Dermocoma toarcensis ( Hess, 1962 ) 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 : 145-146

Dermocoma toarcensis ( Hess, 1962) comb. nov.

Fig. 26 : 7-8

Ophiacantha? toarcensis Hess, 1962: 649 , figs 122-125.

“Seewen Typ IIA” – Hess 1962: 635, fig. 28.

Ophiacantha? toarcensis – Kutscher 1996: 17, pl. 3 figs 1-5. — Kutscher & Villier 2003: 183, pl. 3 figs 5-7.

non Ophiacantha? toarcensis – Thuy 2005: 40, pl. 5 figs 1-6, pl. 6 figs 5-6.

non Ophiacantha? cf. toarcensis – Kutscher & Hary 1991: 45, pl. 1 fig. 4.

Diagnosis

Species of Dermocoma with moderately large LAPs displaying a well-developed vertical striation on outer surface; up to three spurs on outer proximal and inner distal edges; up to six large spine articulations; ventral lobe merged with distalwards pointing tip of outer surface separating notches of spine articulations; ridge on inner side of LAPs slender, dorsal part not thickened.

Material examined

NHMB M11216, NHMB M11217 and 161 dissociated LAPs from the late Toarcian of Seewen, Switzerland, the type material of Hess (1962) termed “Seewen Typ IIA”; 17 dissociated LAPs from the late Pliensbachian of Seewen, Switzerland, the original material of Hess (1962) described as Seewen; the original material of Kutscher (1996) and Kutscher & Villier (2003).

Description

Moderately large, dissociated LAPs, proximal ones nearly twice higher than wide, distal ones almost twice wider than high; dorsal edge straight to slightly concave as a result of a weak constriction; distal edge convex; proximal edge irregularly undulose, with up to three poorly to moderately welldefined, horizontally elongate, prominent and protruding spurs, dorsal one of which slightly smaller than remaining two; ventral quarter to fifth of LAP strongly protruding ventro-proximalwards; outer surface with fine, regular, vertical striation composed of slightly overlapping lamellae and replaced in proximal half of outer surface by finely meshed stereom. Six (proximal LAPs) to three (distal LAPs) large, ear-shaped spine articulations in notches of elevated distal edge; dorsal and ventral lobes of spine articulations merged into continuous volute; ventral lobe merged with distalwards pointing tips of outer surface separating notches; spine articulations proximally sharply delimited by edge of notches rather than single distalmost lamella or well-defined ridge; notches incising vertical striation of outer surface; spine articulations nearly equal sized, with slighty dorsalward increase in size of gaps separating them; gap between spine articulations and distal edge of LAP narrow. Ventral edge of LAP with small but clearly concave tentacle notch in proximal to median LAPs, straight and devoid of tentacle notch in distal LAPs.

Inner side of LAPs with large, conspicuous, sharply defined, prominent and relatively narrow ridge; ventral part of ridge pointing ventro-proximalwards, sharply separated from thickened ventral portion of LAP, oblique central part and near-vertical, slender dorsal part, with all parts connected by very gentle, rounded kinks; ridge in distal LAPs short, with thickened ventral tip and dorso-proximally pointing, slender dorsal tip; inner side of distal edge of LAP with up to three large, moderately well- to welldefined, oval, prominent spurs composed of more densely meshed stereom; inner side of tentacle notch relatively small, with coarsely meshed, horizontally slightly elongate stereom, sharply defined laterally. Small, irregular perforations in poorly defined vertical row dorsally bordering tentacle notch in proximal to median LAPs.

Remarks

A re-examination of the LAPs originally described by Hess (1962) as Ophiacantha ? toarcensis from the Toarcian of Switzerland has now revealed that the species should be transferred to Dermocoma on account of the size, shape and position of the spine articulations, the strongly protruding ventral portion of the LAP and the shape of the ridge on the inner side. Within this genus, D. toarcensis is unique in combining six spine articulations and up to three spurs on the outer proximal and inner distal edges.

Subsequent records of this species from coeval or slightly younger strata by Kutscher (1996) and Kutscher & Villier (2003) have proved to be conspecific. The Hettangian and Sinemurian records by Thuy (2005) and Kutscher & Hary (1991), respectively, belong to different species. The former is here reinterpreted as a new species of Dermocoma , and the latter is a still unknown species which is most probably assignable to Dermocoma – it can only be formally described after re-examination of the original material and/or discovery of new finds. At the present state, this Sinemurian record is best listed as Dermocoma sp.

The LAPs from the Pliensbachian of Seewen, Switzerland, described and illustrated as “Seewen Typ IIA” by Hess (1962) almost certainly belong to Dermocoma toarcensis comb. nov. Kutscher & Hary (1991) synonymised the LAPs termed “Seewen Typ IIA” with those of “Seewen Typ IA”, here re-interpreted as Ophiotoma vadosa sp. nov. (see above), and assigned them to their new species Ophiocten ? seeweni Kutscher & Hary, 1991 , which was based on dissociated LAPs from the Sinemurian of Luxembourg. The latter are, however, neither conspecific with “Seewen Typ IIA” nor with “Seewen Typ IA” (original specimens of “Seewen Typ IB and IIB” were not examined). Ophiocten ? seeweni is here transferred to Dermacantha gen. nov. (see below).

Occurrence

Late Pliensbachian to Late Toarcian of Switzerland, Toarcian of France and Toarcian/Aalenian of Germany.

Figures

Fig. 26. Fossil lateral arm plates (LAPs) of ophiacanthid brittle stars in external (a) and internal (b) views. 1-3. Dermocoma faberi sp. nov. from the Hettangian (Early Jurassic) of Vance, Belgium. 1. MnhnL HE408 (holotype), proximal LAP. 2. MnhnL HE409 (paratype), median LAP. 3. MnhnL HE410 (paratype), distal LAP. 4-6. Dermocoma potti sp. nov. from the late Pliensbachian (Early Jurassic) of Feuguerolles, France. 4. GZG.INV.78675 (holotype), proximal LAP. 5. GZG.INV.78676 (paratype), median LAP. 6. GZG. INV.78677 (paratype), distal LAP. 7-8. Dermocoma toarcensis (Hess, 1962) comb. nov. from the late Toarcian (Early Jurassic) of Seewen, Switzerland. 7. NHMB M11216, proximal LAP. 8. NHMB M11217, distal LAP. 9-11. Dermocoma longwyensis sp. nov. from the early Bajocian (Middle Jurassic) of Longwy, France. 9. GZG.INV.78679 (holotype), proximal LAP. 10. GZG.INV.78680 (paratype), median LAP. 11. GZG.INV.78681 (paratype), distal LAP. One common scale bar per species.