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Journal of Paleontology 91(4):582–603
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., 2015). The EAT is based on embryological information, which
can only be derived from living echinoderms. Paleozoic echi- noderms 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, 1965). 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. 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 (1967c) 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 alsomade better sense of the apical disc of
Bothriocidaris (Fig. 2.3), which apparently differed fromall other sea urchins in having only five radial plates (oculars), one ofwhich wasmodified as themadreporite (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 ofwhich is modified as the madreporite and hence defines the CD inter- radius (as in all other sea urchins). Nevertheless, Bothriocidaris remains puzzling as the supposed primary ambulacral plates apparently obey Lovén’s law (Lovén, 1874) under the conven- tional interpretation (see discussion in Paul, 1967c, p. 538). The principal point here is that the EAT is based on infor-
mation from living echinoderms and may not apply universally to fossil echinoderms. Post-Paleozoic crinoids (see Simms, 1999) and asteroids (e.g., Gale, 2011) may also be derived from one or a few taxa that survived the end Paleozoic mass extinc- tion 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
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 (1967c, 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 (1967c, fig. 6). (3) The apical disc of Bothriocidaris,as reinterpreted by Paul (1967c). 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 (1962, fig. 1) and Solovjev (2009, fig. 1).
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