Krohcoma ampla, 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 : 81-83

Krohcoma ampla sp. nov.

urn:lsid:zoobank.org:act:CE375CEA-89F3-4641-9C52-71B0F7E80517

Fig. 15 : 9-12

Diagnosis

Species of Krohcoma gen. nov. with large LAPs, displaying small, poorly defined, protruding spur in dorsal half of outer proximal edge, paralleled by more clearly defined, round spur in dorsal half of inner distal edges; up to six very large spine articulations; very strong dorsalward increase in size of spine articulations and of gaps separating them; ventral lobe of spine articulations connected with outer surface; row of spine articulations not protruding ventrally; tentacle notch relatively small, accounting for slightly more than one-quarter of the total ventral LAP edge width; ridge on inner side of LAP with widened, triangular dorsal tip.

Etymology

Name formed from amplus, Latin for “large”, “magnificent”, in reference to the large size of the LAPs of the species in comparison to those of its congeners.

Type material

Holotype GZG.INV.78577 .

Paratypes

GZG.INV.78578, same locality as holotype; GZG.INV.78579 and GZG.INV.78580 from the upper Kimmeridgian Amaral Formation of Trancoso, Portugal.

Type locality and horizon

Pointe du Chay near La Rochelle, France; Achilles Subzone, Cymodoce Zone , early Kimmeridgian, Late Jurassic.

Additional material

GZG.INV.78581 (dissociated LAP) from the Kimmeridgian of Pointe du Chay, France; GZG.INV.78582 (12 dissociated LAPs) from the upper Kimmeridgian Amaral Formation of Trancoso, Portugal.

Description

Holotype

GZG.INV.78577 is a very large, dissociated proximal LAP; at least twice higher than wide; dorsal edge nearly straight; distal edge obliquely convex; proximal edge irregularly concave, with small, poorly defined and weakly prominent, yet protruding spur; outer surface with finely meshed stereom, devoid of conspicuous ornament elements. Six very large, ear-shaped spine articulations arranged in dorsoproximally strongly receding row on elevated distal portion of LAP; very strong dorsalward increase in size of spine articulations and of gaps separating them; dorsal and ventral lobes of spine articulations thick, merged into continuous volute encompassing conspicuously large muscle opening; ventral lobe of spine articulations connected with finely meshed outer surface; row of spine articulations not protruding ventrally. Ventral edge of LAP irregularly convex, tentacle notch not discernible in external view.

Inner side of LAP with large, well-defined, prominent ridge; dorsal half nearly straight, oblique, with widened triangular dorsal tip, separated from ventral half by rounded kink; ventral half short, less welldefined than dorsal one, not connected with ventral edge of LAP; inner side of distal edge of LAP with small, moderately well-defined, slightly prominent spur composed of slightly more densely meshed stereom; inner side of tentacle notch large, accounting for slightly more than a quarter of the total ventral LAP edge width. Vertical row of small perforations distally bordering kink between dorsal and ventral portions of the ridge, with dorsalward increase in size of perforations and of gaps separating them.

Paratype supplements and variation

GZG.INV.78578 is a dissociated median LAP; slightly higher than wide; well matching holotype; spur on proximal edge slighty better defined and more prominent. Five very large spine articulations similar to those observed on holotype; connection between ventral lobe of spine articulations and finely meshed stereom of outer surface slightly more prominent.

Inner side of LAP largely obscured by sediment, only ventral portion of ridge discernible, separated from ventral edge of LAP; inner side of distal edge of LAP with small, poorly defined, round and weakly prominent spur composed of slightly more densely meshed stereom. Inner side of tentacle notch moderately large, distally bordered by moderately well-defined, oblique, prominent ridge. No perforations discernible.

GZG.INV.78579 is a dissociated median LAP; slightly higher than wide; dorsal edge concave; outer surface with slightly more coarsely meshed stereom than in holotype, probably as a result of poorer preservation. Four very large spine articulations similar to those observed in holotype but with less welldeveloped connection between outer surface and ventral lobe of spine articulations, again probably due to slightly poorer preservation.

Inner side of LAP with well-defined ridge separated by round kink into dorsal portion with slightly widened dorsal tip, and much shorter ventral portion not connected with ventral edge of LAP; inner side of tentacle notch large, distally bordered by poorly defined, short, oblique ridge-like knob. No perforations discernible.

GZG.INV.78580 is a dissociated distal LAP; slightly wider than high; proximal edge slightly fragmentary. Three very large spine articulations, with very strong dorsalward increase in size; dorsal gap between spine articulations larger than ventral one.

Inner side of LAP with very poorly defined, oblique, weakly prominent ridge composed of more finely meshed stereom; dorsal half of inner side of distal LAP edge with small, round, well-defined, prominent spur.

Remarks

Although, at first sight, these LAPs seem to differ significantly from those assigned to Krohcoma gen. nov., similarities in spine articulation morphology and arrangement are striking. In fact, large spine articulations with thick dorsal and ventral lobes encompassing a conspicuously large muscle opening, arranged in a dorso-proximalwards receding row on the elevated distal portion of the LAPs are a highly distinctive feature which is not found in any other ophiacanthid LAP type. There are admittedly strong differences between these LAPs and those of its congeners in terms of size of tentacle notch, connection between outer surface and ventral lobe of spine articulations and size and shape of the ridge on the inner side. These differences might, in fact, warrant separation at the generic level. On the basis of the striking similarities in spine articulation morphology and arrangement, however, the present LAPs are here considered as a new species of Krohcoma gen. nov.

Occurrence

Early Kimmeridgian of France and late Kimmeridgian of Portugal.

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.