Alternacantha dilionessa, 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 : 134-137

Alternacantha dilionessa sp. nov.

urn:lsid:zoobank.org:act:FD230CAD-9C1E-4C38-B407-1EAB92E10F70

Figs 24 : 3-6; 25: 1-3

Diagnosis

Species of Alternacantha with very large LAPs displaying a moderately well-defined vertical striation on the outer surface; up to six large, well-defined spurs on the outer proximal edge, paralleled by large, relatively short, oval spurs on the inner distal edge; distal edge of ventral arm plates evenly convex; distal edge of dorsal arm plates parabolic.

Etymology

Name composed of diligere, Latin for “to cherish”, and “lioness”, translating into lea in Latin, as an affectionate declaration of esteem to my beloved wife Lea, without whose support the present study would not have been possible.

Type material

Holotype GZG.INV.78662 .

Paratypes

GZG.INV.78663, GZG.INV.78664, GZG.INV.78665, GZG.INV.78666, GZG.INV.78667 and GZG. INV.78668.

Type locality and horizon

Pointe du Chay near La Rochelle, France; Achilles Subzone, Cymodoce Zone , early Kimmeridgian, Late Jurassic.

Additional material

GZG.INV.78669 (512 dissociated LAPs) and GZG.INV.78670 (distal articulated arm fragment).

Description

Holotype

GZG.INV.78662 is a dissociated, very large, proximal LAP; approximately twice wider than high; dorsal edge oblique, nearly straight, with pointed dorso-proximal tip; distal edge slightly convex; proximal edge nearly straight, with six small to moderately large, well-defined, prominent, horizontally elongate, proximally pointed and slightly protruding spurs; second and third ventralmost spurs largest; ventral fifth of LAP strongly protruding ventro-proximalwards; ventro-distal edge of LAP round, weakly protruding ventralwards; outer surface with moderately well-developed, rather coarse, slightly undulose, vertical striation restricted to narrow band near spine articulations and composed of slightly overlapping lamellae of variable width; greater part of outer surface with finely meshed stereom. Five very large, ear-shaped spine articulations in shallow notches of elevated distal edge; ventral and dorsal lobes of spine articulations merged into continuous volute; spine articulations proximally sharply bordered by edge of notches rather than well-defined ridge or single distalmost lamella; notches incising outer surface; gap between spine articulations and distal edge of LAP slightly narrower than one spine articulation; dorsalward increase in size of spine articulations and of gaps separating them; dorsalmost spine articulation separated from the remaining four by very large gap with slightly more coarsely meshed stereom. Ventral edge of LAP with relatively small but deeply concave tentacle notch.

Inner side of LAP with very large, conspicuous, sharply defined, prominent, very narrow ridge; ventral part of ridge bent, pointing ventro-proximalwards; central part of ridge straight, oblique, and dorsal part straight and very slightly oblique, not reaching dorsal edge of LAP; all parts connected by rounded kinks; no thickened parts; inner side of distal edge of LAP with three large, moderately well-defined, prominent, relatively short and high oval spurs in the centre and two smaller, poorly defined spurs ventrally and dorsally bordering the three larger ones; inner side of tentacle notch relatively small, well defined laterally. Shallow, poorly defined furrow dorsally bordering tentacle notch, dorsally obscured by shell fragment.

Paratype supplements and variation

GZG.INV.78663 is a dissociated, very large, proximal LAP; approximately twice wider than high; agreeing closely with holotype; proximal edge with one large, central, well-defined, prominent and protruding spur and two smaller, dorsal and ventral, non-protruding spurs; striation on outer surface limited to a very narrow band near the three ventralmost spine articulations. Four spine articulations similar to those observed on holotype; dorsalmost one even more widely separated from remaining spine articulations.

Inner side similar to that of holotype.

GZG.INV.78664 is a dissociated median LAP; slightly higher than wide; proximal edge with two large, well-defined, prominent, and strongly protruding spurs. Four spine articulations similar to those observed on holotype.

Inner side of LAP with ridge similar to that observed on holotype; inner side of distal edge of LAP with two large, moderately well-defined, weakly prominent spurs. Shallow, moderately well-defined, slightly oblique furrow with two very small spine articulations dorsally bordering tentacle notch.

GZG.INV.78665 is a dissociated distal LAP; slightly wider than high; dorsal edge concave as a result of a weakly developed constriction; proximal edge with single, moderately well-defined, prominent and slightly protruding spur; vertical striation well developed on distal two-fifths of the outer surface. Three spine articulations similar to those observed on holotype; dorsal gap between spine articulations slightly wider than ventral one. Ventral edge of LAP with small, weakly concave tentacle notch.

Inner side of LAP with sharply defined, prominent ridge composed of strongly widened ventral part and ventro-proximally pointing orsal part; inner side of distal edge with single well-defined, slightly prominent spur; inner side of tentacle notch small, sharply defined laterally, pointing ventralwards. No perforations or furrows discernible.

GZG.INV.78666 is a dissociated, large arm spine; cylindrical; slowly tapering; straight; with finely meshed, slightly longitudinally elongate stereom beset with scattered, minute granules; tip broken, exposing lumen filled with loosely meshed stereom.

GZG.INV.78667 is an articulated arm fragment counting five proximal segments; LAPs well in agreement with holotype; ventral arm plates very large, approximately bell shaped, slightly wider than high, widest distally, with strongly and evenly convex distal edge, concave lateral and latero-distal edges and pointed proximal angle; ventral arm plates overlapping, covering ventral portions of LAPs, the latter meeting underneath ventral arm plates; tentacle pores small, no tentacles scales preserved; dorsal arm plates large, nearly as wide as long, with nearly straight, short proximal edge; straight, distalwards diverging lateral edges and convex, parabolic distal edge; short, well-defined and prominent longitudinal ridge in proximal third of dorsal arm plate outer surface; dorsal arm plates overlapping, separating LAPs.

GZG.INV.78668 is an articulated arm fragment counting six proximal to median segments; well in agreement with other arm fragment paratype; tentacle pores with two small, leaf-like scales.

Remarks

D’Orbigny (1850) already mentioned an articulated arm fragment from the Kimmeridgian of Chay peninsula, France, listing it as “ Ophiurella bispinosa d’Orbigny, 1850 ”, and it is more than likely that he was referring to the same species as the one described here. However, since Ophiurella bispinosa has never been described, it must be considered as nomen nudum. The present specimens share greatest similarities with Alternacantha occulta which, however, differs in having smaller proximal LAPs with a pointed to tongue-shaped distal edge, as well as fewer and less well-defined spurs on the outer proximal and inner distal edges. In A. schwermannorum sp. nov. (see above), the distal edge of the ventral arm plates is slightly truncated rather than evenly convex, and the spurs on the inner side of the distal edge of the LAPs are very thin, horizontally elongate and pointed rather than high and oval.

Occurrence

Early Kimmeridgian of France.

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.