Dermacantha, Thuy, 2013

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 : 166-167

Dermacantha sp. nov. innom.

Fig. 30 : 1-3

Material examined

GZG.INV.78714, GZG.INV.78715, GZG.INV.78716 and GZG.INV.78717 (12 dissociated LAPs) from the Bajocian-Bathonian boundary of Touert near Chaudon, France.

Description

GZG.INV.78714 is a dissociated, moderately large, proximal LAP; nearly as high as wide; dorsal edge slightly concave as a result of a weak constriction; distal edge almost straight; ventral sixth of LAP ventroproximalwards protruding; proximal edge slightly undulose, with large, swollen, slightly protruding central part dorsally and ventrally bordered by small, well-defined, horizontally elongate, prominent spurs; outer surface with very fine, regular, vertical striation composed of fine, very weakly overlapping lamellae replaced by finely meshed stereom on proximal third of outer surface. Four relatively small, ear-shaped spine articulations in shallow notches of slightly elevated distal portion of LAP; ventral and dorsal lobes proximally separated by small knob; very weak connection between ventral lobe of spine articulations and distalwards projecting tips of outer surface stereom separating notches; spine articulations of nearly equal size, except for slightly smaller ventralmost one; very weak distalward increase in size of spine articulations; gap between spine articulations and distal edge of LAP narrow. Ventral edge of LAP with small, weakly concave tentacle notch.

Inner side of LAP with relatively small, sharply defined, prominent, slender ridge; ventral half of ridge bent, pointing ventro-proximalwards, not merged with thickened ventral portion of LAP; dorsal half of ridge straight, oblique, with very weakly thickened dorsal tip; inner side of distal edge of LAP with two small, well-defined, prominent, oval, horizontally elongate spurs; inner side of tentacle notch small, laterally well defined. No perforations or furrow discernible.

GZG.INV.78715 is a dissociated median LAP; 1.5 times wider than high; well in agreement with holotype; vertical striation restricted to a smaller area of the outer surface. Four equal-sized spine articulations similar to those observed on holotype; gap separating spine articulations and distal edge of LAP as wide as one spine articulation.

Inner side similar to previous LAP described.

GZG.INV.78716 is a dissociated distal LAP; almost twice wider than high; vertical striation on outer surface slightly less well developed than in holotype. Three equal-sized spine articulations similar to those of holotype; distal edge of LAP slightly fragmentary, width of gap between spine articulations and distal edge of LAP not determinable.

Inner side of LAP with small, sharply defined, prominent, oblique, straight ridge; ventral tip of ridge widened.

Remarks

These LAPs, unambiguously assignable to Dermacantha gen. nov. on account of the general Dermocoma - like morphology combined with a discontinuous volute of the relatively small spine articulations, bear a striking similarity to the LAPs described below as Dermacantha carli sp. nov., to such an extent that both LAP types are almost indistinguishable. Minor differences pertain to the slightly thicker ventral portion and the slightly wider ridge in proximal LAPs of the present form. In addition, the vertical striation in the proximal LAPs of D. carli sp. nov. is composed of slightly larger, more strongly overlapping lamellae. Yet, these differences are so small that a formal description of the present specimens as a new species appears unjustified. To make matters worse, the amount of currently available material is too limited to appreciate fully the variability of the characters on which species separation would be based on. It seems unlikely that the present material is conspecific with D. carli sp. nov., in particular taking into account the long stratigraphic gap (Bajocian-Kimmerdigian) and the fundamentally different depositional settings (mud bottom slope versus deep shelf sponge reef) of both occurrences. In the absence of more diagnostic material, the present specimens are here treated as a probably new, still undescribed species of Dermacantha gen. nov.

Figures

Fig. 30. Fossil lateral arm plates (LAPs) of ophiacanthid brittle stars in external (a) and internal (b) views. 1-3. Dermacantha sp. nov. innom. from the Bajocian-Bathonian boundary of Touert, France. 1. GZG.INV.78714, proximal LAP. 2. GZG.INV.78715, median LAP. 3. GZG.INV.78716, distal LAP. 4-6. Dermacantha carli gen. et sp. nov. from the late Oxfordian (Late Jurassic) of the Plettenberg, Germany. 4. GZG.INV.78718 (holotype), proximal LAP. 5. GZG.INV.78719 (paratype), median LAP. 6. GZG. INV.78720 (paratype), distal LAP. 7-9. Ishidacantha hirokoae gen. et sp. nov. from the middle Toarcian (Early Jurassic) of Le Clapier, France. 7. GZG.INV.78723 (holotype), proximal LAP. 8. GZG.INV.78724 (paratype), median LAP. 9. GZG.INV.78725 (paratype), distal LAP. One common scale bar per species.