Ishidacantha fuersichi, 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 : 174-176

Ishidacantha fuersichi sp. nov.

urn:lsid:zoobank.org:act:26105676-350E-423A-A101-8AEAC88B0FF8

Fig. 31 : 1-4

Diagnosis

Species of Ishidacantha gen. nov. with moderately large LAPs displaying a well-developed, slightly irregular vertical striation on distal third of outer surface and composed of overlapping lamellae; two well-defined, slender, horizontally strongly elongate spurs on the outer proximal and inner distal edges; spurs on the inner distal edge protruding; up to four moderately large, nearly equal-sized spine articulations proximally tightly delimited by distalmost lamellae; ventral and dorsal lobes of spine articulations separated by one to three small, notches; inner side of proximal LAPs with very large dorso-proximalwards bent, continuous proximal ridge and shorter, vertical, distal ridge; tentacle notch relatively small.

Etymology

Species named in honour of Franz T. Fürsich for his contributions on the palaeontology of the type locality and for generously providing the samples yielding the type material of the species.

Type material

Holotype GZG.INV.78728 .

Paratypes

GZG.INV.78729, GZG.INV.78730 and GZG.INV.78731.

Type locality and horizon

Jumara, Kachchh, India; sample 119, upper Chari Formation, Callovian, Middle Jurassic.

Additional material

GZG.INV.78732 (29 dissociated LAPs) from sample 117; GZG.INV.78733 (27 dissociated LAPs) from sample 119; GZG.INV.78734 (88 dissociated LAPs) from sample 121, all from the upper Chari Formation, Callovian.

Description

Holotype

GZG.INV.78728 is a dissociated, moderately large, proximal LAP; fragile; almost twice wider than high; dorsal edge slightly fragmentary, originally nearly straight; distal edge undulose, with two small, pointed, non-prominent protrusions; ventral third of LAP protruding ventro-distalwards; ventro-distal tip of LAP protruding ventralwards, rounded, slightly tongue shaped; ventro-proximal tip of LAP slightly fragmentary, originally oblique; proximal edge with two well-defined, moderately large, equalsized, horizontally strongly elongate, slender, weakly prominent and slightly protruding spurs; outer surface with well-developed, slightly irregular vertical striation composed of overlapping lamellae displaying strong proximalward decrease in size; vertical striation replaced on proximal two-thirds of outer surface by finely meshed stereom; irregular band of very finely meshed stereom along proximal edge of LAP. Four relatively small, nearly equal-sized and equi-distant, ear-shaped spine articulations on slightly elevated distal portion of LAP; dorsal and ventral lobes separated by one to three very small notches; spine articulations proximally tightly delimited by two distalmost lamellae; gap between spine articulations and distal edge of LAP almost as wide as one spine articulation. Ventral edge of LAP nearly straight except for relatively small but very deeply incised, semi-circular tentacle notch.

Inner side of LAP with two sharply defined, prominent, closely spaced but separate ridges; proximal ridge large, conspicuous, slender, slightly dorso-proximalwards bent with weakly widened dorsal and ventral tips; distal ridge shorter than half the length of the proximal one; nearly vertical; inner side of distal edge of LAP with two sharply defined, horizontally elongate, prominent and slightly protruding spurs. No perforations or furrow discernible.

Paratype supplements and variation

GZG.INV.78729 is a dissociated, fragmentary proximal LAP; preserved portions very well in agreement with holotype. Four spine articulations similar to those observed on holotype.

Inner side with very large, oblique, slightly bent, dorsally thickened proximal ridge with small, round knob distally bordering ventral tip of the latter; ridge and knob directly adjacent but not merged.

GZG.INV.78730 is a dissociated median LAP; twice wider than high; well in agreement with holotype; ventral quarter slightly less strongly protruding ventro-proximalwards. Three spine articulations similar to those observed on holotype.

Inner side of LAP with two ridges similar to those of holotype but slightly shorter.

GZG.INV.78731 is a dissociated distal LAP; nearly 2.5 times wider than high; dorsal edge straight; ridge on proximal edge slightly smaller and less well defined than in holotype; ventral portion of LAP not protruding. Three spine articulations similar to those observed on holotype. Ventral edge of LAP concave; tentacle opening developed as perforation emerging at ventro-proximal edge of ventralmost spine articulation.

Inner side of LAP with two sharply defined, prominent, closely spaced but separate knobs; proximal knob nearly triangular; distal knob much smaller, round; very large tentacle perforation in the middle of the distal third of the LAP.

Remarks

The unusually small height/width ratio, the large, strongly protruding ventral portion, the two protruding spurs on the outer proximal and inner distal edges and the two separate ridges on the inner side unambiguously place these LAPs in Ishidacantha gen. nov. The presence of up to four relatively small spine articulations is not found in any other LAP type assigned to this genus. The development of the ridges on the inner side in the present LAPs suggests that the distance between the ridges correlates with the position of the LAPs in the arm and thus with their height/width ratio.

Occurrence

Callovian of India.

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.