Ewin and Thuy—Brittle stars from the British Oxford Clay
large, conical, with coarsely reticulate outer surface, maximum length unknown; inner side of LAPs unknown; dorsal arm plates (Fig. 6.4) large, in contact on all preserved arm segments, widest distally, with strongly convex distal edge and straight proximalward-converging lateral edges, proximal tip unknown; ventral arm plates (Fig. 6.5) nearly as wide as long, separated by LAPs from the third freestanding arm segment, with convex distal edge, concave lateral edges, and obtuse proximal angle; two leaf-shaped, nearly equal-sized distalward-pointing tentacle scales.
Materials.—NHMUK E 54378.
Remarks.—The large, ear-shaped spine articulations with the well-developed sigmoidal fold, in combination with a vertical striation on the outer LAP surface and two to three small, apical oral papillae, unambiguously place the present specimen among the members of the family Ophiacanthidae as formerly understood (e.g., Thuy, 2013). Within this group, the densely granulated disk, the rounded triangular radial shields, the relatively small tentacle openings, the large, contiguous proximal dorsal arm plates, the ventroproximalward-protruding ventral portion of the LAPs, and the regularly spaced spine articulations positioned in notches of the elevated distal LAP portion strongly favor assignment to the extinct genus Dermocoma. Hess (1964) introduced the genus on the basis of articulated ophiuroids from the Bathonian of Malmesbury, Wiltshire, United Kingdom. The genus was recently assigned to the family Ophiacanthidae and thoroughly reassessed by Thuy (2015), who transferred a number of previously known species to Dermocoma and identified numerous new species, thus showing the genus to be one of the most speciose fossil ophiacanthids. Among the currently known species of Dermocoma,
only D. toarcensis (Hess, 1962) from the Toarcian of Switzerland, D. longwyensis Thuy, 2013 from the Bajocian of France, and D. numbergerorum Thuy, 2013 from the Oxfordian of France have up to six spine articulations. The inaccessibility of the outer proximal edge and the inner side of the LAPs in the here-described specimen precludes a thorough comparison with the three previously described species. In D. longwyensis, the LAPs are generally much smaller and have an undulose outer surface striation. The LAPs of D. numbergerorum have a conspicuously long ventral portion. Similarities are greatest with the LAPs of D. toarcensis, although the large stratigraphical gap make an assignment to the species unlikely. Chances are high that the present specimen belongs
to an undescribed species of Dermocoma. Despite its exceptional preservation, however, it cannot be formally described here in the absence of more detailed knowledge of its LAP morphology. Pending the discovery of more insightful material, the specimen is reported here as a member of an ophiuroid group previously unknown from the Oxford Clay and as a new find of an articulated disk assignable to the extinct Dermocoma.
Occurrence.—The Fleet, Weymouth, Dorset; Weymouth Member, early Oxfordian, Late Jurassic.
Genus Ophiotitanos Spencer, 1907
Type species.—Ophiotitanos tenuis Spencer, 1907, by original designation.
Ophiotitanos smithi new species Figure 7
1880 Amphiura prattii Forbes, 1844; Wright, p. 158, pl. 18, fig. 1a–d.
1964 Ophiochiton? pratti (Forbes); Hess, p. 796. Holotype.—NHMUK 24682.
Diagnosis.—Species of Ophiotitanos with stout dorsal disk plating, rounded isosceles-triangular radial shields with slightly concave lateral edges; no trace of removable granules on the disk plates; large, thick, plectrum-shaped oral shields; lateral arm plates with finely meshed stereom with small trabecular intersections; spine articulations on slightly thickened distal portion of LAP; conspicuous dorsalward increase in size of spine articulations and of gaps separating them; large, pointed, slightly flattened arm spines slightly shorter than one arm segment.
Occurrence.—Christian Malford, NE Chippenham, Wiltshire; phaeinum Subzone, athleta Zone, upper part of the Peterborough Member, Oxford Clay Formation, late Callovian, Middle Jurassic.
Description.—NHMUK 24682 is a nearly complete articulated individual (disk diameter of 27.10mm) exposing both ventral and dorsal sides and preserving parts of five arms, three of which include proximal to median segments; dorsal disk has rather thick, rounded, irregularly shaped disk scales; central and radial primary plates indistinguishable from disk scales; radial shields thick, rounded isosceles-triangular, with very weakly concave lateral edges, accounting for approximately half the disk radius, separated by five disk scales; no removable granules discernible. Ventral side of disk with interradial areas covered by small,
round, moderately thick scales; no removable granules discernible; genital slit simple and accounting for at least one-third of the disk radius; oral shields (Fig. 7.2) conspicuous, plectrum-shaped with rounded acute proximal angle, wider, strongly convex distal edge, and convex lateral edges; adoral shields (Fig. 7.2) poorly preserved, straight,with pointed proximal tip, seemingly not abutting near proximal tip of oral shield; oral plates stout; at least six oval to leaf-shaped lateral oral papillae (Fig. 7.2), contiguous, decreasing in size toward the proximal tip of the oral plates; three leaf-shaped, pointed apical oral papillae; teeth unknown; first ventral armplatewithwidth 2.5 times height, with incised proximal edge and pointed distal edge. Arms stout, with relatively short, high proximal segments
and slightly longer and lower median segments; proximal LAPs (Fig. 7.5) higher than wide, devoid of a constriction, with convex distal edge and ventroproximalward-protruding ventral portion; proximal edge of LAPs not observable; outer surface with very finely meshed stereom and small trabecular
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