Ewin and Thuy—Brittle stars from the British Oxford Clay EE 16214 (Fig. 5.5) is a dissociated median LAP with
width approximately 1.5 times height, with concave dorsal edge and evenly convex ventral edge; ridge on inner side wider than in holotype, with sharply defined ventral tip; tentacle opening large and completely encompassed by ventral portion of LAP. EE 16215 (Fig. 5.7) is a dissociated distal LAP with width
nearly two times height, with weakly concave dorsal edge and nearly straight ventral edge; four spine articulations as in holotype, with dorsalward decrease in size of gaps separating the spine articulations; ventral-most spine articulation distally bordered by the relatively small tentacle perforation; inner side with small but well-defined ridge.
Etymology.—After the welsh for star. In honor of the ‘stellar’ support received by the first author from Joanne V. Dallosso, Wales, United Kingdom.
Materials.—NHMUK EE 16213, EE 16214, EE 16215, EE 16216 (five dissociated LAPs).
Remarks.—The above described dissociated LAPs were found together with the remains of Enakomusium whymanae. The spine articulation morphology with the muscle and nerve openings separated by a vertical ridge in combination with the LAPs laterally encompassing the arm suggests an assignment to the ophiurid-Ophiomusium complex within clade A of O’Hara et al. (2014). Similarities with members of the Ophiomusium- Ophiosphalma-Ophiolipus complex are weaker due to the generally more fragile plate architecture and the thin dorsal and ventral contact surfaces with the opposite LAP. Most ophiurids have proximal LAPs that are higher than long and have typically distalward-pointing tentacle notches (Thuy and Stöhr, 2011). Afew recent species, however, share the slightly elongate LAPs with almost completely encompassed tentacle notches in proximal LAPs and tentacle perforations in distal LAPs with the here described specimens. Among these, greatest similarities are shared with the LAPs of Aspidophiura forbesi (Duncan, 1879; Thuy, unpublished observations, 2016), although they differ from the present specimens in having a spur on the proximal edge, prominent spine articulations at some distance from the distal edge, and a much larger dorsal contact surface with the opposite LAP. The assignment of the present material to Aspidophiura should therefore be considered tentative, awaiting further advances in the analysis of the LAP morphologies of similar recent species. No other fossil LAP type currently known shows the
combination of characters displayed by the present material. Superficial similarities are shared with the LAPs of the articulated specimens from the Aptian of Antarctica described by Smith and Crame (2012) as Mesophiomusium paragraysonensis (Taylor, 1966), especially in terms of the relatively thin, rectangular LAPs and the development of the tentacle openings. Differences assessable on the basis of the figures published by Smith and Crame (2012) mainly pertain to the development of the ventrodistal edge of the LAPs. WhetherM. paragraysonensis and the present material belong to the same genus cannot be decided from the evidence at hand. However, neither the present material nor the Aptian specimens in Smith and Crame (2012) are assignable to Mesophiomusium, whose type species is strongly
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reminiscent of modern Ophiomastus Lyman, 1878 (Thuy, 2015). For the time being, we therefore describe the presentmaterial as a newspecies tentatively assigned to the extant genusAspidophiura, stressing, however, that the genus-level placement is likely to change with future advances in LAP morphology studies.
Clade B of O’Hara et al. (2014) Genus Dermocoma Hess, 1964
Type species.—Dermocoma wright Hess, 1964, by original designation.
Dermocoma species indet. Figure 6
Description.—NHMUK E 54378 is an articulated disk (disk diameter 28.35mm) exposing both ventral and dorsal sides and preserving a portion of an arm composed of five proximal segments; disk outline slightly lobed interradially; dorsal disk scales moderately thick, round, imbricate, largely obscured by small, densely spaced removable granules; no variation of granule size or shape discernible on dorsal disk surface; radial shields (Fig. 6.2) also completely covered by disk granules, rounded isosceles triangular in outline, widely separated, accounting for approximately one-third of the disk radius. Interradial areas of ventral disk slightly protruding, covered
by small, rounded, imbricate scales completely obscured by removable granules similar to those of the dorsal disk side; genital slit accounting for at least one-third of the disk radius; oral shields (Fig. 6.3) conspicuous, not covered by granules, approximately as wide as long, with nearly right proximal angle, rounded lateral angles, and a slightly angular convex distal edge; one oral shield slightly larger and thicker than the other four and with proximally pointed depression in the middle of the distal portion, thus probably identifiable as madreporite; adoral shields (Fig. 6.3) narrow, abutting near proximal tip of oral shield, extending around the lateral angles of the oral shield; adoral shields and oral plates covered by removable granules larger than disk granules and slightly elongate; up to seven oval lateral oral papillae (Fig. 6.3), contiguous, becoming smaller and more conical toward the distal tip of the oral plates; two to three apical oral papillae, of similar size as proximal-most lateral oral papillae but more conical; ventral-most teeth two to three times as large as apical oral papillae, rounded triangular; first ventral arm plate with width two times length, oval, with longitudinal central depression. Only proximal-most arm segments preserved, weakly
noded, wider than long and nearly as high as wide; LAPs (Fig. 6.6) with very weak constriction, evenly convex distal edge, and ventrodistalward-protruding ventral portion; proximal edge not observable; outer surface with irregular but well- developed vertical striation, consisting of weakly imbricate vertical lamellae, fining and eventually fading into a finely meshed stereom toward the proximal edge of the LAP; six large, ear-shaped spine articulations in notches of elevated distal portion of the LAPs, proximally bordered by outer surface striation, with dorsalward increase in size of spine articulations and of gaps separating them; spine articulations with continuous volute and well-developed sigmoidal fold; arm spines (Fig. 6.6)
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