diploporites, and paracrinoids differ by so many other characters that recumbent ambulacra are likely to have evolved independently in each. Bockelie (Reference Bockelie1982) has demonstrated an evolutionary trend in caryocystitid rhombiferans from two through four ambulacra, whereas in both the diploporite families Sphaeronitidae and Holocystitidae and in the glyptocystitoid rhombiferans the trend was from five to four ambulacra. Thus, although in all cases ambulacrum A is missing, it seems this came about in two different ways (see the following). Again, caryocystitid rhombiferans and sphaeronitoid diploporites differ by many characters, so it is difficult to accept that ambulacrum A was lost once only.

On a purely practical note, if two different structures are thought to exist, it is better practice to record all relevant information as if they really differ. Then it should become apparent whether the differences are consistent and therefore significant, or whether they are merely end members of a continuum and only one variable structure exists. If alternatively it is initially assumed that only one structure exists, it will not become possible to distinguish between the two alternatives. In the present context, being cautious about assuming homology until there is evidence to support it is a better approach than assuming that homologies exist and not testing this assumption.

The extraxial-axial theory

Briefly, the extraxial-axial theory (EAT) is based on the fact that during echinoderm embryology, the water vascular system and all associated skeletal elements derive from the larval axocoel and first become evident at the rudiment stage. Furthermore, at least during the ontogeny of all known living echinoderms, axial skeletal elements are only added and any branching of the radial water vessels (including to lateral tube feet) only occurs terminally. Jackson (Reference Jackson1912, p. 35–51, pl. 6, 7) first documented in anomalous sea urchins with <5 ambulacra that either the number of oculars coincided with the number of ambulacra, or occasionally, if the ocular corresponding to a missing ambulacrum was present, it lacked a terminal pore. Six ambulacra occurred in sea urchins with two terminal pores in one ocular plate. This led to the ‘ocular plate rule’ of echinoid coronal growth. Four columns of plates are added at the edges of the ocular plates in the apical system: two columns of ambulacral plates plus one column from each of the adjacent interambulacra. This means that echinoid interambulacra are not homologous with interambulacra of any other echinoderms (as pointed out by Mooi and David, Reference Mooi and David2008, p. 47). The only truly interambulacral structures in the echinoid corona are the interradial sutures. In starfish, the terminal radial plate secretes four columns of plates, two adradial ambulacral columns and a column of adambulacral plates on each side of the ambulacrals. The most obvious suggested homology here is that the four columns of plates in sea urchins are homologous with the ambulacral and adambulacrcal plates of starfish (notwithstanding their different positions with respect to the radial water vessels, i.e., internal in echinoids and external in starfish). Hence, position in this paper is referred to as radial or interradial, even though ambulacra are probably homologous in all echinoderms.

Extraxial skeletal elements and the associated body wall derive from the left and right somatocoels. Plates may be added anywhere within the extraxial skeleton making recognizing homologous plates extremely difficult. In early echinoderms, the extraxial skeleton is further divided into a perforate part, which usually includes the ‘cystoid’ theca and commonly has various pore structures in it, and an imperforate part, which commonly includes the stem or column when present and lacks pore structures (Fig. 1). Much early echinoderm evolution can readily be understood in terms of changes in the relative importance of the axial, perforate, and imperforate extraxial skeleton (e.g., Lefebvre et al., Reference Lefebvre, Nardin and Fatka2015).

The EAT is based on embryological information, which can only be derived from living echinoderms. Paleozoic echinoderms may have grown in a different manner. Post-Paleozoic echinoids form a monophyletic group whose roots lie in the late Paleozoic Miocidaridae, the corona of which is characterized by 20 columns of plates: five pairs of columns of ambulacral plates bearing pores for tube feet, and five pairs of ‘interambulacral’ columns. This pattern has since been inherited by all post-Paleozoic sea urchins, which also display the ocular plate rule. Several genera of Paleozoic echinoids have different coronal plate arrangements, including multiple columns of ambulacral and/or interambulacral plates (Kier, Reference Kier1965). Neobothriocidaris (Fig. 2) is relevant here because its radial water vessels and side branches to tube feet lie within the coronal plates, so its water vascular system can be reconstructed accurately. The corona consists of single columns of imperforate plates, beside which are multiple rows of ambulacral plates bearing tube feet and arranged in a chevron pattern (Fig. 2.1). Pore pairs for tube feet are shared by two adjacent ambulacral plates, and each was fed by a lateral branch of the radial water vessel (Fig. 2.2). Figure 2.1 shows at least two new ambulacral plates that do not reach the full width of the row of plates to which they belong (arrows). Hence, ambulacral plates were added adjacent to the plates in the single columns of imperforate plates in Neobothriocidaris.

Figure 2 Morphological features of (1, 2) Neobothriocidaris and (3) Bothriocidaris. (1) Camera lucida drawing of part of the test to show the arrangement of poriferous plates (PP) in rows and nonporiferous plates (NP) in a column. IRS=interradial suture; Pi=pit. The most recently added poriferous plates are arrowed. Redrawn from Paul (Reference Paul1967c, fig. 2). (2) Reconstruction of part of the water vascular system in Neobothriocidaris. CP=central pore; LV=lateral vessel; NP=nonporiferous plates; PP=poriferous plates; RWV=radial water vessel; SP=sutural pore. During growth, new lateral vessels feeding the pore pairs must have been added along the entire length of the radial water vessel. Redrawn from Paul (Reference Paul1967c, fig. 6). (3) The apical disc of Bothriocidaris, as reinterpreted by Paul (Reference Paul1967c). Each radius consists of a central per-radial column of plates (diagonal shading) flanked by two columns of ambulacral plates (stippled). The large plates in the apical disc become genitals (G), one of which is modified as the madreporite (M) and defines the CD interradius. A–E=radii under Carpenter’s system; i and o=inner and outer periproctal cover plates; IRS=interradial sutures defining the limits of ambulacrum A. Redrawn after Männil (Reference Männil1962, fig. 1) and Solovjev (Reference Solovjev2009, fig. 1).

As new ambulacral plates were added, new lateral branches of the radial water vessels were produced (Fig. 2.2). Thus, without doubt in Neobothriocidaris, the radial water vessels were able to branch anywhere along their length. The single columns of imperforate plates in Neobothriocidaris are radial because they contained the radial water vessels, and Paul (Reference Paul1967c) argued that each column was homologous with the single ocular plate of other sea urchins. Thus, addition of ambulacral plates adjacent to a column of ‘ocular’ plates can be said to comply with the ocular plate rule.

This interpretation also made better sense of the apical disc of Bothriocidaris (Fig. 2.3), which apparently differed from all other sea urchins in having only five radial plates (oculars), one of which was modified as the madreporite (hydropore), and therefore lacked not only genital plates but any evidence of gonopores too. However, if in Bothriocidaris the single columns of plates are homologous with ocular plates and radial in position, the five plates in the apical disc become interradial (genitals), one of which is modified as the madreporite and hence defines the CD interradius (as in all other sea urchins). Nevertheless, Bothriocidaris remains puzzling as the supposed primary ambulacral plates apparently obey Lovén’s law (Lovén, Reference Lovén1874) under the conventional interpretation (see discussion in Paul, Reference Paul1967c, p. 538).

The principal point here is that the EAT is based on information from living echinoderms and may not apply universally to fossil echinoderms. Post-Paleozoic crinoids (see Simms, Reference Simms1999) and asteroids (e.g., Gale, Reference Gale2011) may also be derived from one or a few taxa that survived the end Paleozoic mass extinction and form monophyletic groups. Certainly one assumption of the EAT, that branching of radial water vessels was always terminal, does not apply to Neobothriocidaris and possibly to other early Paleozoic echinoids with multiple columns of ambulacral plates. Nevertheless, the EAT and its inherent assumptions are the best theoretical concept on which to investigate homologies of the axial skeleton in early echinoderms.

