588


Journal of Paleontology 91(4):582–603 Kammer et al. (2013) 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., 2010 and Walcottidiscus Bassler, 1935. 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, 1891 (Fig. 3) and the eocrinoid Rhopalocystis (Fig. 8), respectively. TypeA4 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


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 (1982, p. 493, fig. 2).


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., 2010, 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 aster- oid adambulacral plates. The current scheme of plate notation in lenticular paracrinoids (Parsley and Mintz, 1975) 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.


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 (1971, p. 71) introduced the term ‘peri-orals’ for inter- radially positioned ‘orals’ in the diploporite family Holocysti- tidae, the equivalent of Kammer et al.’s (2013) ‘orals.’ Paul (1973, pp. 12, 13, fig. 8) introduced the term ‘circum-orals’ for the radially positioned ‘orals’ in the diploporite family Sphaer- onitidae, the equivalent of Kammer et al.’s (2013) ‘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 pre- viously been referred to as ‘orals’ (e.g., Prokop, 1964, p. 13). Paul (1971, 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 homo- logies 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


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