Ophiacantha, Muller & Troschel, 1842

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 : 101-102

Ophiacantha sp. nov. innom. 1

Fig. 18 : 8-9

Material examined

GZG.INV.78599 and GZG.INV.78600 from the latest Aptian to earliest Albian of Blake Nose, tropical northeast Atlantic.

Description

GZG.INV.78599 is a fragment of a dissociated, medium-sized, proximal LAP of which only dorso-distal portion is preserved; LAP fragile; dorsal edge slightly concave; distal edge convex; preserved portion of proximal edge slightly undulose, devoid of spurs; outer surface with very narrow band of welldeveloped, vertical striation composed of a few fine, overlapping lamellae close to spine articulations; remaining outer surface with finely meshed stereom. Three large, ear-shaped spine articulations preserved, freestanding on strongly elevated distal portion of LAP; dorsal and ventral lobes of spine articulations merged into continuous, round volute; spine articulations proximally sharply bordered by strongly undulose distalmost lamella; gap between spine articulations and distal edge of LAP slightly narrower than one spine articulation.

Inner side of LAP with dorsal tip of ridge, sharply defined, prominent, dorsally pointed, with ventroproximally pointing extension; possible large, poorly defined, non-prominent spur on preserved portion of inner distal LAP edge and composed of slightly more densely meshed stereom; no perforations discernible.

GZG.INV.78600 is a dissociated distal LAP; very fragile, dorsal and ventral edges fragmentary; very well in agreement with proximal LAP described above; distal edge irregularly convex; proximal edge poorly preserved, irregularly undulose, no spur discernible. Four spine articulations similar to those observed in proximal LAP described above. Ventral edge of LAP poorly preserved.

Inner side of LAP with large, conspicuous, sharply defined and prominent, straight, oblique ridge with strongly widened, near-triangular dorsal tip displaying ventro-proximally pointing extension; ventral tip of ridge strongly widened; inner side of distal edge of LAP with large, poorly defined, weakly prominent spur composed of more densely meshed stereom; inner side of tentacle notch relatively small and moderately well defined laterally. No perforations discernible.

Remarks

Although this LAP type is represented by very limited material only, it displays sufficient morphological detail to argue in favour of an assignment to Ophiacantha , based mainly on the shape of the ridge in the distal LAP as well as the round, continuous volute of the large, freestanding spine articulations. In the absence of more complete material, however, this generic placement must be treated with caution. The very fragile nature of the LAPs, the very large, near-circular spine articulations, and the narrow band of very fine vertical striation distally bordered by finely meshed stereom are not found in any other fossil LAP type currently assigned to Ophiacantha . This LAP type most probably represents a new species, the description of which, however, should be based on more complete material.

Figures

Fig. 18. Fossil lateral arm plates (LAPs) of ophiacanthid brittle stars in external (a) and internal (b) views. 1-2. Ophiotreta sp. nov. innom 2 from the Langhian (middle Miocene) of Stotzing (1) and Mannersdorf (2), Austria. 1. NHMW 2012/0139/0003, proximal LAP. 2. NHMW 2012/0139/0001, proximal LAP. 3-4. Europacantha paciphila gen. et sp. nov. from the late Anisian (Middle Triassic) of Felsöörs, Hungary. 3. MHI 2090/1 (holotype), proximal to median LAP. 4. MHI 2091/1 (paratype), proximal LAP. 5-7. Ophiacantha jaegeri sp. nov. from the latest Hauterivian (Early Cretaceous) of Sarstedt, Germany. 5. GZG.INV.78595 (holotype), proximal LAP. 6. GZG.INV.78596 (paratype), median LAP. 7. GZG. INV.78597 (paratype), distal LAP. 8-9. Ophiacantha sp. nov. innom 1 from the latest Aptian to earliest Albian (Early Cretaceous) of Blake Nose, NE Atlantic. 8. GZG.INV.78599, proximal LAP. 9. GZG. INV.78600, distal LAP. 10. Ophiacantha sp. nov. innom 2 from the latest Aptian to earliest Albian (Early Cretaceous) of Blake Nose, NE Atlantic; GZG.INV.78601, distal LAP. One common scale bar per species.