Homology of ambulacra

Several systems of denoting ambulacra in echinoderms have been developed, but recently Carpenter’s system (Carpenter, Reference Carpenter1884, Reference Carpenter1891) (Fig. 3) has found favor. In oral view, the ambulacrum opposite the hydropore is denoted as ambulacrum A; the others are labeled B–E clockwise. Interradii are denoted by the letters of the ambulacra that bound them. So the interradius containing the hydropore is the CD interradius. The hydropore is favored as the prime datum because it arises earliest in echinoderm ontogeny and survives in the same position in adults. However, when there is a single gonopore, this is also located in the CD interradius. Functional gonopores arise much later in ontogeny than the hydropore, and echinoids possess five gonopores so they cannot be used for orientation. In many early echinoderms, the periproct also lies in the CD interradius, but it is much more mobile. Not only does it sometimes appear on the oral surface (in the rhombiferan family Caryocrinitidae), or laterally in the theca (in the related rhombiferan family Hemicosmitidae), but it is also found in the BC interradius in the rhombiferan superfamily Glyptocystitoida and in paracrinoids. Finally, Sprinkle (Reference Sprinkle1973, p. 43, fig. 16) pointed out that many early echinoderms had ambulacra arranged in a 2-1-2 pattern (Fig. 3). Consistently, the single ambulacrum is A. Thus, even when neither hydropore nor gonopore can be detected, it may still be possible to identify homologous ambulacra using this 2-1-2 pattern. A symmetry plane that bisects the A ambulacrum and the CD interradius is useful in discussing positions of hydropore and gonopore in different echinoderm groups. For example, the hydropore is typically to the right of this plane in edrioasteroids, but more commonly on it or to the left in ‘cystoids.’

Figure 3 Camera lucida drawing of the oral surface of the glyptocystitoid rhombiferan Lepadocystis moorei (Meek, Reference Meek1871) (University of Cincinnati 57349). A–E=Carpenter ambulacra; G=gonopore; H=hydropore; L=“first left ambulacral floor plates” of Sumrall and Waters (Reference Sumrall and Waters2012); PO1–PO7=peri-oral plates; 1–6=primary ambulacral cover plates. Modified from Sumrall and Waters (Reference Sumrall and Waters2012, p. 958, fig. 1).

It is now believed that the 2-1-2 pattern of ambulacra reflects the evolutionary history of the echinoderms. Derstler (Reference Derstler1981) and Paul and Smith (Reference Paul and Smith1984, p. 449, fig. 4) independently interpreted helicoplacoids as having three ambulacra, two ascending from the lateral mouth and one descending. Paul and Smith suggested that the single descending ambulacrum might be homologous with the A ambulacrum of pentameral echinoderms and the two ascending ambulacra were homologous with the B+C and D+E ambulacra, even though no evidence of the gonopore, hydropore, or periproct was known in helicoplacoids (and still is not). Smith and Rahman CT scanned a helicoplacoid, which showed no evidence of a lateral periproct (personal communication, A.B. Smith, 2015). They concluded that the periproct of helicoplacoids was apical (Smith and Zamora, Reference Smith and Zamora2013, p. 4), i.e., associated with the two ascending ambulacra, which supports the previously suggested homology of helicoplacoid ambulacra. Although this was an inference not supported by direct observation, the only alternative is that helicoplacoids had a blind gut, which seems even less likely.

Building on Bell’s (Reference Bell1976a, Reference Bellb) earlier work on edrioasteroid ambulacral development, Sumrall and Wray (Reference Sumrall and Wray2007) reviewed both the number and symmetry of the ambulacra in Paleozoic echinoderms. They argued that during ontogeny in most Paleozoic echinoderms, the ambulacra developed in three distinct phases; first the two shared lateral ambulacra (B+C and D+E) appeared, then ambulacrum A was added, and finally both lateral ambulacra divided once to give five (Fig. 4). They explained ‘aberrant’ symmetry with fewer than five ambulacra by their paedomorphic ambulacral reduction (PAR) model, in which combinations of these growth stages were lost during ontogeny. They also distinguished ‘true fivefold’ symmetry, in which all five ambulacra enter the mouth separately, from the 2-1-2 ambulacral symmetry, which is fundamentally triradiate. These subdivisions form a useful basis for discussing homologous ambulacra in ‘cystoids’ and other echinoderms.

Figure 4 True pentamery (1) where all five ambulacra leave the mouth individually versus (2) the 2-1-2 pattern in which three primary ambulacral grooves (A, B+C, and D+E) leave the mouth before the lateral ambulacra bifurcate to give five ambulacra. (3) The supposed order of ambulacral development under the paedomorphic ambulacral reduction (PAR) model of Sumrall and Wray, Reference Sumrall and Wray2007. First, the lateral primary ambulacra (B+C and D+E) develop, then ambulacrum A is added, and finally the lateral ambulacra divide. Redrawn from Sumrall and Wray (Reference Sumrall and Wray2007, p. 150, fig. 1).

Sumrall and Wray (Reference Sumrall and Wray2007, p. 155, fig. 6) summarized the results of their survey by plotting on a phylogenetic tree the occurrence of the seven types of ambulacral symmetry they recognized. They showed that each symmetry type appeared more than once on the tree under any character optimization. Their fundamental conclusion was that distantly related groups of early echinoderms repeatedly modified the basic pentaradial symmetry characteristic of the phylum. Even so, I think the situation is even more complicated. Patterns of ambulacral reduction were achieved in different ways. For example, although all early echinoderms with four ambulacra lack ambulacrum A, this was derived through two evolutionary pathways. It is currently fashionable to ignore stratigraphy in deriving evolutionary relationships, yet stratigraphy does put minimum ages on the appearance of characters as well as taxa. The diploporite families Sphaeronitidae and Holocystitidae both include genera with five and four ambulacra. In sphaeronitids, five ambulacra appear in Glyptosphaerites Müller, Reference Müller1854 and Palaeosphaeronites Prokop, Reference Prokop1964 (Tremadocian, Lower Ordovician), but genera with four ambulacra, such as Tetreucystis Bockelie, Reference Bockelie1984 and Diplosphaeronis Paul, Reference Paul1973 are unknown before the Katian (Upper Ordovician). Similarly, in the Holocystitidae, Brightonicystis (five ambulacra, Hirnantian, Upper Ordovician) appears well before genera with four (Trematocystis Jaekel, Reference Jaekel1899; Pustulocystis Paul, Reference Paul1971, Wenlock, lower Silurian). Thus, in these two families, which are united by the possession of dipores, a palate of six plates, and homologous circum-oral plates (see next section), evolution would seem to have proceeded from five to four ambulacra (Fig. 5). By contrast, Bockelie (Reference Bockelie1982, p. 493, fig. 2) documented parallel evolutionary trends in the caryocystitid rhombiferans Echinosphaerites Wahlenberg, Reference Wahlenberg1821 and Heliocrinites Eichwald, Reference Eichwald1840 from two through four ambulacra (Fig. 6). In a related genus, Caryocystites von Buch, Reference von Buch1846, the trend went only from two to three. These caryocystitid rhombiferans share humatirhombs and only five peri-oral plates (see next section). Thus, Echinosphaerites and Heliocrinites achieved species with four ambulacra by an opposite route to sphaeronitids and holocystitids. Incidentally, no holocystitid has three ambulacra. I presume Sumrall and Wray (Reference Sumrall and Wray2007, fig. 6) were referring to Triamara Tillman, Reference Tillman1967. Although S. A. Miller and coworkers described six nominal species of Triamara under the name Holocystites in the late nineteenth century (e.g., Miller Reference Miller1879, Reference Miller1891), Triamara belongs in the diploporite family Aristocystitidae.

