Ophiotreta striata ( Kutscher & Jagt, 2000 ) 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 : 88-90

Ophiotreta striata ( Kutscher & Jagt, 2000) comb. nov.

Fig. 17 : 1-2

Ophiacantha? striata Kutscher & Jagt, 2000: 64 , pl. 25 figs 8-10.

non Ophiacantha? aff. striata – Štorc & Žítt 2008: 128, fig. 4L-M.

Diagnosis

Species of Ophiotreta with large LAPs, displaying up to ten nearly equal-sized spine articulations; very weak connecting ridge between ventral lobe of spine articulations and distalmost lamella of outer surface striation in all LAPs, including the median and distal ones; no spurs on outer proximal and inner distal LAPs.

Material examined

GZG.INV.78588, GZG.INV.78589 and 216 dissociated LAPs from the Manfred Kutscher Collection in Sassnitz, Germany, the type material of Kutscher & Jagt (2000); 2 dissociated LAPs from the early Maastrichtian of Lägerdorf-Kronsmoor, Germany.

Description

Dissociated moderately large LAPs, more than twice higher than wide (proximal ones) to slightly wider than high (distal LAPs); dorsal edge mostly straight to slightly concave as a result of a weak constriction; distal edge convex; proximal edge irregularly undulose, devoid of spurs; outer surface with vertical striation composed of broad, slightly overlapping lamellae restricted to the proximal edge of the strongly elevated distal portion of proximal LAPs, extending into middle of outer surface in median to distal LAPs; lamellae becoming finer proximally and replaced by moderately finely meshed stereom towards proximal edge of LAP. Ten (proximal LAPs) to five (distal LAPs) large, ear-shaped spine articulations on strongly elevated distal portion of LAPs; dorsal lobe of spine articulations very large, separated from smaller ventral lobe by small, moderately deep notch; spine articulations proximally sharply delimited by thin, slightly undulose distalmost lamella; very weakly defined, slightly prominent connecting ridge between ventral lobes of spine articulations and distalmost lamella; very weak dorsalward increase in size of spine articulations and of gaps separating them. Ventral edge of LAP straight to slightly convex; tentacle notch generally invisible in external view.

Inner side of LAPs with large, conspicuous, sharply defined yet narrow ridge composed of very long, straight, near-vertical dorsal portion very close to the proximal edge of the lap; and shorter and slightly wider, straight, oblique ventral portion with slightly more prominent rounded ventral tip; ventral and dorsal portions of ridge separated by sharp kink with ventro-proximally pointing angle; small, round, sharply defined, prominent knob on ventral portion of inner side, widely separated from ridge; inner side of distal edge of LAP devoid of spurs; inner side of tentacle notch moderately large. Vertical row of very small, seemingly irregularly spaced perforations in very shallow, poorly defined furrow dorsally bordering inner side of tentacle notch.

Remarks

The dissociated LAPs from the early Maastrichtian of Germany and Denmark, on the basis of which Kutscher & Jagt (2000) described the new species, Ophiacantha ? striata , are unequivocally assignable to the Ophiotreta - Ophiopristis group mainly on account of the highly distinctive shape of the ridge on the inner side. The connecting ridges between the ventral lobe of the spine articulations and the distalmost lamella of the outer surface striation are weakly developed in this species, which makes assignment to Ophiopristis equally probable. It is of prime importance here to recall that the LAP morphologies of Ophiopristis and Ophiotreta are almost indistinguishable. In view of the fact that a species can only be assigned to a single genus, Ophiotreta is preferred here, for reasons outlined above in the discussion of the genus. This generic placement is likely to change as soon as more will be known of the general skeletal morphology of Ophiotreta striata comb. nov., but also following a systematic reappraisal of extant species of Ophiotreta and Ophiopristis .

Among the fossil LAPs assigned to Ophiotreta , O. striata comb. nov. is unambiguously characterised by the lack of spurs on the outer proximal edge and the high number of spine articulations. The closest resemblance is with Ophiotreta dendrophyllicola sp. nov. in which, however, the median and distal LAPs display a small, protruding spur on the outer proximal edge, and better-developed connecting ridges between the spine articulations and the outer surface. The LAPs from the Turonian of the Czech Republic described as Ophiacantha ? aff. striata by Štorc & Žítt (2008) are most probably assignable to Ophiotreta . Their slightly better-developed connecting ridge suggests that they belong to an undescribed species close to O. striata comb. nov.

Occurrence

Early Maastrichtian of Germany and Denmark.

Figures

Fig. 17. Fossil skeletal plates of ophiacanthid brittle stars; lateral arm plates (LAPs) in external (a) and internal (b) views. 1-2. Ophiotreta striata (Kutscher & Jagt, 2000) comb. nov. from the early Maastrichtian of Rügen, Germany. 1. GZG.INV.78588, proximal LAP. 2. GZG.INV.78589, distal LAP. 3-11. Ophiotreta dendrophyllicola sp. nov. from the middle Danian (Paleocene) of Fakse, Denmark. 3. MGUH 30236 (holotype), proximal LAP. 4. NHMM 2012 050 (paratype), median LAP. 5. NHMM 2012 051 (paratype), distal LAP. 6. NHMM 2012 052 (paratype), vertebra in distal view. 7. NHMM 2012 053 (paratype), vertebra in proximal view. 8. NHMM 2012 054 (paratype), vertebra in dorsal view. 9. NHMM 2012 055 (paratype), oral plate in adradial (a) and abradial (b) view. 10. NHMM 2012 056 (paratype), arm spine fragment. 11. NHMM 2012 057 (paratype), arm spine fragment. 12-15. Ophiotreta hedone sp. nov. from the middle Lutetian (Eocene) of Grignon, France. 12. GZG.INV.78590 (holotype), proximal LAP. 13. GZG.INV.78591 (paratype), median LAP. 14. GZG.INV.78592 (paratype), distal LAP. 15. GZG.INV.78593 (paratype), median LAP. One common scale bar per species.