Alternacantha schwermannorum, 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 : 129-132

Alternacantha schwermannorum sp. nov.

urn:lsid:zoobank.org:act:08DCE3FA-60AD-4415-80D3-127E2FD02F76

Fig. 23

p.p. Ophiacantha? suprajurassica – Hess 1966: 1030, 1054, figs 10, 70, 72 (material incorrectly referred to Ophiacantha? suprajurassica Hess, 1965 ).

Diagnosis

Species of Alternacantha with large LAPs displaying well-developed, regular vertical striation on outer surface; up to five poorly to moderately well-defined spurs on outer proximal edge; up to six welldefined, horizontally strongly elongate, ridge-like spurs on inner side of distal edge of LAP; up to five spine articulations; distal edge of ventral arm plates truncated.

Etymology

Species named in honour of my friends Leonie and Achim Schwermann.

Type material

Holotype GZG.INV.78654 .

Paratypes

GZG.INV.78655, GZG.INV.78656, GZG.INV.78657 and GZG.INV.78658.

Type locality and horizon

Savigna, France; sample S2a of Gale (2011), Bimammatum Zone, late Oxfordian, Late Jurassic.

Additional material

GZG.INV.78659 (102 dissociated LAPs) from sample S2a of Gale (2011); GZG.INV.78660 (47 dissociated LAPs) from sample S1 of Gale (2011); GZG.INV.78661 (166 dissociated LAPs) from sample S2b of Gale (2011).

Description

Holotype

GZG.INV.78654 is a dissociated, large, proximal LAP; dorsal edge slightly fragmentary distally, originally slightly convex, broad; distal edge irregularly undulose; ventral seventh of LAP protruding ventro-proximalwards, slightly fragmentary; proximal edge nearly straight, with five small, poorly defined, prominent and protruding spurs, three of which are grouped centrally and the fourth one in middle of dorsal half of proximal edge; outer surface with very well-developed, fine, regular to very slightly undulose, vertical striation composed of fine and very slightly overlapping lamellae, restricted to distal third of outer surface near spine articulations, replaced by very finely meshed stereom on remaining outer surface. Five large, ear-shaped spine articulations in shallow notches of elevated distal edge; ventral and dorsal lobes of spine articulations merged into continuous volute; wide gap between spine articulations and distal edge of LAP, nearly as wide as one spine articulation; spine articulations proximally sharply delimited by edge of notches rather than single distalmost lamella or well-defined, prominent ridge; dorsalward increase in size of spine articulations and in gaps separating them; dorsalmost spine articulations separated from remaining four by larger gap. Ventral edge of LAP with small, concave tentacle notch.

Inner side of LAP with large, conspicuous, sharply defined, very narrow ridge; ventral part of ridge slightly 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 very gently rounded kinks; no thickened parts or sharp kinks; inner side of distal edge of LAP with six large, conspicuous, well-defined, slightly prominent, horizontally strongly elongate, ridge-like spurs, becoming smaller and less well-defined dorsalwards; inner side of tentacle notch small, well defined laterally. Very shallow, moderately well-defined, slightly irregular, vertical furrow with single small perforation dorsally bordering tentacle notch.

Paratype supplements and variation

GZG.INV.78655 is a dissociated median LAP; well in agreement with holotype; dorsal edge straight, broad; distal edge convex; proximal edge concave, with single, large, moderately well-defined, prominent and slightly protruding spur. Five spine articulations similar to those observed on holotype; dorsalmost spine articulation smallest, widely separated from remaining four.

Inner side of LAP with ridge similar to that of holotype; inner side of distal edge of LAP with one large ventral, well-defined, prominent, horizontally strongly elongate, lenticular spur, and second dorsal, much smaller, less well-defined spurs. Very shallow and weakly defined, irregular vertical furrow dorsally bordering tentacle notch.

GZG.INV.78656 is a dissociated distal LAP; dorsal edge concave as a result of a well-defined constriction; distal edge convex; proximal edge irregularly wavy, with single moderately well-defined, prominent, lenticular, slightly protruding spur. Three spine articulations similar to those observed in holotype; ventral spine articulation smaller than remaining two; dorsal gap slightly larger than ventral one. Ventral edge of LAP with very small, almost indiscernible tentacle notch.

Inner side of LAP with sharply defined, prominent ridge displaying oblique, near-straight, narrow dorsal part and shorter, strongly widened ventral part with ventro-proximalwards pointing tip; inner side of tentacle notch small, sharply defined laterally. No perforations or furrow discernible.

GZG.INV.78657 is an articulated arm fragment preserving five median to proximal segments; LAPs well in agreement with holotype; position of dorsalmost spine articulation alternating between close to the remaining ones, close to the dorsal edge of the LAP, and intermediate between the latter two positions; a few arm spine fragments preserved, broken near base, exposing lumen filled with coarsely meshed stereom; arm spines cylindrical, with finely meshed and slightly longitudinally elongate stereom; dorsal arm plates missing; LAPs not in contact dorsally; ventral arm plates large, nearly as long as wide, widest distally, not separating LAPs; distal edge of ventral arm plates convex but with slightly truncated, straight tip; lateral edges moderately strongly concave; proximal angle acute; no tentacle scales discernible.

GZG.INV.78658 is an articulated arm fragment preserving four distal segments; several arm spine fragments preserved; most complete arm spine very large, nearly as long as two arm segments, cylindrical, straight, slowly tapering, with finely meshed, slightly longitudinally elongate stereom beset with numerous minute thorns pointing towards the spine tip; dorsal arm plates leaf shaped, longer than wide, with very acute proximal angle, slightly convex lateral edges and convex distal edge; outer surface of dorsal arm plates with very fine, regular transverse striation; ventral arm plates similar to those observed on other paratype arm fragment, slightly longer than wide; tentacle pores very small, single small, leaf-like tentacle scale preserved.

Remarks

This material is unequivocally assignable to the genus Alternacantha on account of the alternating position of the dorsalmost spine articulation. It differs from its congeners in combining five arm spines, a well-developed vertical striation on the outer surface, a slightly truncated distal edge of the ventral arm plates, a parabolic distal edge of the dorsal arm plates, and, most importantly, large, well-defined, thin, horizontally strongly elongate spurs on the inner side of the distal edge of the LAPs. The arm fragments described and illustrated by de Loriol (1872) as a new species, Ophiurella royeri de Loriol, 1872 , from near-coeval strata of Haute-Marne, France, could well be conspecific with the present material. Since, however, Ophiurella royeri is based on arm fragments which do not expose the inner side of the LAPs and in particular the highly diagnostic shape of the spurs on the inner distal edge, it cannot be ruled out that they are rather conspecific with the specimens described below as Alternacantha dilionessa sp. nov. Ophiurella royeri should therefore be considered as a nomen dubium.

Occurrence

Late Oxfordian of France.

Figures

Fig. 23. Fossil lateral arm plates in external (a) and internal (b) views and articulated arm fragments of Alternacantha schwermannorum sp. nov. from the late Oxfordian (Late Jurassic) of Savigna, France. 1. GZG.INV.78654 (holotype), proximal LAP. 2. GZG.INV.78655 (paratype), median LAP. 3. GZG. INV.78656 (paratype), distal LAP. 4. GZG.INV.78657 (paratype), proximal arm fragment in ventral (a) and dorsal (b) views and with detail in dorso-lateral (c) view. 5. GZG.INV.78658 (paratype), distal arm fragment in dorsal (a) and ventral (b) view. One common scale bar for 1-3.