Figure 5 Reduction of ambulacra from five to four and plating around the mouth (M) in the diploporite families (1–3) Sphaeronitidae and four to six Holocystitidae. (1) Eucystis angelini Regnéll, Reference Regnéll1945. (2) Haplosphaeronis oblonga (Angelin, Reference Angelin1878). (3) Diplosphaeronis laevis Paul, Reference Paul1973. (4) Brightonicystis gregaria Paul, Reference Paul1971. (5) Holocystites cylindricus (Hall, Reference Hall1861). (6) Trematocystis globosus (Miller, Reference Miller1878). In the Sphaeronitidae, six circum-oral plates (1–6) surround the mouth; five are radial and the sixth lies in the CD interray. A small seventh plate (7) associated with the gonopore (G) lies below and to the left of the other six plates. Food grooves radiate from the corners of the peristome and frequently end in several small facets (1), but occasionally larger facets are developed (3). In both families, the hydropore (H) is associated with the suture between the two plates in the CD interradius. The mouth (M) is covered by six palatal plates and the anus (An) by a simple anal pyramid. (4) The holocystitid Brightonicystis has five pairs of plates surrounding the mouth (1–10) and an eleventh with the gonopore. (5) Other holocystitids have six interradial peri-oral plates (1–6) and a seventh often associated with the gonopore. (3, 6) In both families, when only four ambulacra are present, ambulacrum A is not developed, but plate CO3 is. Large arrows indicate inferred direction of evolution. Redrawn from Paul (Reference Paul1973, p. 13, fig. 8). A–E=ambulacra; CO1–CO5=circum-oral plates; X1, X2=extra plates in the circum-oral circlet.

Figure 6 Increase in ambulacra from two to four and plating around the mouth (M) in rhombiferan families (1–3) Echinosphaeritidae and (4–6) Caryocystitidae. In caryocystitoid rhombiferans, five interradial plates (here interpreted as peri-orals 1-2 and 4-6) form an oral prominence. Increase in the number of ambulacra does not increase number of peri-orals, but extra plates (x) may contribute to ambulacral facets of Heliocrinites. (1–3) Ambulacral grooves are covered by cover plates, two of which in the CD interray (shaded) may become very large. A hydropore is unknown. B–E=ambulacra; G=gonopore. Arrows indicate direction of evolution. Redrawn from Bockelie (Reference Bockelie1982, p. 493, fig. 2).

Sumrall and Wray (Reference Sumrall and Wray2007, p. 156, fig. 7) also indicated how they thought the primitive 2-1-2 pattern of five ambulacra might have become pseudo-fivefold by shortening the length of the shared portions of ambulacra B+C and D+E. In at least some ‘cystoids,’ this appears to have occurred by the enlargement of the oral opening. For example, in aristocystitids whether with 2, 3, or 4 ambulacra, the mouth is a relatively narrow opening that lies entirely within the two lateral ambulacra (Fig. 7). In the eocrinoid Rhopalocystis Ubaghs, Reference Ubaghs1963 (Fig. 8) and the diploporite families Sphaeronitidae and Holocystitidae (Fig. 5), the oral opening is much larger than the ambulacral grooves, which all arise separately from the edge of the mouth, whether four or five ambulacra are developed. Kammer et al. (Reference Kammer, Sumrall, Zamora, Ausich and Deline2013) make the same point in recognizing the difference between their peristomial border systems type A2 and A3.

Figure 7 Ambulacra and oral plating in aristocystitid diploporites. (1) Aristocystites Barrande, Reference Barrande1887. (2) Calix sedgwicki Rouault, Reference Rouault1851. (3) Glaphocystis Chauvel, Reference Chauvel1966. (4) Lepidocalix Termier and Termier, Reference Termier and Termier1950. (5) Sinocystis Reed, Reference Reed1917. In aristocystitids the mouth (M) lies within a broad food groove, which extends left and right. (2) The food grooves are covered with a double biseries of cover plates. (1) In Aristocystites, a single large facet lies at each end of the food groove. (2) In Calix, both food grooves divide to give four large facets from which up to four brachioles arise. In (3) Glaphocystis, (4) Lepidocalix, and (5) Sinocystis, the four facets apparently gave rise to a single appendage. An=anus; B–E=ambulacra; CO1–CO5=circum-oral plates; F=ambulacral facets; G=gonopore; H=hydropore; PO1–PO6=peri-oral plates. (1) Redrawn from Parsley (Reference Parsley1990, p. 286, fig. 2). (2) Redrawn from Chauvel (Reference Chauvel1977, p. 315, fig. a). (3) Redrawn from Chauvel (Reference Chauvel1966, pl. 4, fig. 1E). (4) Redrawn from Makhlouf et al. (Reference Makhlouf, Lefebvre, Nardin, Nedjari and Paulin press, fig. 6, A2). (5) Redrawn from Bather (Reference Bather1918, p. 535, fig. 9). Dashed plate sutures inferred.

Figure 8 Ambulacra and oral plating in the ‘eocrinoid’ Rhopalocystis. A–E=ambulacra; F=ambulacral facet (only shown in amb D). M=mouth; PO1–PO6=peri-oral plates. Redrawn after Ubaghs (Reference Ubaghs1963, fig. 7, p. 33). Note the peri-orals lack brachiole facets.

Tables 1 and 2 (supplementary material) summarize my interpretation of which ambulacral patterns are present. The tables attempt to include at least one example of every ambulacral pattern known within the major taxa listed. Variation in ambulacral patterns is considerable and there remain numerous early echinoderms whose ambulacral patterns are uncertain.

I think the PAR model of Sumrall and Wray (Reference Sumrall and Wray2007, figs. 6 and 7) was a useful initial concept, but it can be extended. For example, their figure 6 implies that all triradiate echinoderms have an ambulacral pattern of A, B+C, D+E. This is true of the hemicosmitoid rhombiferans Hemicosmites von Buch, Reference von Buch1840, Caryocrinites Say, Reference Say1825, Juglandocrinus von Koenen, Reference von Koenen1886, Paracaryocrinites Chen and Yao, Reference Chen and Yao1993, and Stribalocystites Miller, Reference Miller1891 (Lanc et al., Reference Lanc, McDermott and Paul2015) (Fig. 9), but not of the caryocystitoid rhombiferans Caryocystites, Echinosphaerites, and Heliocrinites with three ambulacra because ambulacrum A is never present in these genera (Bockelie, Reference Bockelie1982) (Fig. 6). Furthermore, the echinoencrinitid rhombiferan Tyrridiocystis Broadhead and Strimple, Reference Broadhead and Strimple1978 has B, C, and D+E (Broadhead and Strimple, Reference Broadhead and Strimple1978) (Fig. 10), whereas the aristocystitid diploporite Triamara has B+C, D and E (Paul, Reference Paul1971). Whether Trimerocystis Schuchert, Reference Schuchert1904 is a teratological specimen of the callocystitid rhombiferan Pseudocrinites Pearce, Reference Pearce1843 (as suggested by Kesling, Reference Kesling1961, p. 258) or a valid genus, it has three ambulacra, B, C, and E. Kesling (Reference Kesling1961, fig. 2b) illustrated a specimen of the callocystitid rhombiferan Jaekelocystis hartleyi Schuchert, Reference Schuchert1903 with ambulacra E, A, and B and (Fig. 3b) another in which ambulacrum D was very short. Finally, I recall a teratological example of the callocystitid rhombiferan Lepadocystis moorei (Meek, Reference Meek1871) in the Field Museum, Chicago, which had only ambulacra A, D, and E. Thus, just with three-rayed genera and anomalous individuals, a variety of patterns exists.

