Ophiacantha, Muller & Troschel, 1842

Thuy, Ben, 2013, Temporary expansion to shelf depths rather than an onshore-offshore trend: the shallow-water rise and demise of the modern deep-sea brittle star family Ophiacanthidae (Echinodermata: Ophiuroidea), European Journal of Taxonomy 48, pp. 1-242 : 103

Ophiacantha sp. nov. innom. 3

Fig. 19 : 1-2

Material examined

GZG.INV.78602, GZG.INV.78603 and GZG.INV.78604 (2 dissociated LAPs) from level 2 of Young et al. (2010) in the middle Albian Loricatus Zone of Folkestone, Great Britain.

Description

GZG.INV.78602 is a dissociated, large, proximal to median LAP; dorsal edge fragmentary; distal edge convex; proximal edge irregularly undulose; small, sharply defined, slightly prominent and protruding, round spur in the dorsal half of the proximal edge, composed of more densely meshed stereom; outer surface with moderately finely meshed stereom; few trabecular intersections, especially in ventral quarter of outer surface, thickened into very small granules; moderately finely meshed stereom of outer surface replaced by much more finely meshed stereom in relatively broad band paralleling proximal edge of LAP. Four large, ear-shaped spine articulations freestanding on strongly elevated distal portion of LAP; dorsal and ventral lobes of spine articulations merged into continuous, round volute; spine articulations proximally bordered by irregularly knobby, undulose and weakly prominent ridge; very weak dorsalward increase in size of spine articulations and of gaps separating them; gap between spine articulations and distal edge of LAP nearly half as wide as one spine articulation. Ventral edge convex, with conspicuous, moderately large, deeply concave tentacle notch.

Inner side of LAP with large, conspicuous ridge composed of a main central, near-straight, oblique part and slightly shorter and wider, oblique, ventro-proximally pointing ventral part separated from the main part by a rounded kink; dorsal tip of main part of ridge fragmentary, but originally widened and with relatively short, pointed ventralward extension; dorsal half of inner side of distal LAP edge with small, moderately well-defined, slightly prominent, dorsally fragmented spur; inner side of tentacle notch deep, relatively short, well-defined laterally. No perforations discernible.

GZG.INV.78603 is a dissociated distal LAP; almost twice wider than high; dorsal edge strongly concave as a result of a well-developed constriction; distal edge nearly straight to slightly convex; spur in dorsal half of proximal edge very weakly defined, almost indiscernible. Four spine articulations similar to those observed on proximal to median LAP described above; ventral edge of LAP slightly concave, tentacle notch invisible in external view.

Inner side of LAP with large, proximally bent, sharply defined ridge; dorsal tip of ridge strongly widened; ventral part of ridge distally bordered by round, prominent, well-defined knob; spur in dorsal half of inner side of distal LAP edge very poorly defined, hardly discernible, slightly prominent; inner side of tentacle notch relatively small, moderately well-defined laterally. No perforations discernible.

Remarks

The ridge displayed by these LAPs is rather atypical of Ophiacantha in that its ventrally pointing extension is unusually short. Nevertheless, the shape of the ridge is basically compatible with the LAP morphology of Ophiacantha in its proper sense, in particular in combination with the shape and arrangement of the spine articulations and the single spur on the outer proximal and inner distal edges. The LAPs in question are unambiguously recognisable on account of the deeply concave tentacle notch visible even in external view, combined with the irregular, undulose ridge proximally bordering the spine articulations, and the short ventrally pointing extension of the ridge on the inner side. Although these LAPs unquestionably represent a new species, the fragmentary condition and limited number of plates currently available precludes a formal description of the species.

Figures

Fig. 19. Fossil skeletal plates of ophiacanthid brittle stars; lateral arm plates (LAPs) in external (a) and internal (b) views. 1-2. Ophiacantha sp. nov. innom 3 from the middle Albian (Early Cretaceous) of Folkestone, Great Britain. 1. GZG.INV.78602, proximal LAP. 2. GZG.INV.78603, distal LAP. 3-5. Ophiacantha reginae sp. nov. from the late Campanian (Late Cretaceous) of Lägerdorf-Alsen, Germany. 3. GZG.INV.78605 (holotype), proximal LAP. 4. GZG.INV.78606 (paratype), median LAP. 5. GZG.INV.78607 (paratype), distal LAP. 6-9. Ophiacantha steffenschneideri sp. nov. from the Rupelian (Oligocene) of Bad Freienwalde, Germany. 6. GZG.INV.78609 (holotype), proximal LAP. 7. GZG.INV.78610 (paratype), median LAP. 8. GZG.INV.78611 (paratype), distal LAP. 9. GZG. INV.78612 (paratype), arm spine. 10-12. Ophiogaleus sp. nov. innom 1 from the late Sinemurian to early Pliensbachian (Early Jurassic) of the Glasenbach Gorge, Austria. 10. NHMW 2012/0137/0017, proximal LAP. 11. NHMW 2012/0137/0018, proximal to median LAP. 12. NHMW 2012/0137/0019, proximal to median LAP. One common scale bar per species except for 9.