Ophiomitrella, Verrill, 1899

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 : 178-179

Ophiomitrella ? sp. 1

Fig. 31 : 10-12

Material examined

GZG.INV.78735, GZG.INV.78736, GZG.INV.78737 and GZG.INV.78738 (8 dissociated LAPs) from the Callovian of Bauer-Wehrland, Germany; GZG.INV.78739 (1 dissociated LAP) from the Callovian of Althüttendorf, Germany.

Description

GZG.INV.78735 is a dissociated, small, median LAP; slightly higher than wide; dorsal edge straight; distal edge convex; proximal edge concave, devoid of spurs; distal half of outer surface with densely meshed stereom; trabeculae of densely meshed stereom merged into few, thick, irregular vertical ridges; proximal half with moderately coarsely meshed stereom. Five moderately large, nearly equal-sized, equi- distant, ear-shaped spine articulations freestanding on elevated distal portion of LAP; spine articulations proximally sharply bordered by near-straight, prominent, strongly thickened ridge; gap between spine articulations and distal edge of LAP very narrow; ventral and dorsal lobes of spine articulations merged into continuous volute but with very gentle kink proximally resulting in slightly oval spine articulations. Ventral edge of LAP nearly straight; tentacle notch invisible in external view. Row of spine articulations slightly protruding ventralwards.

Inner side of LAP with rather inconspicuous, well defined, prominent, oblique ridge with slightly widened, ventro-proximalwards pointing ventral tip, widened dorsal tip with ventralward projecting extension; inner side of distal edge of LAP with poorly defined, round area of slightly more densely meshed stereom; inner side of tentacle notch relatively small. Irregular row of perforations dorsally bordering tentacle notch; ventralmost perforation large, horizontally elongate, irregular.

GZG.INV.78736 is a dissociated median LAP; slightly higher than wide; well in agreement with the LAP described above; outer surface slightly less well preserved, with finely meshed stereom probably originally covered by coarsely meshed stereom. Five spine articulations similar to those observed on previous LAP; proximally bordered by slightly undulose, sharply defined, thick, prominent ridge.

Ridge on inner side of LAP similar to that of previous specimens, slightly better preserved; no area of slightly more densely meshed stereom on inner side of distal edge of LAP; inner side of tentacle notch smaller. Perforations on inner side loosely arranged in vertical row dorsally bordring tentacle notch; nearly of equal size.

GZG.INV.78737 is a dissociated proximal LAP fragment; originally clearly higher than wide; all edges broken; outer surface with coarsely and densely meshed stereom. At least five spine articulations similar to those observed on specimens described above; proximally bordered by sharply defined, prominent, broad ridge.

Inner side poorly preserved; shape of oblique, rather broad ridge not discernible; inner side of distal edge of LAP with moderately large, poorly defined, round area of slightly more densely meshed stereom. Irregular vertical row of moderately large, poorly preserved perforations.

Remarks

Although the material described above is scanty, it does display a combination of characters which is reminiscent of at least ome of the extant species of Ophiomitrella . Strong similarities are also shared with the LAPs of Ophiacantha , especially on account of the shape of the ridge on the inner side and the absence of well-developed spurs on the outer proximal edge. The unusually broad, sharply defined and straight ridge proximally bordering the spine articulations, however, is more typical of Ophiomitrella . Thus, pending a revision of the highly heterogeneous genera Ophiacantha and Ophiomitrella , the LAPs described above are here considered as a possible, indeterminate fossil record of Ophiomitrella .

Figures

Fig. 31. Lateral armplates (LAPs) of fossil and Recent ophiacanthidbrittle stars in external (a) and internal (b) views. 1-4. Ishidacantha fuersichi gen. et sp. nov. from the Callovian (Middle Jurassic) of Jumara, India. 1. GZG.INV.78728 (holotype), proximal LAP. 2. GZG.INV.78729 (paratype), proximal LAP. 3. GZG.INV.78730 (paratype), median LAP. 4. GZG.INV.78731 (paratype), distal LAP.5-6. Ishidacantha trispinosa (Hess, 1965) comb. nov. from the early Oxfordian (Late Jurassic) of Longecombe, France. 5. NHMB M11222, proximal LAP. 6. NHMB M11223, distal LAP. 7-9. Ophiomitrella conferta (Koehler, 1922), Recent. 7. Proximal LAP. 8. median LAP. 9. distal LAP. 10-12. Ophiomitrella? sp. 1 from the Callovian of Bauer-Wehrland, Germany. 10. GZG.INV.78735, proximal LAP. 11. GZG.INV.78736, median LAP. 12. GZG.INV.78737, distal LAP. One common scale bar per species.