Figure 9 Oral areas in hemicosmitoid rhombiferans. (1) Hemicosmites pyriformis von Buch, Reference von Buch1840 (holotype). (2) H. pyriformis to show oral plating. (3) Juglandocrinus sdzuyi (Chauvel and le Menn, Reference Chauvel and le Menn1979). (4) Paracaryocrinites sp. (5) Thomacystis tuberculata Paul, Reference Paul1969. (6) Caryocrinites ornatus Say, Reference Say1825. (7) Stribalocystites tribrachiatus (Frest, Reference Frest1975). (8) Diagrammatic representation of ambulacra and mouth in Caryocrinitidae. Apart from (5) Thomacystis, all hemicosmitoids have three ambulacra (A, B+C, D+E). (6) In Caryocrinites s.s., all three ambulacra branch, so lateral groups of facets do not represent separate ambulacra B–E. Ac=accessory plates; AF=accessory facet; An=anus; AO=ambulacral orifice; AP=accessory pore; Ce=central tegminal plate; F=facetal plates; G=gonopore; H=hydropore; IR=interradial plates; L1–L9=lateral plates; M=mouth; PO1–PO6=peri-oral plates; n=nerve pore; R1–R9=radial plates; W=wedge plates; 1–8=pericentral tegminal plates. Outlines of facets and food grooves dashed lines. (1, 2) Redrawn from Bockelie (1979c, fig. 8c, 8b, respectively, p. 375). (3) Redrawn from Chauvel and le Menn (Reference Chauvel and le Menn1979, p. 554, fig. 2e). (4) Redrawn from Lanc et al. (Reference Lanc, McDermott and Paul2015, p. 7, fig. 6). (5) Redrawn from Paul (Reference Paul1969, p. 193, fig. 2). (6) Redrawn from Frest (Reference Frest1975, p. 89, fig. 4a). (7) Redrawn from Frest (Reference Frest1975, p. 103, fig. 15). (8) Redrawn from Lanc et al. (Reference Lanc, McDermott and Paul2015, p. 5, fig. 3c).

Figure 10 Ambulacra and oral plating of the glyptocystitoid rhombiferan Tyrridiocystis Broadhead and Strimple, Reference Broadhead and Strimple1978, to show three ambulacra, B, C, and D+E. Contrast with the aristocystitoid diploporite Triamara, which also has three ambulacra, but B+C, D, and E. Br=brachiolar plates; G=gonopore; H=hydropore; M=mouth; PO1–PO7=peri-oral plates; ?=an additional plate between PO1 and PO2. Redrawn from Broadhead and Strimple (Reference Broadhead and Strimple1978, p. 172, fig. 3a).

To grow all the variety of ambulacral structures seen in primitive echinoderms requires only three instructions: ‘grow,’ ‘branch,’ and ‘stop growing.’ Thus, in one sense, all cases of echinoderms with fewer than five ambulacra must arise by paedomorphosis, since the ‘stop’ instruction must happen very early in development. Nevertheless, the simple pattern of ambulacral addition indicated by Sumrall and Wray (Reference Sumrall and Wray2007, fig. 1) (Fig. 4) did not exist as a developmental pattern in early echinoderms. If it did, echinoderms with four and one ambulacrum would not exist. I suspect that ambulacral growth was controlled by developmental genes, which not only could be turned off to reduce the number of ambulacra, but acted entirely independently in each ambulacrum. So, for example, to get the three-rayed pattern of Tyrridiocystis (Fig. 10) and Triamara required turning ambulacrum A off in both, but then branching only the B+C ambulacrum in the former and only the D+E ambulacrum in the latter. Whether all two-rayed forms have B+C and D+E, or some other pattern, such as the C and E pattern of the callocystitid rhombiferan Pseudocrinites (Jaekel, Reference Jaekel1899; Kesling, Reference Kesling1961) depends on the oral plating, which now needs to be considered more fully. Pseudocrinites is a particularly interesting case in the context of the PAR model. It has only two ambulacra, in both of which the first two brachioles branch to the left. Thus, developmental arguments suggest the two ambulacra are homologous with the B and D ambulacra of pentameral callocystitids, which also have the first two facets to the left. However, one of the ambulacra passes between the periproct and the gonopore and hydropore. Positional arguments show that this ambulacrum in callocystitids with five (or four) ambulacra is always ambulacrum C and the ambulacrum opposite is E. Thus, if one accepts the developmental argument, these two ambulacra have rotated on the theca. Alternatively, if one accepts the positional argument, the two ambulacra have developed extra brachioles. Knowledge of the oral plating could settle the argument since O1 is always associated with ambulacrum D, O2 with E, and so on, but unfortunately the only illustration of the orals in Pseudocrinites (Paul, Reference Paul1967a, p. 325, fig. 16) is completely wrong. It resulted from a combination of fortuitous cracks and wishful thinking, but is obviously wrong. The orals surround the mouth; they cannot all be visible on one side of it.

Homology of oral plates

Without knowledge of the oral plating, the taxonomic affinities of most early echinoderms remain uncertain. Sumrall (e.g., Reference Sumrall2010, p. 269 and elsewhere) has rightly pointed out that giving a series of plates the same name does not mean the plates are homologous in different echinoderm groups, but merely implies it. This is just as true of oral plates as of basals, laterals, or deltoids. Equally, however, assigning different names may obscure homology, as with echinoid interambulacral and asteroid adambulacral plates. The current scheme of plate notation in lenticular paracrinoids (Parsley and Mintz, Reference Parsley and Mintz1975) is particularly unhelpful in identifying the oral plates. Thus, the homology of oral plates in different early echinoderms needs to be tested, and those with unusual arrangements can be used as a test. It is immediately apparent that the number of oral plates does not correlate with the number of ambulacra in several major groups (e.g., the caryocystitoid rhombiferans, Fig. 6). Thus, the assumption that the orals of ‘cystoids’ are homologous with primary ambulacral plates (as in other echinoderm classes) needs consideration.

Kammer et al. (Reference Kammer, Sumrall, Zamora, Ausich and Deline2013) have recently considered this aspect in detail. They made a fundamental distinction between oral frames composed exclusively of interradially positioned plates, which they called ‘orals’ and which constituted their type A peristomial border system (PBS), and those composed of radially positioned plates, which they called ‘oral frame plates’ (their PBS type B). They recognized four different subtypes of type A oral frames and two of type B. Type A1 involved paired ambulacral flooring plates as well as the usual seven ‘orals.’ It was recognized only in the middle Cambrian edrioasteroids Kailidiscus Zhao et al., Reference Zhao, Sumrall, Parsley and Peng2010 and Walcottidiscus Bassler, Reference Bassler1935. Types A2 and A3 were distinguished by having four or six ‘orals’ forming the mouth frame, respectively. In the former, the 2-1-2 pattern of ambulacra is obvious, but it is absent in the latter, where all five food grooves enter the mouth separately. Types A2 and A3 were characterized by the glyptocystitid rhombiferan Lepadocystis Carpenter, Reference Carpenter1891 (Fig. 3) and the eocrinoid Rhopalocystis (Fig. 8), respectively. Type A4 also had six plates forming the peristome frame, but differed in having the mouth covered by just five primary peristomial cover plates. It was characterized by three genera of crinoids.

Type B1 was characterized by having five oral frame plates closest to the mouth and in contact with the ‘orals’ more distally, whereas type B2 had an oral frame composed of just the five oral frame plates. Both were exemplified by edroasteroid genera.

