Ophiotreta stefaniae, 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 : 84-87

Ophiotreta stefaniae sp. nov.

urn:lsid:zoobank.org:act:91A11ADF-3122-45D4-856B-A42B1C989127

Fig. 16 : 7-10

Diagnosis

Species of Ophiotreta with large LAPs displaying a relatively small, protruding, pointed spur on the outer proximal edge, paralleled by a large, round, slightly prominent but not protruding spur on the inner distal edge composed of densely meshed stereom; up to six nearly equal-sized spine articulations, all with well-developed connecting ridge between ventral lobe and strongly undulose distalmost lamella of outer surface striation; ridge on inner side composed of three parts, the ventralmost of which relatively short, straight, oblique, pointing ventro-proximalwards; no knob on the ventral portion of the inner side.

Etymology

Species named in honour of my friend Steffi Lutz, in memory of the superb years spent together in Tübingen, not far from the type locality of the species.

Type material

Holotype GZG.INV.78583 .

Paratypes

GZG.INV.78584, GZG.INV.78585 and GZG.INV.78586.

Type locality and horizon

Plettenberg near Balingen, southern Germany; clay pockets between sponge associations in the lowest bed exposed in the quarry, Bimammatum Zone, late Oxfordian, Late Jurassic.

Additional material

GZG.INV.78587 (53 dissociated LAPs).

Description

Holotype

GZG.INV.78583 is a dissociated, large, proximal LAP; more than twice higher than wide; dorsal edge slightly concave as a result of a well-developed constriction; distal edge evenly convex; proximal edge concave, with small, poorly defined, slightly prominent and protruding spur in the middle; ventral half of proximal edge slightly fragmentary; outer surface with rather irregular vertical striation composed of broad lamellae separated by single very thin lines of stereom pores and covering the proximal edge of the strongly elevated distal portion of the LAP and the ventral portion of the outer surface; vertical striation replaced by densely to finely meshed stereom in other parts of the outer surface. Six large, earshaped spine articulations freestanding on strongly elevated distal portion of LAP, proximally sharply bordered by undulose distalmost lamella; dorsal lobe of spine articulations very large, separated from smaller ventral lobe by very small, shallow notch; short but well-developed and prominent connecting ridge between distalmost lamella and ventral lobe in every spine articulation; very weak dorsalward increase in size of spine articulations and of gaps separating them. Ventral edge of LAP irregularly convex, tentacle notch not visible in external view.

Inner side of LAP with large, sharply defined, prominent and relatively narrow ridge separated by two angular kinks into long, straight and near-vertical dorsal part close to the proximal edge of the LAP, a much shorter, oblique, straight central part, and a similarly short slightly bent, oblique ventral part; inner side of distal edge of LAP with large, oval, moderately well-defined, prominent spur composed of densely meshed stereom; inner side of tentacle notch moderately large, with coarsely meshed stereom. Vertical row of small, irregular perforations in shallow, poorly defined furrow dorsally bordering tentacle notch.

Paratype supplements and variation

GZG.INV.78584 is a dissociated proximal LAP; ventro-proximal portion missing; generally very well in agreement with holotype; spur on proximal edge better preserved, strongly protruding, pointed. Six spine articulations similar to those observed in holotype.

Inner side of LAP with dorsal and central parts of ridge very well preserved, similar to those observed in holotype except for ventro-proximally pointing angle in kink separating them; inner side of distal edge of LAP with two moderately large, well-defined, prominent spurs composed of densely meshed stereom.

GZG.INV.78585 is a dissociated median LAP; nearly as high as wide; dorsal edge strongly concave as a result of a strong constriction; proximal edge with moderately large; rather poorly defined, weakly prominent but protruding and pointed spur. Five spine articulations similar to those observed in holotype; weak dorsalward increase in size of spine articulations and of gaps separating them but dorsalmost spine articulation as small as ventralmost one. Ventral edge irregularly convex with relatively small, concave tentacle notch near ventralmost spine articulation.

Inner side of LAP with well-defined ridge separated by kink into long, straight, oblique dorsal part with widened, poorly defined dorsal tip, and much shorter oblique ventral part with well-defined ventral tip; inner side of distal edge of LAP with large, round, moderately well-defined and weakly prominent spur composed of densely meshed stereom. Three irregular perforations in vertical row dorsally bordering inner side of tentacle notch.

GZG.INV.78586 is a dissociated distal LAP; approximately 1.5 times wider than high; dorsal edge clearly concave as a result of a strong constriction; proximal edge with moderately large; rather poorly defined, weakly prominent, protruding and pointed spur. Five spine articulations; ventral lobe of spine articulations thickened into small, round knob, connected with the distalmost lamella of the outer surface striation by a very short, moderately prominent connecting ridge; weak dorsalward increase in size of spine articulations and of gaps separating them but dorsalmost spine articulation as small as ventralmost one.

Inner side of LAP with sharply defined, straight, oblique ridge with widened, rounded and well-defined dorsal tip, and slightly widened, ventrally pointing ventral tip. Single moderately large perforation dorsally bordering inner side of tentacle notch.

Remarks

These LAPs display the highly distinctive combination of spine articulations with a well-developed connecting ridge with the undulose distalmost lamella, and the ridge on the inner side composed of a long, near-vertical dorsal part and a shorter oblique ventral part separated from the dorsal one by a kink with a ventro-proximally pointing angle, exclusively found in the LAPs of Ophiotreta and Ophiopristis . In contrast to the LAPs of most extant species of these two genera, however, these fossil ones display a third part of the ridge in addition to the usual dorsal and ventral ones, pointing ventro-proximally. Whether this difference in ridge morphology warrants separation at the generic level is questionable. It seems highly probably, however, that the third, ventralmost part of the ridge in the present fossil LAP is the equivalent of the knob observed on the ventral portion of the LAPs in other species of Ophiotreta and Ophiopristis .

Awaiting the discovery of articulated specimens that can supply additional grounds for separation at the generic level, the present specimens are assigned to Ophiotreta (see above for a detailed discussion on a morphological analysis of LAPs of Ophiotreta and Ophiopristis ). Confusion with other fossil LAPs assigned to this genus can be ruled out on account of the protruding, pointed spur on the outer proximal edge of these LAPs.

Occurrence

Late Oxfordian of Germany.

Figures

Fig. 16. Skeletal plates of fossil and Recent ophiacanthid brittle stars; lateral arm plates (LAPs) in external (a) and internal (b) views. 1-3. Ophiotreta durbanensis (Mortensen, 1933), Recent. 1. Proximal LAP. 2. Distal LAP. 3. Arm spine. 4. Ophiotreta valenciennesi (Lyman, 1879), Recent; proximal to median LAP. 5-6. Ophiopristis procera (Koehler, 1904), Recent. 5. Proximal LAP. 6. Distal LAP. 7-10. Ophiotreta stefaniae sp. nov. from the late Oxfordian (Late Jurassic) of the Plettenberg, Germany. 7. GZG.INV.78583 (holotype), proximal LAP. 8. GZG.INV.78584 (paratype), proximal LAP. 9. GZG. INV.78585 (paratype), median LAP. 10. GZG.INV.78586 (paratype), distal LAP. One common scale bar per species is given, except for 3.