Paul—Testing for homologies in echinoderms
583
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., 2011) and in paracrinoids (Malocystites Billings, 1858). Rhombiferans, diploporites, and paracrinoids differ by so many other characters that recumbent ambulacra are likely to have evolved independently in each. Bockelie (1982) 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 rhombi- ferans and sphaeronitoid diploporites differ bymany characters, so it is difficult to accept that ambulacrum A was lost once only. On a purely practical note, if two different structures are
Figure 1. Illustration of extraxial-axial theory (EAT) using lower Cambrian Camptostroma Ruedemann, 1933 as an example. A–E = ambulacra in the Carpenter system (Carpenter, 1884, 1891); AR = axial region; IER = imperforate extraxial region; PER = perforate extraxial region. Redrawn from Paul and Smith (1984, fig. 19).
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, 1982, 1988 for a full discussion). In effect, homologous characters are synapomor- phies, 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, 1900, p. 11, fig. 9; Sprinkle, 1973, 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 particu- larly obvious in echinoderms with oral frames composed of just four plates (the peristomial border system type A2 of Kammer et al., 2013) and enables identification of individual ambulacra. The undivided ambulacrum is A under Carpenter’s system (Carpenter, 1884, 1891), and clockwise in oral view the others are B, C, D, and E. The idea that these ambulacra are homo- logous 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 para- crinoids. See Sumrall (1997, p. 270, fig. 1) for another example of the positional argument applied to echinoderms. Evolutionary tests include the distribution of characters in
different major groups. For example, in glyptocystitoid rhom- biferans, recumbent ambulacra (restricted here to ambulacra
thought to exist, it is better practice to record all relevant infor- mation 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 term- inally. Jackson (1912, p. 35–51, pl. 6, 7) first documented in anomalous sea urchins with <5 ambulacra that either the num- ber of oculars coincided with the number of ambulacra, or occasionally, if the ocular corresponding to a missing ambula- crum 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 inter- ambulacra. This means that echinoid interambulacra are not homologous with interambulacra of any other echinoderms (as pointed out by Mooi and David, 2008, p. 47). The only truly interambulacral structures in the echinoid corona are the inter- radial 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 col- umns of plates in sea urchins are homologouswith the ambulacral and adambulacrcal plates of starfish (notwithstanding their different positions with respect to the radial water vessels,
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