Ophiacantha steffenschneideri Thuy 2013, sp. nov.

Ophiacantha steffenschneideri sp. nov.

urn:lsid:zoobank.org:act: 4FB6425F-54A5-4285-88EE-8FD4911F63AB

Fig. 19: 6-9

Diagnosis

Species of Ophiacantha with relatively large LAPs displaying a well-developed vertical striation confined to a very narrow band close to the spine articulations; up to seven relatively small spine articulations; well-developed spur on the outer proximal and inner distal edges.

Etymology

Species named in honour of Steffen Schneider, who kindly provided the type material of the species.

Type material

Holotype GZG.INV.78609.

Paratypes

GZG.INV.78610, GZG.INV.78611 and GZG.INV.78612.

Type locality and horizon

Bad Freienwalde, Germany; “Septarienton”, Rupelian, Oligocene.

Additional material

GZG.INV.78613 (710 dissociated LAPs); GZG.INV.78614 (11 dissociated arm spines).

Description

Holotype

GZG.INV.78609 is a dissociated, large, proximal LAP; approximately 1.5 times higher than wide; dorsal edge strongly concave as a result of a well-developed constriction; distal edge convex; proximal edge irregularly undulose, with small, moderately well-defined, slightly prominent and protruding, angular spur; outer surface with very narrow band of a well-developed, regular, vertical striation composed of very few fine lamellae proximally bordering spine articulations; lamellae replaced by moderately finely meshed stereom on almost entire outer surface; trabecular intersections thickened into small granules on ventral sixth of outer surface. Seven relatively small,<ear-shaped spine articulations freestanding on strongly elevated distal portion of LAP; dorsal and ventral lobes of spine articulation merged into continuous, round volute; spine articulations proximally sharply bordered by distalmost lamella, becoming increasingly undulose dorsalwards; spine articulations nearly equal-sized but gap separating them with dorsalward increase in size; gap between spine articulations at least as wide as one spine articulation. Ventral edge of LAP nearly straight except for protruding row of spine articulations; tentacle notch invisible in external view.

Inner side of LAP with very large, conspicuous, sharply defined and prominent ridge; main part of ridge nearly straight, oblique, dorsally strongly widened and merged with nearly vertical ventrally pointing extension close to proximal edge and almost as long as main part of ridge; ventral part of ridge with slightly thickened kink and short proximally pointing part; inner side of distal edge of LAP with small, round, well-defined, weakly prominent spur composed of densely meshed stereom; inner side of tentacle notch poorly defined, small, rather inconspicuous. Small perforations, round to vertically elongate, in irregular vertical row dorsally bordering tentacle notch.

Paratype supplements and variation

GZG.INV.78610 is a dissociated median LAP; slightly wider than high; agreeing well with holotype; spur on proximal edge much better defined and more strongly protruding. Five spine articulations similar to those observed on holotype. Ventral edge of LAP concave; tentacle notch invisible in external view.

Inner side of LAP with conspicuous ridge similar to that observed on holotype; spur on inner side of distal edge of LAP better defined. Two small, well-defined perforations dorsally bordering inner side of tentacle notch.

GZG.INV.78611 is a dissociated distal LAP; almost twice wider than high; very well in agreement with holotype; spur on proximal edge better defined and more strongly protruding. Five spine articulations on strongly elevated distal portion of LAP, slightly more horizontally elongate than those of holotype; dorsal and ventral lobes merged into continuous volute but with small, shallow notch proximally. Ventral edge strongly concave; tentacle notch invisible in external view.

Inner side of LAP with large, well-defined, prominent ridge; dorsal part of ridge similar to that observed on holotype; ventral part of ridge distally rather than proximally pointing and longer than dorsal part; inner side of distal edge of LAP with relatively large, poorly defined, oblique spur composed of more densely meshed stereom; inner side of tentacle notch small, oblique, moderately well defined. Three relatively large, irregular perforations in oblique row dorsally bordering inner side of tentacle notch.

GZG.INV.78612 is a dissociated arm spine; cylindrical, slowly tapering; few coarse, regular longitudinal ribs irregularly beset with minute granules, separated by narrow rows of pores with occasional short thorns pointing towards the apex of the spine; tip of spine broken.

Remarks

These LAPs fit the LAP morphology diagnosis of Ophiacantha in its proper sense so markedly well that assignment to that genus is unquestionable. The LAPs in question clearly differ from all other fossil LAP types assigned to Ophiacantha in combining a fine vertical striation, which is confined to a very narrow band of the outer surface, with relatively small spine articulations and the well-developed (at least in median and distal LAPs) spur on the outer proximal and inner distal edges. It is thus described here as a new species.

This LAP type bears striking similarities to the LAPs of extant Ophiacantha bidentata and its morphologically closely similar congener O. fraterna Verrill, 1885 (Martynov & Litvinova 2008). In terms of the shape of the ridge on the inner side, the size of the spine articulations and the presence of a spur on the outer proximal and inner distal edges, the Oligocene species is closer to O. fraterna. The slightly better-developed constriction and the more strongly undulose distalmost lamella proximally bordering the spine articulations, in contrast, place the Oligocene species closer to O. bidentata. These striking similarities in LAP morphology suggest that the Oligocene taxon from the North Sea Basin shares close phylogenetic ties with the North Atlantic O. bidentata and O. fraterna. Interestingly, the latter two are morphologically so similar that they were long considered to be conspecific (Martynov & Litvinova 2008), implying a recent divergence date and/or very little morphological differentiation following divergence. In this light, Ophiacantha steffenschneideri sp. nov. would be a plausible candidate for an ancestor of O. bidentata and O. fraterna. However, such a claim cannot, of course, be substantiated on the basis of LAP morphology alone.

Occurrence

Oligocene of Germany.