586


Journal of Paleontology 91(4):582–603


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’ sym- metry, 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. Sumrall and Wray (2007, 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


that lies entirely within the two lateral ambulacra (Fig. 7). In the eocrinoid Rhopalocystis Ubaghs, 1963 (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. (2013) make the same point in recognizing the difference between their peristomial border systems type A2 and A3. 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 ambu- lacral 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 (2007, figs. 6


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


include genera with five and four ambulacra. In sphaeronitids, five ambulacra appear in Glyptosphaerites Müller, 1854 and Palaeosphaeronites Prokop, 1964 (Tremadocian, Lower Ordo- vician), but genera with four ambulacra, such as Tetreucystis Bockelie, 1984 and Diplosphaeronis Paul, 1973 are unknown before the Katian (Upper Ordovician). Similarly, in the Holo- cystitidae, Brightonicystis (five ambulacra, Hirnantian, Upper Ordovician) appears well before genera with four (Tremato- cystis Jaekel, 1899; Pustulocystis Paul, 1971, 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 (1982, p. 493, fig. 2) documented parallel evolutionary trends in the caryocystitid rhombiferans Echino- sphaerites Wahlenberg, 1821 and Heliocrinites Eichwald, 1840 from two through four ambulacra (Fig. 6). In a related genus, Caryocystites von Buch, 1846, the trend went only from two to three. These caryocystitid rhombiferans share humatirhombs and only five peri-oral plates (see next section). Thus, Echino- sphaerites and Heliocrinites achieved species with four ambu- lacra by an opposite route to sphaeronitids and holocystitids. Incidentally, no holocystitid has three ambulacra. I presume Sumrall and Wray (2007, fig. 6) were referring to Triamara Tillman, 1967. Although S. A. Miller and coworkers described six nominal species of Triamara under the name Holocystites in the late nineteenth century (e.g., Miller 1879, 1891), Triamara belongs in the diploporite family Aristocystitidae. Sumrall and Wray (2007, p. 156, fig. 7) also indicated how


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, 1840, Caryocrinites Say, 1825, Juglandocrinus von Koenen, 1886, Paracaryocrinites Chen and Yao, 1993, and Stribalocystites Miller, 1891 (Lanc et al., 2015) (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, 1982) (Fig. 6). Furthermore, the echinoencrinitid rhombiferan Tyrridiocystis Broadhead and Strimple, 1978 has B, C, and D+E (Broadhead and Strimple, 1978) (Fig. 10), whereas the aristocystitid diplo- porite Triamara has B+ C, D and E (Paul, 1971). Whether Trimerocystis Schuchert, 1904 is a teratological specimen of the callocystitid rhombiferan Pseudocrinites Pearce, 1843 (as sug- gested by Kesling, 1961, p. 258) or a valid genus, it has three ambulacra, B, C, and E. Kesling (1961, fig. 2b) illustrated a specimen of the callocystitid rhombiferan Jaekelocystis hartleyi Schuchert, 1903 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 Lepa- docystis moorei (Meek, 1871) 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.


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 (2007, fig. 1) (Fig. 4) did not exist as a developmental pattern in early echinoderms. If it did, echinoderms with four and one ambula- crum 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


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