Ishidacantha trispinosa (Hess, 1965) 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 : 176-177

Ishidacantha trispinosa (Hess, 1965) comb. nov.

Fig. 31 : 5-6

p.p. Ophiopholis? trispinosa Hess, 1965a: 1075 , figs 36-37 [non figs 16, 38-41, referable to Lapidaster etteri gen. et sp. nov.]

Diagnosis

Species of Ishidacantha gen. nov. with large LAPs displaying coarsely meshed stereom on the outer surface with a very narrow band of poorly developed vertical striation near spine articulations; two moderately well-defined, horizontally elongate spurs on outer proximal and inner distal edges in proximal to median LAPs, dorsal one of which protruding; three large spine articulations in all LAPs, with continuous volute and strong dorsalward increase in size; inner side of proximal LAPs with two round, slightly oblique knobs in vertical row connected by poorly defined, irregular, slender, weakly prominent vertical ridge in proximal half, and short, near-vertical ridge in distal half; tentacle notch large.

Material examined

NHMB M11222, NHMB M11223; 5 dissociated LAPs from the Renggeri Member, early Oxfordian of Longecombe, France, the type material of Hess (1965a).

Description

Large dissociated LAPs; proximal ones almost twice wider than high, distal ones slightly more than twice wider than high; dorsal edge strongly concave as a result of a well-developed constriction; distal edge convex with a conspicuous, pointed protrusion in dorsal half of distal edge, best developed in proximal LAPs; proximal edge irregularly undulose, in proximal to median LAPs with two moderately welldeveloped, prominent, protruding, horizontally elongate, ridge-like spurs, dorsal one of which larger than ventral one; spurs almost indiscernible in distal LAPs; ventro-distal tip of LAPs strongly protruding ventralwards, tongue shaped; ventro-proximal tip of LAP strongly oblique; ventral half to third of LAP strongly protruding ventro-proximalwards, large, wide; outer surface of LAPs with coarsely meshed stereom, replaced by finely meshed stereom in narrow band along proximal edge of LAP; trabeculae of coarsely meshed stereom merged into poorly developed vertical striation in very narrow band near spine articulations in proximal LAPs. Three large, ear-shaped spine articulations freestanding on strongly elevated distal portion in all LAPs; ventral and dorsal lobes of spine articulations merged into continuous volute; spine articulations not sharply bordered proximally; very strong dorsalward increase in size of spine articulations; weaker dorsalward increase in size of gaps separating spine articulations; gap between spine articulations and distal edge of LAP almost as wide as one spine articulation. Ventral edge of LAPs nearly straight, except for large, conspicuous, very deeply incised almost semi-circular tentacle notch.

Inner side of LAPs with two sharply defined, prominent ridge-like structures, proximal one separated into two, round, oblique knobs arranged in vertical row and connected by very poorly defined, narrow, weakly prominent irregular ridge; distal ridge-like structure consisting of single vertical, slightly oblique ridge in proximal to median LAPs, and of a small, round knob in distal LAPs; inner side of distal edge of proximal to median LAPs with two well-defined, prominent, horizontally elongate spurs composed of more densely meshed stereom, dorsal one of which slightly larger and protruding; spurs very poorly defined in distal LAPs; inner side of tentacle notch very large, widely encompassed by non-thickened ventral portion of LAPs. No perforations or furrow discernible.

Remarks

Hess (1965a) described Ophiopholis ? trispinosa Hess, 1965 on the basis of dissociated LAPs and articulated arm fragments from the Oxfordian of France, which are here shown to belong to two different species. The smaller LAPs and the articulated arm fragments are assignable to Lapidaster gen. nov., a probably close relative of extant large-pored Ophiologimus (see above). Most of the larger LAPs, including the holotype, are here demonstrated to belong to the new small-pored ophiacanthid genus Ishidacantha gen. nov. Among the LAP types assigned to this genus, the above-mentioned ones are unique in displaying three large spine articulations with a strong dorsalward increase in size, a coarsely meshed stereom with a very poorly developed vertical striation, and a proximal ridge on the inner side separated into a ventral and a dorsal knob. Thus, they are described here as a new species which displays the highly distinctive characters of the new genus best and is therefore selected as the type species of Ishidacantha gen. nov.

Occurrence

Early Oxfordian of France.

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.