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Fig 1. Introduction to the ophiuroid interbrachial frame.Scanning electron micrographs of whole interbrachial frames and their isolated skeletal components of (A) Ophiacantha vivipara, (B) Ophiura lymani, (C) Amphiura eugeniae and (D) Ophioplocus januarii. (a) Whole interbrachial frame (aboral side). Arrows: green, tooth; yellow, dental plate; blue, oral plate; red, peristomial plate. White arrowhead (in Da), advertebral groove. (b) Dental plate (proximal side, aboral end at top). Arrowhead, tooth socket. (c) Tooth. Arrowhead, cap of imperforate (A. eugeniae) or more densely fenestrated (O. januarii) stereom. (d) Oral plate (adradial side, aboral end at top). Arrow (in Ad), neural groove; white arrowhead, adradial articular area; red arrowhead, advertebral groove (showing attachment area of distal radial ligament); asterisk, adradial muscle attachment area. (e) Oral plate (abradial side, aboral end at top). Arrow (in Ae), neural groove; arrowhead, interruption of differentiated stereom associated with bursal diverticulum; asterisk, abradial muscle attachment area. Scalebars: 2 mm (Aa, Ba, Ca, Da); 0.5 mm (Ad,e, Bb-e, Db-e); 0.2 mm (Ab,c, Bc, Cb-e).
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Fig 2. Scanning electron micrographs showing aboral side of a single interbrachial frame unit (“jawâ€) of (A) Ophiacantha vivipara (see Fig 1Aa) and (B) Ophioplocus januarii (see Fig 1Da). aa, adradial articulation; dep, dental plate; do, distal part of oral plate; eim, external interradial muscle (between abradial sides of adjacent oral plates); L, maximum length of peristomial plate; po, proximal part of oral plate; ovm, orovertebral muscle (between oral plate and first vertebral ossicle); pep, peristomial plate; rm, radial muscle (between adradial sides of adjacent oral plates); t, tooth; arrowhead, ligament connecting peristomial plate to oral plate; W, maximum width of peristomial plate; asterisk, pore for water vascular canal that connects circumoral water vascular ring to buccal podia; red dots, lateral edges of aboral region of dental plate, the distance between which is the maximum transverse width, as defined herein. Scalebars: 1 mm.
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Fig 3. Old and new phylogenetic trees of class Ophiuroidea.(A) Cladogram derived by Smith et al. [23] from the phylogenetic diagrams of Matsumoto [21] (slightly modified). (B) Cladogram derived from the phylogenetic tree of O’Hara et al. [9] (slightly modified).
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Fig 4. Character states of peristomial plates (PEPs): Summary of results.The cladogram is derived from O’Hara et al. [9] (slightly modified). Regarding fragmentation state, a = single, b = two parts, c = three parts, d = other; the values are the numbers of species showing each state; where a species shows two states, each state is assigned a value of 0.5. The numerals in parentheses refer to the following comments: (1) One species has small PEPs and one has large PEPs. (2) One species has small PEPs. (3) One species (Ophioplocus januarii) has inclined PEPs and two species (O. januarii and Sigsbeia murrhina) have small PEPs. (4) One species has small PEPs and one has large PEPs. (5) One species (Ophiocomella ophiactoides) has large horizontal PEPs. (6) One species (Ophiocten sericeum) has small PEPs. (7) One species (Ophiolipus agassizii) has small PEPs.
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Fig 5. Light micrographs of peristomial plates in situ and detached.(A-D, I-P) Aboral views of single interbrachial frame units (“jawsâ€) of partly digested specimens. (E-H) Detached peristomial plates: distal side, aboral end at top. (A, E) Ophiothrix fragilis (Ophiotrichidae); (B, F) Ophiopholis aculeata (Ophiopholidae); (C, G) Amphiura filiformis (Amphiuridae); (D, H) Ophiocoma pumila (Ophiocomidae); (I) Amphilepis ingolfiana (Amphilepididae); (J) Ophiopsila aranea (Ophiopsilidae); (K) Ophionereis reticulata (Ophionereididae); (L) Ophioplocus esmarki (Hemieuryalidae); (M) Ophiocomina nigra (Ophiotomidae); (N) Ophiomusa lymani (Ophiomusaidae); (O) Ophiura ophiura (Ophiuridae); (P) Astrophyton muricatum (Gorgonocephalidae). In B and C the proximally projecting edges of the abradial muscle attachment areas (which are present in A) have been removed to enhance visualisation of the peristomial plates. do, distal part of oral plate; po, proximal part of oral plate; asterisk, dental plate; red arrowhead, peristomial plate; white arrowhead, abradial muscle attachment area. Scalebars: 1 mm (A, B, L, M, O); 0.5 mm (D-F, H, J, K, N, P); 0.4 mm (C); 0.25 mm (G, I).