My own ideas are remarkably similar, despite being based on different genera of early echinoderms. I certainly agree that radial versus interradial positioning is a fundamental distinction in recognizing types of oral frame plates in early echinoderms. Paul (Reference Paul1971, p. 71) introduced the term ‘peri-orals’ for interradially positioned ‘orals’ in the diploporite family Holocystitidae, the equivalent of Kammer et al.’s (Reference Kammer, Sumrall, Zamora, Ausich and Deline2013) ‘orals.’ Paul (Reference Paul1973, pp. 12, 13, fig. 8) introduced the term ‘circum-orals’ for the radially positioned ‘orals’ in the diploporite family Sphaeronitidae, the equivalent of Kammer et al.’s (Reference Kammer, Sumrall, Zamora, Ausich and Deline2013) ‘oral frame plates.’ Although not explicitly stated, this was to avoid any implied homology between the two types of oral frames and because the oral cover plates of sphaeronitids had also previously been referred to as ‘orals’ (e.g., Prokop, Reference Prokop1964, p. 13). Paul (Reference Paul1971, p. 6, fig. 1) also introduced the term ‘palate’ for the cover of the mouth in both holocystitids and sphaeronitids, which is composed of six primary oral cover plates or ‘palatals.’ All three terms, peri-orals, circum-orals, and palatals, are still useful and will be used here.

In the 1960s and 1970s, it was fashionable to emphasize differences between major groups of echinoderms. However, with the rise of cladistics, it has become more common and more profitable to seek similarities between such groups. So, in the early 1970s, I was concerned not to imply unwarranted homologies by using the same term ‘orals’ for plates I did not think were homologous. Now, I would go further and suggest that radially positioned oral plates, the circum-orals, may well be homologous in whatever echinoderm they occur. Similarly, interradially positioned orals, my peri-orals, may also be homologous wherever they occur, although there are some reasons to doubt this. Thus, I now think the diploporite family Sphaeronitidae is characterized by the loss of the peri-oral circlet and the retention of circum-orals to form the mouth frame (Fig. 5.1–5.3). The diploporite family Holocystitidae has both peri-orals, forming the mouth frame, and circum-orals (previously called facetal plates, Paul, Reference Paul1971) (Fig. 5.5, 5.6). If one accepts this idea, then any suggested homology between circum-orals and peri-orals can be rejected on the conjunction test of Patterson (Reference Patterson1982, Reference Patterson1988), which states that two supposedly homologous structures cannot be present in the same organism. I would also suggest a further test of whether the new interpretation improves our understanding of early echinoderm relationships. Here I would argue that it does because I can suggest new interpretations of the oral plating of the diploporite families Aristocystitidae (Fig. 7) and Parasphaeronitidae (Fig. 11), both of which have at least eight plates associated with their oral areas.

Figure 11 Ambulacra and oral plating in the parasphaeronitid diploporite Pachycystis Bockelie, Reference Bockelie1984. Note that the mouth (M) is surrounded by eight circum-oral plates (CO1–CO6, X1, X2); the gonopore (G) and hydropore (H) occur in plate CO1. An - anus. Redrawn from Bockelie (Reference Bockelie1984, p. 15, fig. 11g). Dashed plate sutures inferred.

As far as Kammer et al.’s (Reference Kammer, Sumrall, Zamora, Ausich and Deline2013) types of peristomial border systems are concerned, among ‘cystoids,’ the unique holocystitid diploporite Brightonicystis Paul, Reference Paul1971 (Fig. 5.4) and those hemicosmitoid rhombiferans in which the oral surface is not hidden by a tegmen (Fig. 9.1, 9.2, 9.4) have oral frames in which pairs of ambulacral plates are involved. They constitute a type of oral plating at least analogous to Kammer et al.’s type A1. I would emphasize the distinction between incorporating pairs of ambulacral plates and only incorporating the first ambulacral plate into the oral frame rather more than Kammer et al. (Reference Kammer, Sumrall, Zamora, Ausich and Deline2013) did.

I agree with Kammer et al.’s (Reference Kammer, Sumrall, Zamora, Ausich and Deline2013) types A2 and A3, although I would point out that Paul and Donovan (Reference Paul and Donovan2011, p. 448, fig. 14) illustrated two examples of the same rhombiferan species, Glansicystis glans Paul (in Paul and Donovan, Reference Paul and Donovan2011), which fall into both groups (see Fig. 12.1, 12.2). I suspect the difference arises because the oral area retains more or less sediment, but it could also be due to changes during growth.

Figure 12 Ambulacra and oral plating in some ‘cystoids’ with four ambulacra. (1, 2) The glyptocystitoid rhombiferan Glansicystis Paul, Reference Paul1967a. (3) The paracrinoid Bistomiacystis Sprinkle and Parsley, Reference Sprinkle and Parsley1982. AP=anal pyramid; B–E=ambulacra; G=gonopore; H=hydropore; M=mouth; PO1–PO7=peri-oral plates. (1, 2) Food grooves and ambulacral facets dotted. Although ambulacrum A is undeveloped, the full complement of seven peri-orals is present. (1, 2) Two examples of Glansicystis glans Paul, in Paul and Donovan (Reference Paul and Donovan2011) with six and four plates framing the mouth, respectively. In Bistomiacystis, four of the peri-orals have overgrown the mouth. (1, 2) Redrawn from Paul and Donovan (Reference Paul and Donovan2011, p. 448, fig. 14). (3) Redrawn from Sumrall and Deline (Reference Sumrall and Deline2009, p. 136, fig. 2).

Kammer et al.’s (Reference Kammer, Sumrall, Zamora, Ausich and Deline2013) type A4 differs primarily from A3 by their interpretation of the oral cover plates; that is, a palate is present. I suspect palates simply represent paedomorphic cessation of the introduction of new cover plates during growth. However, I would make a distinction between those peristome border systems (and palates) that are composed of six peri-oral plates (and six palatals) and those where only five peri-orals (and five palatals) occur. The former appear to characterize ‘cystoid’ groups, although the unusual diploporite Eumorphocystis Branson and Peck, Reference Branson and Peck1940 apparently has five peri-orals. The latter are typical of crinoids.

Types B1 and B2 differ in having both circlets of plates in B1 and only the circum-orals in B2. However, here I think the proposed homology of the two circlets breaks down due to the positional test. In holocystitids, the peri-oral circlet forms the mouth frame and is surrounded by the circum-oral circlet. In Kammer et al.’s (Reference Kammer, Sumrall, Zamora, Ausich and Deline2013) example of type B1 peristomial border system, the edrioasteroid Edriophus Bell, Reference Bell1976b, the supposed circum-orals surround most of the mouth and the supposed peri-orals are more distal.

Mouth frames with single ambulacral plates

Helicocystis Smith and Zamora, Reference Smith and Zamora2013 (Fig. 13) was regarded as the latest common ancestor of all pentaradial echinoderms and has seven interradial peri-orals, five of which align with the spiral ambulacra and two extra plates that occur in the CD interray associated with the gonopore. Similar patterns, with the oral frame composed of six peri-oral plates with or without a seventh plate associated with the hydropore and/or gonopore, are found repeatedly in ‘cystoids’ with five ambulacra (e.g., Figs. 3, 5, 8, 14). In some taxa with fewer than five ambulacra, the same basic pattern is retained (Figs. 10–12, 15) and was present in the recently described Sanducystis Zamora et al., Reference Zamora, Sumrall., Zhu and Lefebvrein press. It is tempting to suggest that this is a fundamental pattern in ‘cystoid’ groups and other early echinoderms, as Sumrall (Reference Sumrall2010, Reference Sumrall2015) has done.

Figure 13 Oral area of Helicocystis Smith and Zamora, Reference Smith and Zamora2013, to show seven oral plates and spiral ambulacra (above and right).

