Dermocoma wrighti Hess, 1964

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 : 148-149

Dermocoma wrighti Hess, 1964

Fig. 27 : 1-2

Dermocoma wrighti Hess, 1964: 78 , figs 42-45, pl. 6, pl. 10 fig. 2.

p.p. “Liesberg Typ I” – Hess 1963: 1151, figs 19-21.

p.p. Dermocoma wrighti – Hess & Holenweg 1985: 146, fig. 2.

Dermocoma wrighti – Kutscher 1987a: 62, pl. 2 fig. 2, pl. 4 fig. 1.

non Dermocoma wrighti – Hess 1972a: 36, figs 39-41, pl. 13 fig. 1.

Diagnosis

Species of Dermocoma with moderately large LAPs displaying two poorly to moderately well-defined spurs on the outer proximal edge, dorsal one of which larger, developed as swollen part of proximal edge rather than well-defined spur; outer surface with well-defined, moderately fine vertical striation; up to five large spine articulations in notches of distal portion of LAP, and with distalward increase in size; ridge on the inner side with long, near-straight, oblique dorsal portion devoid of thickened parts.

Material examined

NHMB M11218, NHMB M11219 and 13 dissociated LAPs from the Koenigi Zone, early Callovian of Liesberg, Switzerland, the original material of Hess (1963); GZG.INV.78684 (133 dissociated LAPs) from the Humphriesianum Zone, early Bajocian of Cirey-lès-Nolay, France; GZG.INV.78685 (8 dissociated LAPs) from the Humphriesianum Zone, early Bajocian of Delkhofen, Germany; GZG. INV.78686 (3 dissociated LAPs) from the Callovian of Bauer-Wehrland, Germany, the original material of Kutscher (1987a).

Description

Moderately large, dissociated LAPs; proximal ones slightly higher than wide, distal ones slightly wider than high; dorsal edge straight to weakly concave as a result of a poorly developed constriction; distal edge convex; proximal edge concave, with two poorly to moderately well-defined spurs, dorsal one large, prominent, strongly protruding, developed as swollen part of proximal edge rather than sharply defined spur; ventral spur much smaller, less strongly protruding; ventral fifth of LAPs strongly ventro-proximalwards protruding; outer surface with well-developed, moderately fine vertical striation composed of slightly irregular, weakly overlapping lamellae, and replaced by finely meshed stereom on proximal half of outer surface. Five large, ear-shaped spine articulations in notches of elevated distal portion of LAP; ventral and dorsal lobes of spine articulations merged into continuous volute; ventral lobe connected with distalwards projecting tips of outer surface separating notches; spine articulations proximally sharply separated by edge of notches rather than single distalmost lamella or ridge; dorsalward increase in size of spine articulations and of gaps separating them; moderately wide gap between spine articulations and distal edge of LAP. Ventral edge of LAP with small, concave tentacle notch in all LAPs. Inner side of LAPs with large, conspicuous, sharply defined, prominent, slender ridge; ventral part of ridge ventro-proximalwards bent, not merged with thickened ventral portion of LAP; dorsal part of ridge much longer than ventral one, almost straight, slightly oblique devoid of thickened part; ridge in distal LAPs moderately short, strongly oblique, with dorso-proximally pointing dorsal tip, and widened ventral tip; inner side of distal edge of LAP with two moderately well-defined spurs, dorsal one of which slightly larger than ventral one; inner side of tentacle notch relatively small, well defined laterally. Shallow, poorly defined vertical furrow with irregular, scattered minute perforations dorsally bordering tentacle notch in proximal to median LAPs.

Remarks

The original description of Dermocoma wrighti , the type species of the genus, by Hess (1964) provided only limited data on LAP morphology. A careful re-examination of the type specimens has now enabled a detailed, comprehensive LAP morphological diagnosis to be worked out as a basis of comparison with other records based exclusively on dissociated LAPs. In this way it has been possible to confirm Kutscher’s (1987a) observation that the dissociated LAPs from the Callovian of Switzerland described earlier by Hess (1963) as “Liesberg Typ I” were assignable to Dermocoma wrighti . The LAPs of this species are characterised by the combination of up to five spine articulations and two moderately welldefined spurs on the outer proximal and inner distal edges, the dorsal one which being larger and better defined. This combination is also found in the LAPs originally described as Ophiacantha ? biformis Hess, 1975 , here shown to be assignable to Dermocoma (see below). In that species, however, the spine articulations are nearly of equal size, whereas in D. wrighti they display a dorsalward increase in size. In addition, the ridge of D. wrighti is devoid of thickened parts.

Occurrence

Early Bajocian of France and Germany; early Callovian of Switzerland and Callovian of Germany.

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.