Ophiocamax dorotheae, 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 : 199-203

Ophiocamax dorotheae sp. nov.

urn:lsid:zoobank.org:act:412FC94A-B8E6-48B2-915D-E719B0AD10CC

Fig. 35 : 7-11

Ophiacantha? suprajurassica Hess, 1965a: 1065, 1077 .

Ophiacantha? sp. or Ophiothrix ? sp. – Hess 1960: 419, fig. 39.

Ophiacantha? suprajurassica – Hess 1966: 1030, 1054, figs 76-77 (material incorrectly assigned to Ophiacantha? suprajurassica Hess, 1965 ).

Diagnosis

Species of Ophiocamax with large LAPs displaying large, well-defined, strongly protruding spur on the outer proximal edge paralleled by a large, non-prominent, rounded area of densely meshed stereom on the inner distal edge; up to five spine articulations, two dorsalmost of which always largest; ridge on inner side of LAP relatively short, with strongly widened dorsal tip; dorsal arm plates irregularly pentagonal to blazon-shaped, with small granules or thorns scattered on outer surface.

Etymology

Species named in honour of Dorothea Hause-Reitner for her friendship and generous assistance in scanning electron microscopy.

Type material

Holotype GZG.INV.78763 .

Paratypes

GZG.INV.78764, GZG.INV.78765, GZG.INV.78766 and GZG.INV.78767.

Type locality and horizon

Savigna, France; sample S2b of Gale (2011), Bimammatum Zone, late Oxfordian, Late Jurassic.

Additional material

GZG.INV.78768 (29 dissociated LAPs) from sample S2b of Gale (2011), GZG.INV.78769 (6 dissociated LAPs) from sample S1 of Gale (2011), GZG.INV.78770 (8 dissociated LAPs) from sample S2a of Gale (2011), all from the Bifurcatus Zone, late Oxfordian of Savigna, France; dissociated LAP from the Bifurcatus Zone, late Oxfordian of Guldental, Switzerland, the original material of Hess (1966); 26 dissociated LAPs from the Renggeri Member, early Oxfordian of Chapois, France, the original material of Hess (1965a); 2 dissociated LAPs from the Renggeri Member of Longecombe, France, the original material of Hess (1965a).

Description

Holotype

GZG.INV.78763 is a dissociated, large, proximal LAP; nearly as high as wide; dorsal edge strongly concave as a result of a well-developed constriction; distal edge strongly convex; proximal edge irregularly undulose, with a large, sharply defined, prominent and strongly protruding, oval, horizontally elongate spur composed of densely meshed stereom; weakly defined, shallow depression in ventral half of proximal edge, composed of more finely meshed stereom; outer surface with finely meshed stereom, fining towards proximal edge. Five large ear-shaped spine articulations freestanding on strongly elevated distal half of LAP; strong dorsalward increase in size of spine articulations and of gaps separating them; ventral and dorsal lobes merged into continuous volute; dorsal lobe with conspicuous central notch; gap between spine articulations and distal edge of LAP as wide as one spine articulation, strongly increasing in width dorsalwards, parallel to dorsalward increase in size of spine articulations; dorsalmost spine articulation below dorsal edge of LAP, pointing dorsalwards; spine articulations proximally sharply bordered by undulose edge of elevated distal half of LAP. Ventral edge of LAP nearly straight, with very small, weakly concave, almost indiscernible tentacle notch.

Inner side of LAP with very large, broad, sharply defined, prominent ridge with evenly bent, ventroproximally pointing ventral part, and strongly widened, near-triangular dorsal part; inner side of distal edge of LAP with large, well-defined, rounded, non-prominent spur composed of densely meshed stereom; inner side of tentacle notch very small, semi-circular, well defined laterally. No perforations or furrow discernible.

Paratype supplements and variation

GZG.INV.78764 is a dissociated median LAP; slightly wider than high; very well in agreement with holotype; dorsal edge less strongly concave; spur on proximal edge slightly smaller than in holotype. Five spine articulations similar to those of holotype; gap between spine articulations and distal edge of LAP slightly narrower. Ventral edge of LAP weakly concave, with very small, almost indiscernible tentacle notch.

Inner side of LAP with very large, well-defined, short ridge with strongly widened ventral and dorsal edges; inner side of tentacle notch very small, sharply defined, semi circular.