Figure 14 Ambulacra and oral plating in some pentameral ‘cystoids.’ (1) The cryptocrinitid Cryptocrinites von Buch, Reference von Buch1840. (2) The protocrinitid diploporite Tristomiacystis Sumrall et al., Reference Sumrall, Brett, Dexter and Bartholomew2009. (3) The gomphocystitid diploporite Celticystis Bockelie, Reference Bockelie1979a. A–E=ambulacra; An=anus; G=gonopore; H=hydropore; M=mouth; PO1–PO6=peri-oral plates. Food grooves and ambulacral facets dotted. Three ambulacral orifices of Tristomiacystis black. (1) Redrawn from Bockelie (Reference Bockelie1981b, p. 139, fig, 10b). (2) Redrawn from Sumrall et al. (Reference Sumrall, Brett, Dexter and Bartholomew2009, p. 745, fig. 5.1). (3) Redrawn from Bockelie (Reference Bockelie1979a, p. 160, fig. 5a). The larger, presumed first ambulacral facet in Cryptocrinites is to the left (counterclockwise) in each ambulacrum; the peri-orals lack facets; and the anus lies in the BC interray. In Tristomiacystis, the first facets are to the left and shared by a peri-oral plate and an ambulacral plate; four of the peri-orals have overgrown the mouth. Celticystis has all facets to the left of the ambulacra, and the first facets are shared by a peri-oral and an ambulacral plate except in ambulacrum C.

Figure 15 Ambulacra and oral plating in some glyptocystitoid rhombiferan ‘cystoids’ with two ambulacra. (1, 2, 5) Pleurocystites Billings, Reference Billings1854. (3, 4, 6) Praepleurocystis Paul, Reference Paul1967b. (7) Schizocystis Jaekel, Reference Jaekel1895. 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 (Reference Paul1984, p. 119, fig. 76); (5, 6) from Paul (Reference Paul1967b, p. 113, figs. 8, 9), respectively; (7) from Kesling (Reference Kesling1968a, p. S185, fig. 89, 1b). The ambulacral facets in Schizocystis (7) are largely to the left in both ambulacra.

Variations in this pattern of orals occur. For example, the ‘eocrinoid’ Rhopalocystis has six peri-orals around the mouth, which lack ambulacral facets (Fig. 8), as does Cryptocrinites von Buch, Reference von Buch1840 (Fig. 14.1), whereas only four of the peri-orals of glyptocystitoid rhombiferans form the mouth frame (Fig. 3). Glyptocystitoid peri-orals always share the first facet with an adjacent ambulacral plate (Figs. 3, 10) or with each other (Fig. 12.1, 12.2). The unusual diploporite Tristomiacystis Sumrall et al., Reference Sumrall, Brett, Dexter and Bartholomew2009 (Fig. 14.2) and the paracrinoid Bistomiacystis Sprinkle and Parsley, Reference Sprinkle and Parsley1982 (Fig. 12.3) have peri-orals that have grown over the ambulacral grooves producing multiple ambulacral orifices. The peri-orals of Tristomiacystis share ambulacral facets with the first ambulacral plates; those of Bistomiacystis do not. Finally, when ambulacra curve, ambulacral facets are always aligned on the outside of the curve as in the gomphocystitid diploporite Celticystis Bockelie Reference Bockelie1979a (Fig. 14.3) and the paracrinoid Bistomiacystis (Fig. 12.3).

In sphaeronitid (Fig. 5.1–5.3) and parasphaeronitid diploporites (Fig. 11), five of the ‘orals’ are radial in position as are those of the ‘eocrinoid’ Lichenoides Barrande, Reference Barrande1846. Here these are distinguished as circum-orals, and they occur in a mouth frame of six plates in sphaeronitids, but eight in parasphaeronitids (Fig. 11). It is believed these circum-orals are homologous with the outer circlet of plates in holocystitids, originally called facetals by Paul (Reference Paul1971). In the holocystitid Pustulocystis Paul, Reference Paul1971, there are six facetal plates, whereas in Holocystites Hall, Reference Hall1864, Trematocystis, and Pentacystis Paul, Reference Paul1971, there are eight (Fig. 5.5, 5.6). By contrast, aristocystitid diploporites have at least eight plates surrounding the mouth (Fig. 7), which are here interpreted as a mixture of four peri-orals and four circum-orals.

Recently, Sheffield and Sumrall (Reference Sheffield and Sumrall2015) have discussed oral plate homology in holocystitid diploporites. The paper contains some taxonomic errors; Trematocystis was described by Jaekel (Reference Jaekel1899) not Miller (Reference Miller1878), Pustulocystis by Paul (Reference Paul1971) not Miller (Reference Miller1891), and Pentacystis gibsoni by Frest and Strimple (in Frest et al., Reference Frest, Strimple and Paul2011) not by Paul, Reference Paul1971. More important, Sheffield and Sumrall (Reference Sheffield and Sumrall2015) suggest two new interpretations of the oral plating in the Holocystitidae. First, they suggest that the first two circlets surrounding the mouth consist of seven orals and seven facetals, not six and eight as originally suggested by Paul (Reference Paul1971, p. 13). Then they suggest that Pentacystis possessed a complete oral circlet with six plates surrounding the mouth, but that these were lost taphonomically. The first suggestion is not controversial, especially given the large number of early echinoderms that are now known to have the basic pattern of six peri-orals forming the mouth frame, plus a seventh often bearing the gonopore in the CD interray.

The suggestion that Pentacystis originally had the six orals surrounding the mouth is attractive. It would certainly make the anatomy of almost all holocystitid genera more consistent. In support of this interpretation, the type species Pentacystis simplex Paul, Reference Paul1971, which was originally thought to lack all six plates, lacks the distinctive oral pores that usually line the peristome frame in other holocystitids, suggesting the six peri-oral plates are indeed missing. However, Frest and Strimple (in Frest et al., Reference Frest, Strimple and Paul2011, p. 89, fig. 46c) illustrated a specimen of Osgoodicystis cooperi with just the two orals in the CD interray and oral pores developed on the five ‘facetals’ (equivalent to CO2-4, X1-2 in Fig. 5.5) that complete the oral frame. Sheffield and Sumrall (Reference Sheffield and Sumrall2015, p. 161, fig. 1c) illustrate a specimen of Pentacystis gibsoni in which they interpret some loose plates in the E, A, and B radii as displaced peri-orals. It is difficult to see 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., Reference Frest, Strimple and Paul2011, p. 2). I think Sheffield and Sumrall’s (Reference Sheffield and Sumrall2015) 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 rhombiferan Thomacystis Paul, Reference Paul1969 (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 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, Reference Bockelie1981a, p. 199, fig. 10; Paul, Reference Paul1997, 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 rhombiferans may have from one to five ambulacra, and in some genera, e.g., Callocystites Hall, Reference Hall1852, the ambulacra may branch (Paul, Reference Paul2015), 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 associated with the mouth (Fig. 16), although Parsley (Reference Parsley1978, p. 474, fig. 1) illustrated a specimen of Comarocystites Billings, Reference Billings1854, with a very small mouth surrounded by just three ‘orals.’ Many paracrinoids have just two ambulacra, which may define two faces currently referred to as ‘anterior’ and ‘posterior’ (see Parsley and Mintz, Reference Parsley and Mintz1975; Frest and Strimple, Reference Frest and Strimple1976; Frest et al., Reference Frest, Strimple and McGinnis1976; Parsley, Reference Parsley1982b). Two plates of the anterior face (A3, A4) and two of the posterior face (P6, P7) usually frame the mouth (Fig. 16.2). The gonopore consistently occurs in plate P7, and the hydropore occurs at the triple junction between plates P5–P7 (Fig. 16.2). This arrangement is present in globular genera such as Canadocystis Jaekel, Reference Jaekel1900 (Fig. 16.1). Insofar as it can be determined, these plates are interradial and are here accepted as peri-orals. An unusual feature of many paracrinoids is that the first ambulacral facet in both ambulacra is significantly larger than subsequent facets. If one includes these two ambulacral plates (PO in Fig. 16), then many paracrinoids have seven peri-orals. Unfortunately, not all these peri-orals can be homologous with the seven ‘orals’ identified in Bistomiacystis (Fig. 12.3).

