Dermocoma subtilirugosa ( Kristan-Tollmann & Gramann, 1992 ) 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 : 138-139

Dermocoma subtilirugosa ( Kristan-Tollmann & Gramann, 1992) comb. nov. Fig. 25 : 4-5

Ophiacantha? subtilirugosa Kristan-Tollmann & Gramann, 1992: 468 , pl. 3 figs 1-6.

Diagnosis

Species of Dermocoma with moderately large LAPs displaying a very fine vertical striation on almost entire outer surface; two very poorly defined spurs on the outer proximal and inner distal edges; up to four small spine articulations in tight notches of non-elevated distal edge.

Material examined

GZG.INV.78671, GZG.INV.78672 and GZG.INV.78673 (3 dissociated LAPs) from the Rhaetian of Fischerwiese, Austria..

Description

Moderately large LAPs; rounded, crescentic (proximal LAPs) to rectangular (distal LAPs) outline; slightly higher than wide (proximal LAPs) to more than twice wider than high (distalmost LAPs); dorsal edge straight to slightly convex, distal edge gently convex; ventral quarter to fifth large and protruding ventro-proximalwards in proximal to median LAPs; proximal edge evenly concave, with up to two small, very poorly defined, weakly prominent and not protruding, almost indiscernible spurs; outer surface almost entirely covered by very fine vertical striation, composed of thin lamellae, distalwards slightly overlapping, proximalwards decreasing in size and fading into finely meshed stereom close to proximal edge of LAP; striation mostly regularly vertical, with occasional irregularities (slightly oblique and/or branching lamellae) in ventral half of outer surface. Three (distal LAPs) to four (proximal LAPs) small, ear-shaped, nearly equal-sized spine articulations in tight notches of distal edge, interrupting vertical striation; gaps separating spine articulations increasing in size dorsalwards; dorsal and ventral lobes of spine articulations forming continuous volute; very narrow gap between spine articulations and distal edge of LAP. Ventral edge of proximal LAPs with large but weakly concave tentacle notch; tentacle perforation ventro-distally bordering ventralmost spine articulation in distal LAPs.

Inner side of proximal and median LAPs with narrow, well-defined, prominent ridge separated by rounded kink into dorsal and ventral portions; dorsal portion oblique, bent dorso-proximalwards, with widened slightly less prominent and less well-defined dorsal tip; ventral portion much shorter than dorsal one, pointing ventro-proximalwards, sharply separated from thickened ventral edge of LAP; ridge in distal LAPs short, oblique, well-defined, prominent, without kink, bone-shaped to triangular. Very faint vertical row of small, irregular perforations close to distal edge of LAPs, separating inner side into coarsely meshed main portion, and narrow, finely meshed distal portion. Tentacle notch in proximal and median LAPs large, distally bordered by narrow, oblique ridge.

Remarks

LAPs similar to the ones described here were first recorded from the Rhaetian of rocks dredged from the northern Exmouth Plateau and described as Ophiacantha ? subtilirugosa by Kristan-Tollmann & Gramann (1992). The new material from the Rhaetian of Austria now enables a reassessment of this species in accordance with the most recently set standards in ophiuroid micropalaeontology ( Thuy & Stöhr 2011). The shape, size and position of the spine articulations, the outer surface ornament and the shape of the ridge on the inner side strongly suggest assignment to Dermocoma . Among the LAP types assigned to this genus, the present one differs in combining small spine articulations with a fine vertical striation which covers almost the entire outer surface and two almost indiscernible spurs on the outer proximal and inner edges.

Occurrence

Rhaetian of Austria and the Wombat Plateau off northern Australia.

Figures

Fig. 25. Fossil skeletal plates and articulated arm fragments of ophiacanthid brittle stars; lateral arm plates (LAPs) in external (a) and internal (b) views. 1-3. Alternacantha dilionessa sp. nov. from the early Kimmeridgian of the Pointe du Chay, France. 1. GZG.INV.78666 (paratype), arm spine. 2. GZG. INV.78667 (paratype), median arm fragment in ventral view. 3. GZG.INV.78668 (paratype), proximal arm fragment in lateral (a) and dorsal (b) views. 4-5. Dermocoma subtilirugosa (Kristan-Tollmann & Gramann, 1992) comb. nov. from the Rhaetian (Late Triassic) of Fischerwiese, Austria. 4. GZG.INV.78671, proximal LAP. 5. GZG.INV.78672, median LAP. 6. Dermocoma sp. nov. innom. 1 from the Rhaetian (Late Triassic) of Fischerwiese, Austria; GZG.INV.78674, median LAP. One common scale bar per species except for 1.