GZG.INV.78765 is a dissociated distal LAP; almost twice wider than high; very well in agreement with holotype; depression in ventral half of outer surface smaller and even less well defined than in holotype, almost indiscernible. Four spine articulations similar to those of holotype; dorsalmost spine articulation not pointing dorsalwards; gap between spine articulations and distal edge of LAP much narrower. Ventral edge of LAP gently concave; tentacle notch invisible in external view.

Inner side of LAP with large, poorly defined, broad, weakly prominent ridge displaying strongly widened ventral part and dorso-proximalwards pointing dorsal tip; spur on inner side of distal edge slightly less well defined; inner side of tentacle notch very small, more than semi circular.

GZG.INV.78766 is an articulated arm fragment preserving two median segments; LAPs well in agreement with dissociated ones described above; dorsal arm plates large, widely separated by LAPs, slightly longer than wide, widest distally, irregularly pentagonal to blazon shaped, with gently convex distal edge, slightly proximalwards converging lateral edges and nearly right proximal angle; outer surface of dorsal arm plates with small, scattered granules or thorns; ventral arm plates widely separated by LAPs; slighty wider than long, widest distally, with gently convex to slightly angular distal edge, short, straight lateral edges, small half-circular tentacle notches and obtuse proximal angle; few very short arm fragments preserved, originally cylindrical, with moderately coarsely meshed outer surface stereom, no further details preserved.

GZG.INV.78767 is an articulated arm fragment preserving three median to distal segments; one second ventralmost arm spine nearly completely preserved, straight, cylindrical, almost as long as one arm segment, with moderately coarsely meshed and slightly longitudinally elongate stereom.

Remarks

The highly distinctive shape and position of the spine articulations, combined with the lack of a conspicuous outer surface ornament, the distinctively shaped ridge on the inner side and the very small, well-defined, semi-circular tentacle notch unambiguously place these LAPs in the genus Ophiocamax or at least in a very closely related form (more closely related than to Ophiomitra , on account of the much closer similarities in LAP morphology). The low number of spine articulations and the very weakly ventralwards protruding row of spine articulations precludes assignment to the closely related Late Jurassic Ophiosternle crinitum (Quenstedt, 1876) . The present LAPs are by far the oldest record that is assignable to Ophiocamax , predating the Miocene records from Japan ( Ishida 2001) and the Grenadines (Jagt et al. in press).

The present LAPs differ in that the two dorsalmost spine articulations invariably are the largest, rather than much smaller than the remaining ones as in the LAPs of most extant species of Ophiocamax . Greatest similarities are shared with the LAPs of extant O. vitrea , especially on account of the shape and position of the spine articulations, the presence of a well-developed spur on the outer proximal and inner distal edge, and the development of the tentacle notch. In addition, the shape and outer surface ornament of the dorsal arm plates is markedly similar in O. vitrea and these fossil specimens. The shape of the ridge on the inner side, in contrast, is more reminiscent of that observed in the LAPs of O. austera . In the light of these minor, yet significant, morphological differences and the huge stratigraphic gap, the fossil LAPs are here described as a new species.

Occurrence

Early to late Oxfordian of France, late Oxfordian of Switzerland.

Figures

Fig. 35. Skeletal plates and arm fragments of fossil and Recent ophiacanthid brittle stars; lateral arm plates (LAPs) in external (a) and internal (b) views. 1-3. Ophiocamax vitrea Lyman, 1878, Recent. 1. Proximal LAP. 2. Median LAP. 3. Dorsal arm plate. 4-5. Ophiocamax hystrix Lyman, 1878, Recent. 4. Proximal LAP. 5. Median LAP. 6. Ophiocamax austera Verrill, 1899, Recent; proximal LAP. 7-11. Ophiocamax dorotheae sp. nov. from the late Oxfordian (Late Jurassic) of Savigna, France. 7. GZG.INV.78763 (holotype), proximal LAP. 8. GZG.INV.78764 (paratype), median LAP. 9. GZG.INV.78765 (paratype), distal LAP. 10. GZG.INV.78766 (paratype), proximal arm fragment in ventral view. 11. GZG.INV.78767 (paratype), median arm fragment in dorsal view. One common scale bar per species except for 10 and 11.