Figure 16 Oral regions of paracrinoids (1) Canadocystis Jaekel, Reference Jaekel1900 and (2) Platycystites Miller, Reference Miller1889. A3, A4=‘anterior’ plates; F=pinnule facet; G=gonopore; H=hydropore; M=mouth; P5–P7=‘posterior’ plates; Pe=periproct; PO=primary ossicle of recumbent arm. Labeling follows Parsley and Mintz, Reference Parsley and Mintz1975. (1), Redrawn from Kesling, Reference Kesling1968b (p. S280, fig. 162.2g). (2) Redrawn from Parsley and Mintz, Reference Parsley and Mintz1975 (p. 65, fig. 5).

Ambulacral structure

This is probably the part of this paper where we have the least information. In the taphonomy of ‘cystoids,’ it appears that after death, loss of erect ambulacral structures occurred first. Furthermore, the loss was commonly at the junction with the theca, so that entire structures were lost immediately rather than being degraded bit by bit from their tips. For example, in the Diploporita, no sphaeronitid is known with ambulacral appendages preserved, and in the Holocystitidae, Frest and Strimple have described a handful of specimens in which the first ambulacral plate is preserved in some ambulacra (Frest et al., Reference Frest, Strimple and Paul2011, p. 31, pl. 4), where the plates are all uniserial. Chauvel (Reference Chauvel1977) described four or five examples in the Aristocystitidae in which any evidence of brachioles is preserved. Thus, in this paper, unknown ambulacral structures are referred to as ‘appendages.’ Historically, the most significant feature of ambulacral structures was whether the plating was uniserial or biserial. Here, terminal uniserial structures are called ‘pinnules,’ biserial structures are called ‘brachioles,’ and complex, branched structures are called ‘arms’ without inferring any phylogenetic implications. All three may be erect or recumbent on the theca (Supplementary material, Table 3). In addition, whatever the ambulacral plating, they can be developed in three positions. Those forming part of the thecal wall are here referred to as ‘mural.’ Those developed outside the theca are ‘exothecal’ and may be either erect or recumbent on the thecal surface. In the latter case, pathways develop on thecal plate surfaces to accommodate the ambulacral structures. Such ambulacral structures have frequently been described as ‘epithecal,’ but this term has been used for both mural and recumbent structures in the past and is best abandoned.

All parts of the ambulacral system including flooring and cover plates bear the same topological relationship to the inferred radial water vessels or tube feet (Fig. 17) and so can be regarded as homologous. In effect, this is restating the EAT in a different way—all such plates are part of the axial skeleton and therefore homologous at that level of homology. However, some oral cover plates were added at intermediate positions during growth rather than always terminally (Fig. 3). For example, Sumrall (Reference Sumrall2010, p. 270, fig. 1) has identified five cover plates at the interrays between the ambulacra as ‘primary oral cover plates’ in the glytocystitoid rhombiferan Lepadocystis. I prefer to include a second in the CD interray, making six primary oral cover plates in all (Fig. 3), but either way, if these cover plates were the first to be secreted then the other oral cover plates between primaries 2 and 5 in Figure 3 were added later and could not have been added terminally as the ambulacra grew. Similar patterns occur in the oral cover plates of edrioasteroids (Bell, Reference Bell1976a, Reference Bellb). This is not an anomaly under the EAT, as cover plates have not been considered to comply universally with the ocular plate rule of exclusively terminal addition (see, for example, Mooi et al., Reference Mooi, David and Wray2005, p. 546).

Figure 17 Ambulacral structure and inferred water vascular system in helicoplacoids to show that both flooring and cover plates have the same topological position relative to the radial water vessel and tube feet, respectively. Redrawn from Paul and Smith (Reference Paul and Smith1984, fig. 4D).

Modern crinoid ambulacra differ from those of all other living classes in lacking a tube foot/ampulla system and in having tube feet arranged in triplets (Nichols, Reference Nichols1960) (Fig. 18). They also lack calcified cover plates but have lappets alternately on either side of the food groove instead. When feeding, the long tube feet are extended horizontally, between the lappets (Fig. 18.3). The intermediate and short tube feet are attached to the lappets proximally and extend at different angles to the horizontal (Fig. 18.2, 18.3). When the latter two bend to deposit food in the food groove, they move the lappets. There is, thus, a 1:1 relationship between tube-feet triplets and lappets. It is possible that a similar 1:1 relationship occurred between ‘cystoid’ tube feet and cover plates or cover plate sets. It is equally possible that the structure of modern crinoid tube feet is a post-Paleozoic innovation and that such uniformitarian arguments do not apply to the Paleozoic.

Figure 18 Water vascular system in a pinnule of the Recent comatilid crinoid Antedon. (1) Ventral view showing the triplets of tube feet (black) and their relationship to the lappets. C – C=position of the cross section in (3). (2) Oblique ventral view. (3) Cross section to show orientation of tube feet with respect to the long axis of a pinnule. Redrawn from Nichols (Reference Nichols1960, fig. 1).

Sumrall (Reference Sumrall2008, Reference Sumrall2010) used the ambulacra of glyptocystitoid rhombiferans as typical in developing his universal elemental homology (UEH) model, so it is appropriate to consider glyptocystitoid ambulacral structure as a whole (Fig. 3). In glyptocystitoid rhombiferans, the first, horseshoe-shaped, brachiole facet is developed to the left (as viewed in the direction of growth) in all ambulacra (Fig. 3) and is shared by one of the peri-orals (PO1-PO5) and the plates labeled L (“first left ambulacral floor plates” of Sumrall and Waters, Reference Sumrall and Waters2012). Thereafter, each brachiole facet is supported by a smaller adoral ambulacral flooring plate and a larger distal flooring plate. Here, I suggest these pairings are homologous and represent the first pair of brachiolar plates modified to form a peri-oral plus an L plate or pairs of ambulacral flooring plates. Glyptocystitoids with all five ambulacra have an unusual arrangement in which ambulacra B and D have the first two brachioles to the left, and thereafter brachioles alternate right and left to the tip of the ambulacrum. In ambulacra A, C, and E, only the first brachiole lies to the left of the ambulacrum, and brachioles alternate regularly throughout these ambulacra (Fig. 3). Irrespective of this unexpected ‘B D different’ pattern of primary brachioles in glyptocystitoids, in early forms the ambulacral flooring plates form part of the thecal wall. When extensive ambulacra are developed, as in the cheirocrinid Coronocystis Paul, Reference Paul1972, clefts develop in the extended radial plates to accommodate the ambulacra. Furthermore, the oral frame is composed of the first ambulacral plates secreted on the left side of all five ambulacra plus a sixth (PO6) in the CD interray. Brachioles are biserial, and in extensive ambulacra the main trunk is composed of a double biseries of flooring plates. Here, this structure is interpreted as resulting from the first pair of brachiolar plates on either side being modified as flooring plates. A similar structure occurs in the main trunk of the erect arms of the hemicosmitoid rhombiferan Caryocrinites Say, Reference Say1825 (see Sprinkle, Reference Sprinkle1975; Lanc et al., Reference Lanc, McDermott and Paul2015) and the recumbent ambulacra of blastoids (Beaver, Reference Beaver1968, p. S322, fig. 190).

Main ambulacral trunks occasionally branch in the glyptocystitoid families Callocystitidae and Glyptocystitidae. In Callocystites and Coelocystis Schuchert, Reference Schuchert1903 (Callocystitidae), the branching is equal (Paul, Reference Paul2015), whereas in Sphaerocystites Hall, Reference Hall1859, Strobilocystites White, Reference White1876 (Callocystitidae), and Hesperocystis Sinclair, Reference Sinclair1945 (Glyptocystitidae) the branching is unequal (Sprinkle, Reference Sprinkle1982a; Paul, Reference Paul2015).

