Alternacantha arges, 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 : 132-134

Alternacantha arges sp. nov.

urn:lsid:zoobank.org:act:B1D9DEC0-8F10-4D8D-AAB0-15949F6DB8CA

Fig. 24 : 1-2

Ophiothrix? royeri – Hess 1960: 396, figs 7-8 [specimen incorrectly assigned to Ophiothrix? royeri (de Loriol, 1872) ].

Diagnosis

Species of Alternacantha with extremely large LAPs devoid of vertical striation on the outer surface; up to three large, well-defined spurs on the outer proximal edge; up to four spine articulations; distal edge of ventral arm plates truncated; distal edge of dorsal arm plates Gauss-curve-shaped.

Etymology

Species named after Arges, one of the three giant cyclopes in Greek mythology, in reference to the extremely large size of the species in combination with the large dorsalmost spine articulation resembling the single eye of the cyclopes.

Holotype

NHMB M 610 .

Type locality and horizon

Raedersdorf, France; Humeralis Member, late Oxfordian, Late Jurassic.

Description of holotype

M 610 is an extremely large, articulated arm fragment preserving nine proximal segments; LAPs more than twice higher than wide; dorsal edge pointed to tongue shaped; distal edge irregularly undulose; proximal edge concave, with up to three very large, well-defined, strongly prominent, horizontally elongate, protruding spurs; ventral portion of LAP small, protruding ventro-proximalwards; outer surface of LAPs with finely meshed stereom, devoid of vertical striation. Three to four very large, earshaped spine articulations in notches of strongly elevated distal edge; dorsal and ventral lobes of spine articulations merged into continuous volute; strong dorsalward increase in size of spine articulations and of gaps separating them; position of dorsalmost spine articulation alternating between close to the remaining ones and near the dorsal edge of the LAP, separated from the remaining spine articulations by an extremely wide gap; spine articulations proximally sharply bordered by edge of notches; gap between spine articulations and distal edge of LAP relatively large.

Inner side of LAPs not exposed.

Ventral arm plates large, wider than long, distal half widest; distal edge of ventral arm plates convex but with a sharply truncated, straight tip; lateral and latero-proximal edges of ventral arm plates concave; proximal angle of ventral arm plates right to slightly acute; ventral arm plates overlapping but LAP most probably in contact underneath ventral arm plates; tentacle pores small, no tentacle scales preserved; dorsal arm plates large, nearly as wide as long, with straight distalwards diverging lateral edges and conspicuously Gauss-curve-shaped distal edge with tongue-shaped tip; dorsal arm plates broadly overlapping, separating LAPs on all preserved segments.

Remarks

Hess (1960) already provided a detailed description and excellent figures of the present specimen, which he considered to be conspecific with de Loriol’s (1872) Ophiurella royeri which was tentatively transferred to the extant genus Ophiothrix Müller &Troschel, 1840 . It obviously belongs to Alternacantha on account of the alternating position of the dorsalmost spine articulation. A detailed re-examination of Hess’s (1960) specimen, however, has now revealed a highly distinctive arm morphology which differentiates it unambiguously from all other species assignable to Alternacantha , including the type material of O.? royeri . In fact, apart from its extremely large size, the specimen lacks a vertical striation on the outer surface, and displays only up to four spine articulations and a conspicuously Gauss-curveshaped distal edge of the dorsal arm plates. Even if it could be argued that the lack of a vertical striation on the outer surface is related to the larger size and thus greater heigth/width ratio of the LAPs of the specimen, the number of spine articulations and shape of the distal edge of the dorsal arm plate are sizeindependent features. This particular specimen is thus described here as a new species.

Figures

Fig. 24. Fossil lateral arm plates in external (a) and internal (b) views and articulated arm fragment of ophiacanthid brittle stars. 1-2. Alternacantha arges sp. nov. from the late Oxfordian (Late Jurassic) of Raedersdorf, France; M 610 in dorsal (1a) and ventral (1b) views and with details of dorsal (2a) and ventral (2b) arm plating. 3-6. Alternacantha dilionessa sp. nov. from the early Kimmeridgian of the Pointe du Chay, France. 3. GZG.INV.78662 (holotype), proximal LAP. 4. GZG.INV.78663 (paratype), proximal LAP. 5. GZG.INV.78664 (paratype), median LAP. 6. GZG.INV.78665 (paratype), distal LAP. One common scale bar for 1, for 2 and for 3-6, respectively.