Dermocoma biformis (Hess, 1975) 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 : 151-153

Dermocoma biformis (Hess, 1975) comb. nov.

Fig. 27 : 5-8

Ophiacantha? biformis Hess, 1975a: 594 , figs 10-12, pl. 1 fig. 6 [non pl. 1 fig. 5, which is not an ophiacanthid].

Ophiacantha ? biformis – Hess 1975b: 608, figs 8-9.

Diagnosis

Species of Dermocoma with very large LAPs displaying two well-defined spurs on outer proximal and inner distal edges; dorsal spur very large, prominent and strongly protruding, ventral spur smaller and less strongly protruding; fine, well-developed vertical striation on distal half of outer surface; up to five moderately large, equal-sized spine articulations in tight notches of distal portion of LAP; ridge on inner side large, relatively slender, with long, near-straight, almost vertical dorsal part slightly thickened at its centre.

Material examined

NHMB M11220, NHMB M11221 and 120 dissociated LAPs from the Günsberg Member, late Oxfordian of Guldental, Switzerland; GZG.INV.78690 and GZG.INV.78691 (137 dissociated LAPs) from sample S2a of Gale (2011) from the late Oxfordian of Savigna, France; GZG.INV.78692, GZG.INV.78693 (51 dissociated LAPs) from sample S2b of Gale (2011) from the late Oxfordian of Savigna, France; GZG. INV.78694 (36 dissociated LAPs) sample S1 of Gale (2011) from the late Oxfordian of Savigna, France; 14 dissociated LAPs from the Humeralis Member, late Oxfordian of Raedersdorf, France, original material of Hess (1975b); 33 dissociated LAPs from the Bifurcatus Zone, late Oxfordian of Guldental, Switzerland, the original material of Hess (1966); 11 dissociated LAPs from the Renggeri Member, early Oxfordian of Chapois, France, the original material of Hess (1965a).

Description

Very large, dissociated LAPs; proximal ones 1.5 times higher than wide, distal ones almost twice wider than high; dorsal edge slightly convex (proximal LAPs) to slightly concave (distal ones) as a result of a weak constriction; distal edge convex; proximal edge concave, with two spurs; dorsal spur very large, well-defined, oval, prominent and strongly protruding at least in proximal to median LAPs; ventral spur much smaller, less well-defined, weakly prominent and protruding; ventral fifth of LAPs strongly protruding ventro-proximalwards; outer surface with well-developed fine, vertical striation composed of slightly wavy, fine, weakly overlapping lamellae close to spine articulations, and replaced by very finely meshed stereom on proximal half of outer surface. Five (proximal LAPs) to three (distal ones) moderately large, ear-shaped spine articulations in relatively tight notches of the slightly elevated distal portion of the LAPs; ventral and dorsal lobes of spine articulations merged into continuous volute; ventral lobe with very weak and thin connection with distalwards projecting tips of outer surface separating notches; spine articulations proximally sharply separated by edge of notches; spine articulations nearly equal sized; dorsalward increase in size of gaps separating spine articulations; narrow gap between spine articulations and distal edge of LAP. Ventral edge of LAP with small, concave tentacle notch in all but the distalmost LAPs.

Inner side of LAPs with large, conspicuous, sharply defined, prominent, moderately slender ridge; ventral part of ridge ventro-proximalwards bent, confluent with thickened ventral portion of LAP; dorsal part of ridge longer than ventral one, almost straight, slightly oblique with weakly thickened central part; ridge in distal LAPs short, oblique, with dorso-proximalwards pointing dorsal tip, widened ventral tip, and distally bordered by small round knob; inner side of distal edge with two moderately well-defined, oval spurs composed of densely meshed stereom; dorsal spur much larger than ventral one in proximal to median LAPs; inner side of tentacle notch small, deeply incised and sharply bordered laterally. Very shallow, poorly defined vertical furrow with few, minute perforations dorsally bordering tentacle notch and distally bordered by moderately well-defined, narrow and weakly prominent ridge; no perforations or furrow discernible in distal LAPs.

Articulated arm fragments preserving median to proximal segments; ventral arm plates large, slightly longer than wide, polygonal, with straight to weakly concave distal edge, straight distalwards converging latero-distal edges; concave lateral and latero-proximal edges and a right to slightly acute proximal angle; LAPs in contact ventrally but not separating ventral arm plates; tentacle pores small, covered by at least one small, leaf-like tentacle scale; dorsal arm plates large, slightly longer than wide, widest distally separating LAPs, nearly trapezoid in outline, with convex distal edge, distalwards diverging, straight lateral edges and slightly convex proximal edge; dorsal arm plates with very fine transverse striation.

Remarks

Hess (1975a) originally described a set of dissociated LAPs from the Oxfordian of Switzerland as Ophiacantha ? biformis , the species name being chosen to reflect the fact that median and distal LAPs display a vertical striation which the proximal ones completely lack. However, as suggested by the observations of Thuy & Stöhr (2011), the type of ornament on the outer surface of the LAPs within one species is very unlikely to change fundamentally according to the position along the arm. Indeed, a careful re-examination of the type material has revealed that it included more than one type of LAPs and that a number of proximal LAPs, including the holotype, display the same vertical striation, although limited to a narrow band near the spine articulations, as observed on the median and distal LAPs. The original description and diagnosis of Ophiacantha ? biformis are based on at least two different types of LAPs, which is a prime example of why, for new species based on dissociated LAPs, the holotype and the paratypes should always be described individually ( Thuy & Stöhr 2011).

The LAP type corresponding to the holotype of Ophiacantha ? biformis can unambiguously be assigned to the genus Dermocoma on account of the shape, size and position of the spine articulations, the strongly protruding ventral portion of the LAPs and the shape of the ridge on the inner side. Closest similarities are shared with the LAPs of D. wrighti which, however, differ in displaying a dorsalward increase in size of the spine articulations, a slender ridge devoid of thickened parts, and a slightly less sharply defined dorsal spur on the outer proximal edge.

New finds assignable to Dermocoma biformis comb.nov. from the Oxfordian of France include articulated arm fragments which complement the morphological data based on dissociated LAPs. The additional data on arm morphology are in agreement with the assignment to Dermocoma and furthermore suggest that the dorsal arm plate illustrated by Thuy (2005) and originally assigned to Ophiacantha ? toarcensis (see above), indeed, belongs to Dermocoma , most probably to D. faberi sp. nov.

Occurrence

Early Oxfordian of France; late Oxfordian of Switzerland and France.

Figures

Fig. 27. Fossil lateral arm plates in external (a) and internal (b) views and articulated arm fragment of ophiacanthid brittle stars. 1-2. Dermocoma wrighti Hess, 1964 from the early Callovian (Middle Jurassic) of Liesberg, Switzerland. 1. NHMB M11218, proximal LAP. 2. NHMB M11219, distal LAP. 3-4. Dermocoma sp. 1 from the Callovian (Middle Jurassic) of Jumara, India. 3. GZG.INV.78687, proximal LAP. 4. GZG.INV.78688, distal LAP. 5-8. Dermocoma biformis (Hess, 1975) comb. nov. from the late Oxfordian (Late Jurassic) of Guldental, Switzerland (5-6) and Savigna, France (7-8). 5. NHMB M11220, proximal LAP. 6. NHMB M11221, distal LAP. 7. GZG.INV.78690, proximal LAP. 8. GZG. INV.78692, proximal arm fragment in ventral (a) and dorsal (b) views. One common scale bar for 1-2, 3-4, 5-6, 7 and 8 respectively.