The suggestion that peri-orals and L plates are basically modified first brachiolar plates can be tested by seeing whether a similar pattern can be found in other groups of ‘cystoids’ besides glyptocystitoids and the hemicosmitoid Caryocrinites. The eocrinoid Rhopalocystis (Fig. 8) has peri-orals that do not bear or share brachiole facets. Thereafter, a simple biserial set of mural ambulacral flooring plates bear brachiole facets contained entirely within the individual plates. Such an ambulacral structure could be derived in an analogous way to the ambulacral structure of glyptocystitoids if the appendages were pinnules and the first pinnular plates were modified as ambulacral flooring plates. Unfortunately, Rhopalocystis has biserial brachioles (Ubaghs, Reference Ubaghs1963, p. 36, pl. 1, fig. 4).

Recumbent biserial ambulacra occur in the palaeocystitids Ulrichocystis Bassler, Reference Bassler1950, and Bromidocystis Sprinkle, Reference Sprinkle1982b, with each ambulacral facet confined to a single flooring plate (Paul, Reference Paul1988; Sprinkle, Reference Sprinkle1982b). In Bromidocystis, the peri-orals lack ambulacral facets. Ulrichocystis had three ambulacra with unknown appendages, whereas Bromidocystis had five ambulacra and recumbent brachioles (Sprinkle, Reference Sprinkle1982b, p. 290, fig. 72i). Rhopalocystis and Bromidocystis alone are enough to 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, Reference Conrad1842 and Cupulocorona Donovan and Paul, Reference Donovan and Paul1985 are known to have had isotomously branched erect biserial arms (Fay, Reference Fay1962; Brett et al., Reference Brett, Frest, Sprinkle and Clement1983), but the first plate of each ambulacrum is uniserial and analogous to a crinoid axilliary plate (Donovan and Paul, Reference Donovan and Paul1985, 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 Trachelocrinus Sprinkle, Reference Sprinkle1973 and Bockia Bockelie, Reference Bockelie1981b. Trachelocrinus is unique in having lateral brachioles that branch off every third trunk plate (Sprinkle, Reference Sprinkle1973, 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, Reference Bockelie1981b, p. 129, fig. 2g, j). Both Bockia and Trachelocrinus have erect arms arising from an oral prominence, which Bockelie (Reference Bockelie1981b, 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, Reference Parsley and Mintz1975, pl. 1, figs. 2, 3) or recumbent (e.g., Sinclairocystis, see Parsley, Reference Parsley1982b). 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, Reference Sprinkle and Parsley1982). 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, Reference Parsley and Mintz1975, 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, Reference Sprinkle and Parsley1982, p. 225, fig. 60), and this probably explains why it has its facets confined to the right side of each ambulacrum. The flattened eocrinoid Haimacystis Sumrall et al., Reference Sumrall, Sprinkle and Guensburg2001, has only two ambulacra curved in a rough semicircle, apparently composed of biserial trunk plates possibly with biserial brachioles arising from the outer edge of the curved ambulacra (Sumrall et al., Reference Sumrall, Sprinkle and Guensburg2001, p. 988, fig. 2.2, p. 990, fig. 4.3). Thus, one ambulacrum 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 disproportionately large (see, for example, Parsley and Mintz, Reference Parsley and Mintz1975, pl. 3, fig. 5, pl. 7, figs. 14, 15). Finally, Malocystites murchisoni Billings, Reference Billings1858, is unique among paracrinoids in having recumbent pinnules (Parsley and Mintz, Reference Parsley and Mintz1975, p. 86).

Rozhnov (Reference Rozhnov2012, Reference Rozhnov2015) has recently summarized available information on two possible paracrinoid genera from the Baltic region. Heckerites Rozhnov, Reference Rozhnov1987 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, Reference Rozhnov2012, p. 310, 311, fig. 3). Similarly, Achradocystites has erect biserial arms with alternating biserial brachioles. Superficially, Heckerites resembles the North American paracrinoid Amygdalocystites Billings, Reference Billings1854, and the erect 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 structures 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 1 mm 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, Reference Jaekel1899, Tholocystis Chauvel, Reference Chauvel1941, Herpetocystis Termier and Termier, Reference Termier and Termier1972, and Finitiporus Frest and Strimple (in Frest et al., Reference Frest, Strimple and Paul2011), 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., Reference Frest, Strimple and Paul2011, p. 32, fig. 19). Frest and Strimple described such ambulacra as ‘epipanniculate’ and assigned them to a new subfamily, Herpetocystinae (Frest et al., Reference Frest, Strimple and Paul2011, p. 59, table 19, p. 62), although Termier and Termier (Reference Termier and Termier1972) had previously erected a family Herpetocystidae.

Second, in some species of Eucystis Angelin, Reference Angelin1878 and all species of Glyptosphaerites Müller, Reference Müller1854, some food grooves extend over the theca to plates beyond the circum-oral circlet. The arrangement of these longer food grooves was apparently random, especially in Glyptosphaerites. Regnéll (Reference Regnéll1945, pl. 9, fig. 5) illustrated a specimen with two parallel ambulacral branches that produce two facets each on three plates. Similarly, Kesling (Reference Kesling1968a, 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 ‘ambulacral 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, Reference Eichwald1840 and Dactylocystis Jaekel, Reference Jaekel1899, 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, Reference Bockelie1984, p. 28, fig. 16). Thus, the ambulacra appear to be composed of regularly arranged biserial thecal plates. Bockelie (Reference Bockelie1984, 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 alternating plates that bear a single facet and all the diplopores. No interradial plates bear pores (Kesling, Reference Kesling1968a, fig. 146, 4a, b).

The Gomphocystitidae bear spiral ambulacra that coil in a clockwise manner so appendages arise from the left side only. 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 (Reference Bockelie1979a, p. 160, fig. 5a) showed an unexpected arrangement in Celticystis Bockelie, Reference Bockelie1979a, 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, Reference Bockelie1979a, p. 161, fig. 6). Uniserial, mural ambulacra also occur in Gomphocystites Hall, Reference Hall1864 (Bockelie, Reference Bockelie1979a, 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, Reference Parsley1982a; Paul and Fone, Reference Paul and Fone1997). 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 (Reference Parsley1982a, 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 (Reference Paul and Fone1997, 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 (Reference Parsley1982a, 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, Reference Paul and Fone1997, the ratio of aboral to lateral arm plates was exactly 2:1 (Paul and Fone, Reference Paul and Fone1997, p. 157). Parsley (Reference Parsley1982a, 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, Reference Branson and Peck1940. However, Parsley’s illustrations (Reference Parsley1982a, 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 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 (Reference Sprinkle1973, 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, Reference Sprinkle1973, with the first five to 10 proximal brachiolar plates biserial opposite but more distal ones biserial alternate (Sprinkle, Reference Sprinkle1973, 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., Reference Frest, Strimple and Paul2011, 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., Reference Frest, Strimple and Paul2011), the ambulacra were recumbent on the theca and appear to have been biserial (Frest et al., Reference Frest, Strimple and Paul2011, pl. 4, fig. 2).

Summary of ambulacral structure

The ambulacral structure found in glyptocystitoids with a double 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, helicocystoids, 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 ambulacral 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 sphaeronitid diploporite Glyptosphaerites. In Lichenoides, the appendages were brachioles, and more than one arose from the ‘radial’ plates. However, Sprinkle (Reference Sprinkle1973, p. 110) described the brachioles 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.

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 (Reference Sprinkle1973) 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 pinnules arose are a logical ambulacral structure if the flooring plates are modified first pinnulars. Truly uniserial main ambulacral trunks occur in North American paracrinoids whose pinnules 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 (Reference Sprinkle and Parsley1982, 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, Reference Paul1967a, p. 305–306). In some rhombiferans (e.g., Echinosphaerites, see Barrande, Reference Barrande1887, 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.