Ewin and Thuy—Brittle stars from the British Oxford Clay
intersections, no striation discernible; up to six spine articula- tions in deep notches of distal edge, proximally sharply bordered by slightly thickened outer surface stereom; spine articulations slightly oblique, composed of a crescent-shaped dorsal lobe and a slightly smaller and lower similarly crescent- shaped ventral lobe merged proximally into a continuous, dorsoventrally flattened volute and distally connected by a weakly developed sigmoidal fold; muscle opening slightly larger than nerve opening; conspicuous but regular dorsalward increase in size of spine articulations and of gaps separating them; arm spines stout, pointed, slightly flattened, with very small scale-like ornamentation on outer surface, almost as long as an arm segment and with dorsalward increase in thickness but not in length; inner side of LAPs unknown. Dorsal arm plates (Fig. 7.3) very large, wider than high, widest distally, with incised distal edge and slightly convex, proximalward- converging lateral edges, proximal edge not observable; dorsal arm plates widely separating LAPs in all observable arm segments. Ventral arm plates (Fig. 7.4) nearly as wide as long, irregularly hexagonal, with strongly obtuse distal angle and truncated distal tip, slightly concave lateral edges, and proximalward-converging latero-proximal edges; proximal tip of ventral arm plates unknown; tentacle openings small, covered by two small, oval scales.
Etymology.—Species named in honor of Andrew B. Smith (Kelso, Scotland, United Kingdom), one of the most influential echinoderm paleontologists of the last few decades, in recog- nition of his enthusiasm to promote research on ophiuroid evolution.
Materials.—Only the holotype NHMUK 24682.
Remarks.—The present specimen was originally published by Wright (1880) but misidentified as Amphiura pratti Forbes, 1844. Hess (1964) redescribed the specimen in detail but also considered it as another specimen of A. pratti, which he tentatively transferred to the extant genus
Ophiochiton.It is interesting to note that Hess (1964) mentioned a superficial similarity between specimen 24682 and the Cretaceous ophiodermatid Ophiotitanos tenuis Spencer, 1907. Indeed, the spine articulation morphology of the present specimen leaves no doubt as to its ophiodermatid affinities. Fossil ophiodermatids were generally assigned to extant
genera, in particular the type genus Ophioderma Müller and Troschel, 1840. Thuy (2015), however, argued that most of the fossil ophiodermatid reported, in fact, belong to a different genus: Ophiotitanos. The present specimen is no exception in this respect. Its LAP morphology, in particular the conspicuous dorsalward increase in size of the spine articulations and of the gaps separating them, the low number of spine articulations, the long, stout arm spines, and the seeming absence of removable granules on the disk plates and scales preclude assignment to any currently known extant ophiodermatid. Ophiotitanos and in particular its type species O. tenuis still await a thorough reassessment in terms of higher systematics and phylogenetic relation with modern ophiodermatids. For the time being, however, it seems best to treat the present specimen as another Jurassic record of Ophiotitanos.
795 On the species level, the specimen described herein stands
out in having rounded isosceles-triangular radial shields with slightly concave lateral edges and, most important, in having LAPs with a strong dorsalward increase in size of the spine articulations and of the gaps separating them, a pattern found in no other currently known species of Ophiotitanos or related forms assigned to other genera. We thus consider it a new species.
Clade C of O’Hara et al. (2014) Genus Ophioplax Lyman, 1875
Type species.—Ophioplax ljungmani Lyman, 1875, by original designation.
Ophioplax pratti (Forbes, 1844) Figure 8
1880 Amphiura prattii Forbes, 1844, figs. 49, 50. 1964 Ophiochiton? pratti (Forbes); Hess, p. 796.
Material.—BGS 3096 A (holotype).
Remarks.—The type specimen of A. pratti was duly redescribed and figured by Hess (1964). Our reexamination of the holotype yielded no further insights with respect to the poorly preserved disk skeleton (Fig. 8.1). As for the arms, however, scanning electron microscopy revealed previously overlooked but diagnostically significant microstructural characters. The spine articulations (Fig. 8.2, 8.3) were already found by Hess (1964) to be horseshoe-shaped. We can now show that the dorsal and ventral lobes of the spine articulation are proximally connected by three to four small, slightly elongate spurs. In combination with a finely meshed outer surface stereom with very small, inconspicuous trabecular intersections, the spine articulation morphology strongly suggests assignment to the extant genus Ophioplax, albeit tentatively. With the reidentification of specimen EE 24682 as an
ophiodermatid, the only currently known specimen of Ophioplax pratti is the holotype. Its poor preservation, in particular with respect to the accessibility of the LAPs, unfortunately precludes a comprehensive comparison with other fossil occurrences of the Ophioplax-Ophiodoris group (e.g., Thuy et al., 2013; Thuy, 2015). Proper characterization of the holotype would require a more thorough investigation of the LAP morphology, which is difficult to obtain without damaging the type.Wetherefore treat O. pratti as a valid species, stressing, however, that the present state of knowledge precludes any systematic conclusions beyond genus level.
Occurrence.—Christian Malford, NE Chippenham, Wiltshire; phaeinum Subzone, athleta Zone, upper part of the Peterborough Member,Oxford Clay Formation, late Callovian,Middle Jurassic.
Paleoecological considerations
While ophiuroids are not the most iconic fossils of the British Oxford Clay, locally they can occur in great numbers and constitute a reasonable component of the Oxford Clay benthos.
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