788
Journal of Paleontology 91(4):781–798
articulations in proximal and median LAPs, three in distal one; spine articulations relatively large, prominent, on slightly elevated part of distal LAP portion, tightly encompassed by outer surface stereom; strong dorsalward increase in size of spine articulations and in distance separating them; spine articulation morphology as in holotype but better preserved. Inner side of LAPs with large, rounded triangular dorsal contact surface with opposite LAP; ventral contact surface much smaller; short, well-defined, prominent ridge in central part of inner side, with tongue-shaped, dorso-proximalward-pointing dorsal tip; inner side of distal LAP edge with slightly more densely meshed stereom but devoid of discernible spurs; proximal and median LAPs with large, deep, sharply defined ventralward-pointing tentacle notch; distal LAP with large tentacle perforation distally bordering the central ridge; no other perforations discernible.
Etymology.—Species named in honor of Janet Whyman (Coventry, United Kingdom), who discovered the type material and generously donated it to the Natural History Museum London.
Materials.—NHMUK EE 16206, EE 16207, EE 16208, EE 16209, EE 16210, EE 16211 (paratypes); NHMUK EE 15466 (28 complete specimens and a collection of several hundred arm and disk fragments).
Remarks.—The Ophiomusium-like form, with stout overall plating and at least part of the tentacle pores developed as within-plate openings, is one of the most widespread in Jurassic ophiuroids. The taxonomic status and systematic position of these forms was subject to considerable controversy, amend- ments of genus diagnoses, and the introduction of the regretta- bly misrouted genus Mesophiomusium Kutscher and Jagt, 2000 (Thuy and Meyer, 2013; Thuy, 2015). To put at least some of the Jurassic Ophiomusium-like forms on solid taxonomic ground, in accordance with the latest insights on ophiuroid morphology, Thuy (2015) introduced the genus Enakomusium to accommodate Ophiomusium-like forms with rounded rectangular LAPs, relatively large spine articulations bearing relatively long spines, and tentacle pores developed as between- plate openings in proximal and often median arm segments. The present material fully accords with the diagnosis of
Enakomusium. Among the currently known species of the genus, it most closely resembles the type species E. weymouthiense (see below), particularly in respect to the radial shields, the single interradial plates, and the mouth plating. The present specimens differ, however, in their smaller size, a less robust plating, and particularly, possession of lateral arm plates with a constriction (Figs. 4.4 to 4.6) and larger, more prominent spine articulations that are not sunken into depres- sions and which bear spines that exceed the length of an arm segment. The LAPs of the here described material furthermore lack any discernible spurs on the outer distal edge, have a less well-defined furrow paralleling the distal edge, and are generally less robust. At least part of these differences cannot be explained by ontogenetic variation and thus preclude assignment to E. weymouthiense, despite their stratigraphical and geographical proximity.
Other species assigned to Enakomusium generally have
stouter LAPs, smaller and less prominent spine articulations, and discernible spurs on the outer distal edge and therefore cannot be confused with the present material. It thus seems warranted to erect a new species. Within the morphological spectrum of Enakomusium, the new species is the least Ophiomusium-like, with the generally more fragile plating, the conspicuous and prominent spine articulations, and the unusually long arm spines. Modern representatives of the Ophiomusium- Ophiosphalma-Ophiolipus complex tend tobe muchmore morphologically conservative. The Jurassic forms generally contrast with their modern relatives in showing a much larger range of morphologies, with the present one even superficially reminiscent of ophiacanthid morphologies with the large, prominent spine articulations, the constricted LAPs, and the long spines (e.g., Thuy, 2013).
Enakomusium sp. indet.
Materials.—LEIUG 73036. One specimen containing three poorly preserved articulated individuals and three sections of disarticulated arm.
Occurrence.—Stewartby quarry, StewartbyBrickworks, Bedford, Bedfordshire, UK Bed 6a (Duff), lower Oxford Clay, medea Subzone, jason Zone (Callovian), Middle Jurassic.
Remarks.—This poorly preserved specimen was figured by Martil and Hudson (1991). The articulated ophiuroids from the Peterborough Member of Stewartby, Great Britain, are too poorly preserved to enable a confident species-level identifica- tion. With respect to the few morphological features discernible on the specimens, assignment to Enakomusium seems warranted, and there are indeed superficial similarities to E. whymanae n. sp. from Coln Quarry, Gloucestershire, but we nevertheless prefer treating the specimens as an unidentified record of Enakomusium.
Enakomusium weymouthiense (Damon, 1880) Figure 5.1–5.3
1880 Ophioderma weymouthiense Damon, plate 17, fig. 10, 10A.
1963 Ophiomusium calloviense Hess, p. 1143. 1964 Ophiomusium weymouthiense Hess, p. 780. 2015 Enakomusium weymouthiense Thuy, p. 95.
Materials.—NHMUKE 1607b (neotype), specimens figured by Hess (1964) from the same locality include E 1607a, E 3500. Eighteen other specimens from the Weymouth area containing over 50 individuals are also known in the NHM collections but did not provide any new information.
Description.—Redescription of the original LAPs of Hess (1963): LAPs moderately large, thick, proximal ones slightly higher than wide, distal ones slightly wider than high, rounded rectangular outline with very weakly convex dorsal and distal edges; proximal edge strongly concave, with two poorly defined, vertically elongate, thin spurs dorsally and a much
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