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Fig 6. Peristomial plates of Ophiocomina nigra: relationship between (A) disc diameter and relative width and (B) disc diameter and relative length. Results from nine animals, four with disc diameter 11 mm and the others with disc diameters 3.5, 5.0, 8.0, 14.0 and 17.0 mm respectively. The relative dimensions of four or five PEPs of each animal were estimated; blue diamonds indicate individual values and brown squares indicate means for each animal; the values for the four animals with disc diameter 11 mm are staggered slightly along the x-axis. Pearson product-moment correlation indicates no significant association between disc diameter and relative width (r = 0.0016, d.f. 41, P > 0.05) or relative length (r = 0.0328, d.f. 41, P > 0.05).
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Fig 7. Relative dimensions of the peristomial plates of directly examined species.Each symbol represents one species. Ophioblenna antillensis (formerly placed in the Ophiacanthidae) is currently incertae sedis. The broken lines are the upper limits of the relative dimensions of ‘small’ PEPs as defined herein. The red oval encloses all species with inclined PEPs (and a single species with horizontal PEPs–Ophionereis reticulata). The black arrow indicates a cluster of three species–Acrocnida brachiata, Ophionereis reticulata and Ophiopholis aculeata) and the red arrow indicates Ophiocomella ophiactoides (the only ophiocomid that does not have small inclined PEPs). The red bars superimposed on the symbol for Ophiocomina nigra show the ranges of values obtained from nine individuals of this species (disc diameters 3.5–17.0 mm).
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Fig 8. Relative dimensions of the peristomial plates of species from families not represented in Fig 7 (data obtained from the published literature).The broken lines are the upper limits of the relative dimensions of ‘small’ PEPs as defined herein.
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Fig 9. Relative dimensions of the peristomial plates of additional species from families represented in Fig 7 (data obtained from the published literature).The broken lines are the upper limits of the relative dimensions of ‘small’ PEPs as defined herein.
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Fig 10. Familial distribution of IF character states, based on information accumulated to date (S5 Table).The cladogram is derived from O’Hara et al. [9] (modified by the separation of Ophioplocus januarii from other hemieuryalids and the representation of family Ophionereididae as subfamilies Ophionereidinae and Ophiochitoninae). Characters are numbered as in Table 1. Blue denotes type A and red type B character states. White denotes lack of data. The numbers of species for which information is available are also given. Lower case italicised letters indicate clades to which reference is made in the text. Asterisks and numerals in parentheses refer to the following comments: (1) The dental plates of two species are type A and that of the third (Amphilepis ingolfiana) has a B-like aboral perforation. The teeth of this last species also have an imperforate median cusp (apparently also present in certain other Amphilepis species [26,45]), which may be a vestige of the imperforate cap present in most other families of clade m (suborder Gnathophiurina). (2) There is information on the abradial muscle attachment area of four species, three of which are type B and one type A. (3) There is information on the abradial muscle attachment area of five species, four of which are type B and one type A. (4) The adradial muscle attachment area approaches the type B state, but lacks a pronounced aboral expansion and narrow oral region. (5) Most hemieuryalids have type A dental plates. However, Ophioplocus januarii and four Ophiozonella species have type B dental plates. (6) Although most ophiolepidids have type A dental plates, one species has an “imperfect†perforation and another has distinctly type B perforations. (7) Of the five characters for which there is information, only the type A adradial muscle attachment area is consistent across the three species. All other characters are type A in one species (Clarkcoma australis) and type B in the other two (C. bollonsi and C. canaliculata). (8) The IF traits of the Ophiomyxidae are consistently type A, with the exception of the teeth of the six Ophioconis species for which information was obtained. These have broad teeth with a cap of imperforate stereom, which in most (but not all) species has a denticulate edge. The latter feature suggests that these have evolved from Ophiomyxa-like teeth, which have a proximal edge of imperforate spikes ([36]; Wilkie pers. obs.). The abradial and adradial muscle attachment areas of Ophioconis cincta and O. cupida also approach the B condition. (9) The character states of all ophiocomids are consistently type B, with the exception of Ophiocomella ophiactoides, which has type A peristomial plates. (10) Some ophiohelids have broad, though flattened, teeth, those of Ophiothauma heptactis also having an imperforate proximal edge [46]. (11) The adradial muscle attachment area of a few gorgonocephalid and euryalid species approaches the type B spoon-with-narrow-handle shape.
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