Krohcoma, 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 : 80-81

Krohcoma sp. nov. innom.

Fig. 15 : 7-8

Material examined

GZG.INV.78574, GZG.INV.78575 and GZG.INV.78576 (dissociated LAP) from the Patcham Formation, late Bathonian, of Jumara, India.

Description of proximal LAP

GZG.INV.78574 is a dissociated relatively small, proximal LAP; slightly higher than wide; dorsal edge strongly concave as a result of a well-developed constriction; distal edge obliquely convex; proximal edge irregularly concave, devoid of spurs or thickened parts; outer surface with moderately finely meshed stereom, no conspicuous ornament elements discernible; six very large, ear-shaped to horseshoeshaped spine articulations in continuous, dorso-proximalwards conspicuously receding row on strongly elevated distal portion of LAP; dorsal and ventral lobes of spine articulations thick, proximally merged by relatively thin connection into continuous, horizontally slightly elongate volute; dorsalward increase in size of spine articulations and of gaps separating them; row of spine articulations protruding ventrally. Ventral edge of LAP slightly concave, tentacle notch not visible in external view.

Inner side of LAP largely obscured by sediment; ridge not clearly discernible; inner side of distal edge of LAP devoid of spurs; inner side of tentacle notch large, accounting for slightly less than one-third of the total ventral LAP edge width. No perforations dicernible.

Description of median to distal LAP

GZG.INV.78575 is a dissociated, small, median to distal LAP, with slightly fragmentary dorsoproximal tip; nearly as wide as high; proximal edge devoid of spurs or thickened parts; outer surface with moderately finely meshed stereom. Five very large spine articulations, similar to those observed of holotype but muscle opening not obscured by sediment, very large; spine articulations arranged in dorso-proximally strongly receding row on elevated distal portion of LAP; row of spine articulations very strongly protruding ventrally. Ventral edge of LAP strongly concave as a result of the strongly protruding row of spine articulations.

Inner side of LAP with large, moderately well-defined, wide, bent, prominent ridge; dorsal tip of ridge pointing dorso-proximalwards; ventral tip of ridge merged with ventral LAP edge; inner side of distal edge of LAP devoid of spurs; inner side of tentacle notch very large, accounting for at least one-third of the total ventral LAP edge width. No perforations discernible.

Remarks

These LAPs unquestionably belong to Krohcoma gen. nov. on account of the position and morphology of the spine articulations. The combination of up to six large spine articulations with slight dorsalward increase in size, moderately large tentacle notches, the absence of spurs on the outer proximal and inner distal edges, and the strongly protruding ventral tip of the row of spine articulations distinguishes these two LAPs from the two other species here assigned to Krohcoma gen. nov. These two LAPs clearly represent a new species, the formal description of which, however, requires better-preserved material.

Figures

Fig. 15. Fossil lateral arm plates (LAPs) of ophiacanthid brittle stars in external (a) and internal (b) views. 1-3. Geromura teckliformis gen. et sp. nov. from the late Valanginian (Early Cretaceous) of the Ternberg Nappe, Austria. 1. NHMW 2012/0138/0005 (holotype), proximal LAP. 2. NHMW 2012/0138/0006 (paratype), median LAP. 3. NHMW 2012/0138/0007 (paratype), distal LAP. 4-6. Krohcoma mira gen. et sp. nov. from the late Sinemurian to early Pliensbachian (Early Jurassic) of the Glasenbach Gorge, Austria. 4. NHMW 2012/0137/0013 (holotype), proximal LAP. 5. NHMW 2012/0137/0014 (paratype), median LAP. 6. NHMW 2012/0137/0015 (paratype), distal LAP. 7-8. Krohcoma sp. nov. innom from the late Bathonian (Middle Jurassic) of Jumara, India. 7. GZG.INV.78574, proximal LAP. 8. GZG.INV.78575, median to distal LAP. 9-12. Krohcoma ampla gen. et sp. nov. from the early Kimmeridgian of the Pointe du Chay (9-10), France, and the late Kimmeridgian of Trancoso, Portugal (11-12). 9. GZG.INV.78577 (holotype), proximal LAP. 10. GZG.INV.78578 (paratype), median LAP. 11. GZG.INV.78579 (paratype), median LAP. 12. GZG.INV.78580 (paratype), distal LAP. One common scale bar per species is given.