Paul—Testing for homologies in echinoderms
ambulacral plates. Nevertheless, the EAT and its inherent assumptions are the best theoretical concept on which to investi- gate 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, 1884, 1891) (Fig. 3) has found favor. In oral view, the ambulacrum opposite the hydropore is denoted as ambulacrumA; 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 datumbecause it arises earliest in echinodermontogeny 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 rhom- biferan 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 Glyptocys- titoida and in paracrinoids. Finally, Sprinkle (1973, 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 ambula- cra using this 2-1-2 pattern. A symmetry plane that bisects the A ambulacrum and the CD interradius is useful in discussing posi- tions of hydropore and gonopore in different echinoderm groups.
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For example, the hydropore is typically to the right of this plane in edrioasteroids, butmore commonly on it or to the left in ‘cystoids.’ It is now believed that the 2-1-2 pattern of ambulacra
reflects the evolutionary history of the echinoderms. Derstler (1981) and Paul and Smith (1984, 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 echino- derms 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 helicopla- coid, 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, 2013, p. 4), i.e., associated with the two ascending ambulacra, which supports the previously suggested homology of helico- placoid ambulacra. Although this was an inference not sup- ported by direct observation, the only alternative is that helicoplacoids had a blind gut, which seems even less likely. Building on Bell’s (1976a, b) earlier work on edrioasteroid
ambulacral development, Sumrall and Wray (2007) 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).
Figure 3. Camera lucida drawing of the oral surface of the glyptocystitoid rhombiferan Lepadocystis moorei (Meek, 1871) (University of Cincinnati 57349). A–E = Carpenter ambulacra; G = gonopore; H = hydropore; L = “first left ambulacral floor plates” of Sumrall and Waters (2012); PO1– PO7 = peri-oral plates; 1–6 = primary ambulacral cover plates. Modified from Sumrall and Waters (2012, p. 958, fig. 1).
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, 2007. 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 (2007, p. 150, fig. 1).
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