Alternacantha occulta Thuy & Meyer, 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 : 127-128

Alternacantha occulta Thuy & Meyer, 2013

Fig. 22 : 7-8

Synonymy

Reference is made to Thuy & Meyer (2013) for a complete synonymy of the species.

Diagnosis

[complementary to that provided by Thuy & Meyer (2013)]

Species of Alternacantha with large LAPs displaying a pointed to tongue-shaped, relatively narrow dorsal edge; outer surface with a well-developed vertical striation; up to four moderately well-developed spurs on the outer proximal and inner distal edges; up to five spine articulations; distal edge of ventral arm plates evenly convex; distal edge of dorsal arm plates parabolic.

Material studied

GZG.INV.78648, GZG.INV.78649 and GZG.INV.78650 (48 dissociated LAPs) from the early Bajocian of Longwy, France; GZG.INV.78651 (23 dissociated LAPs) from the early Bajocian of Cirey-lès-Nolay, France.

Description

Large, dissociated LAPs; more than twice wider than high (proximal LAPs) to 1.5 times wider than high (distal LAPs); dorsal edge convex, pointed to round and tongue like in proximal LAPs, concave as a result of a well-developed constriction in median to distal LAPs; distal edge convex; ventral quarter to fifth of LAP strongly protruding ventro-proximalwards; ventro-distal tip of LAP slightly protruding ventralwards, tongue shaped; proximal edge concave with up to four, medium-sized to large, moderately well-developed, prominent, horizontally elongate, pointedly to bluntly protruding spurs of variable position; outer surface with well-developed, fine, regular vertical striation composed of narrow, slightly overlapping lamellae; striation replaced by finely meshed stereom in distal half of outer surface in all LAPs and additionally in dorsal fifth of proximal LAPs. Five (proximal LAPs) to three (distal LAPs) large to very large, ear-shaped spine articulations in moderately shallow notches of elevated distal portion of LAP; dorsal and ventral lobes of spine articulations merged into continuous lobe, with small, very shallow notch at proximal point of contact; spine articulations sharply bordered by edge of notches rather than a well-defined ridge of distalmost lamella; notches deeply incising outer surface striation; strong dorsalward increase in size of spine articulations and of gaps separating them in all LAPs; position of dorsalmost spine articulation varying in position between close to the remaining spine articulations to widely separated from the latter by a gap equalling up to twice the height of the dorsalmost spine articulation in proximal to median LAPs. Ventral edge of LAP with relatively small but deeply incised, concave tentacle notch in all LAPs.

Inner side with large, relatively narrow (proximal and median LAPs) to moderately broad (distal LAPs), sharply defined, prominent ridge devoid of kink or thickened part; ventral tip of ridge pointing ventroproximally, not merged with ventral portion of LAP; dorsal half of ridge straight and oblique, not reaching dorsal edge of LAP; inner side of distal edge of LAP with up to four variably well-defined, oval to lenticular, slightly prominent spurs composed of more densely meshed stereom; inner side of tentacle notch relatively small, with coarsely meshed and slightly horizontally stretched stereom. Small, irregular perforations densely grouped in short vertical row dorsally bordering the tentacle notch in proximal to median LAPs; single small perforation dorsally bordering tentacle notch in distal ones.

Remark

The original description of Alternacantha occulta on the basis of several articulated specimens ( Thuy & Meyer 2013) is here complemented by a detailed description of the LAP morphology of the species, including the inner side of the LAPs which is not exposed on the articulated type specimens. As noted above, species of Alternacantha share nearly indistinguishable LAP morphologies, which makes a distinction of A. occulta from species known exclusively from arm fragments and dissociated LAPs a major challenge. In terms of the shape of the ventral and dorsal arm plates, A. occulta is closest to A. dilionessa sp. nov. (see below). In the former, however, the proximal LAPs are smaller, the dorsal edge is pointed to tongue shaped, rather than broad and oblique, and the outer proximal and inner distal edges display up to four, rather than six, spurs. In A. schwermannorum sp. nov., the ventral arm plates have a truncated rather than evenly convex distal edge, and the LAPs are dorsally broad rather than tongue like and display very thin, horizontally elongate rather than oval spurs on the inner distal edge. Alternacantha arges sp. nov. is a much larger species with LAPs devoid of vertical striation, truncated rather than evenly convex distal edges of the ventral plates and distinctively Gauss-curve-shaped rather than parabolic distal edges of the dorsal arm plates.

Figures

Fig. 22. Lateral arm plates (LAPs) of fossil and Recent ophiacanthid brittle stars in external (a) and internal (b) views. 1-2. Hanshessia sp. from the Callovian (Middle Jurassic) of Jumara, India. 1. GZG. INV.78641, proximal to median LAP. 2. GZG.INV.78642, proximal LAP. 3-4. Ophiocopa spatula Lyman, 1883, Recent. 3. Proximal LAP. 4. Distal LAP. 5. Alternacantha? sp. from the late Pliensbachian (Early Jurassic) of Amellago, Morocco; GZG.INV.78645, proximal LAP. 6. Alternacantha sp. nov. innom. 1 from the middle Toarcian (Early Jurassic) of Le Clapier, France; GZG.INV.78647, proximal to median LAP. 7-8. Alternacantha occulta Thuy & Meyer, 2013, from the early Bajocian (Middle Jurassic) of Longwy, France. 7. GZG.INV.78648, proximal LAP. 8. GZG.INV.78649, distal LAP. 9-10. Alternacantha sp. nov. innom. 2 from the Callovian (Middle Jurassic) of Jumara, India. 9. GZG.INV.78652, proximal to median LAP. 10. GZG.INV.78653, median LAP. One common scale bar per species.