Krohcoma mira, 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 : 79-80

Krohcoma mira sp. nov.

urn:lsid:zoobank.org:act:79C26A85-E693-4AF7-8A41-36FB956BE7ED

Fig. 15 : 4-6

Diagnosis

Species of Krohcoma gen. nov. with relatively small LAPs, displaying very poorly defined, large spur in the middle of the proximal edge; up to five very large, equal-sized spine articulations; row of spine articulations protruding ventrally; tentacle notch very large, equalling half of the total ventral LAP edge width; ridge on inner side of LAPs merged with ventral LAP edge.

Etymology

Name derived from mirus, Latin for “astonishing”, in reference to the palaeohabitat of the species suggesting a deep-water origin of the genus and thus challenging widely accepted concepts on the role of the deep sea in macroevolutionary patterns.

Type material

Holotype

NHMW 2012/0137/0013 .

Paratypes

NHMW 2012/0137/0014 and NHMW 2012/0137/0015.

Type locality and horizon

Glasenbach Gorge, Austria; Hauptknollenbrekzie, late Sinemurian to early Pliensbachian, Early Jurassic.

Additional material

NHMW 2012/0137/0015 (10 dissociated LAPs).

Description

Holotype

NHMW 2012/0137/0013 is a dissociated, small, proximal LAP; nearly as high as wide; dorsal edge slightly convex; distal edge oblique, convex; proximal edge irregularly undulose, with very poorly defined, prominent and slightly protruding central spur composed of slightly more densely meshed stereom; ventro-proximal tip of LAP thickened into very poorly defined, prominent spur composed of slightly more densely meshed stereom; outer surface with moderately finely meshed stereom, devoid of conspicuous ornament elements. Five very large, ear- to horseshoe-shaped spine articulations, freestanding in dorso-proximally receding row on strongly bulging distal portion of LAP; dorsal and ventral lobes of spine articulations very thick, proximally merged by relatively thin connection into continuous lobe encompassing conspicuously large muscle opening; spine articulations nearly equal sized and equi-distant; row of spine articulations protruding ventrally. Ventral edge of LAP straight except for protruding ventral tip of spine articulation row; tentacle notch not visible in external view.

Inner side of LAP with relatively small, narrow, sharply defined, prominent and slightly bent ridge; dorsal tip of ridge pointing dorso-proximalwards, not widened; ventral portion of ridge merged with thickened ventral edge of LAP; inner side of distal edge of LAP with poorly defined, slightly prominent but not protruding spur composed of slightly more densely meshed stereom; inner side of tentacle notch very large, accounting for approximately half of the total ventral LAP edge width. No perforations discernible.

Paratype supplements and variation

NHMW 2012/0137/0014 is a dissociated median LAP; slightly wider than high; dorsal edge straight to slightly concave. Four very large, nearly equal-sized spine articulations similar to those observed on holotype; very weak dorsalward increase in size of gaps separating spine articulations; row of spine articulations only slightly protruding ventrally. Ventral edge of LAP nearly straight.

Inner side of LAP well in agreement with that of holotype; no spur discernible on inner side of distal edge of LAP.

NHMW 2012/0137/0015 is a dissociated distal LAP; almost twice wider than high; dorsal edge clearly concave as a result of a well-developed constriction; proximal edge with rather poorly defined, prominent and very slightly protruding central spur composed of slightly more densely meshed stereom; ventral sixth of LAP slightly protruding ventro-proximalwards. Four very large, equal-sized, ear- to horseshoeshaped spine articulations on elevated distal portion of LAP, arranged in dorso-proximally receding row; very slight dorsalward increase in size of gaps separating spine articulations. Ventral edge with very large, weakly concave tentacle notch.

Inner side of LAP obscured by sediment.

Remarks

These LAPs can be easily differentiated from almost every other type of LAPs on account of the distinctive dorso-proximally receding row of very large spine articulations. Within the group of LAPs that display the latter character, here assigned to Krohcoma gen. nov., the above-described ones are unique in having equal-sized spine articulations and very large tentacle notches accounting for nearly half of the total ventral LAP edge width.

Occurrence

Late Sinemurian to early Pliensbachian of Austria.

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.