Ophiojagtus irimurai, 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 : 216-219

Ophiojagtus irimurai sp. nov.

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Fig. 38 : 1-4

Diagnosis

Species of Ophiojagtus gen. nov. with wide, but comparatively short, ventral protrusion accounting for one-fifth to less than one-third of the total LAP height; proximal edge with weakly to very well-developed protrusion; four to seven spine articulations; weak dorsalward increase in size of spine articulations and of gaps separating them; ridge on inner side relatively slender, with weakly widened dorsal tip; ventral part of ridge extending well onto the ventral portion of the LAP; conspicuously angular kink between the ventral and dorsal parts of the ridge.

Etymology

Species named in honour of my friend and colleague Seiichi Irimura, in admiring recognition of his contributions to ophiuroid systematics.

Type material

Holotype GZG.INV.78808 .

Paratypes

GZG.INV.78809, GZG.INV.78810 and GZG.INV.78811.

Type locality and horizon

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

Additional material

GZG.INV.78812 (3 dissociated LAPs) from the early Kimmeridgian of the Pointe du Chay, France; GZG.INV.78813 (1 dissociated LAP) from the Amaral Formation, late Kimmeridgian of Trancoso, Portugal.

Description

Holotype

GZG.INV.78808 is a dissociated, large, proximal LAP; of massive, stout and strongly curved general aspect; almost three times higher than wide; dorsal edge rounded, convex; distal edge convex; proximal edge weakly concave, devoid of spurs, with very weak, rounded protrusion; ventral portion of LAP moderately broad, widened ventrally, relatively short, accounting for one-fifth of the total LAP height; outer surface with finely meshed stereom. Seven large, nearly equal-sized, ear-shaped spine articulations freestanding on bulging distal portion of LAP and composed of thick, continuous volute; weak dorsalward increase in size of gaps separating spine articulations; gap between spine articulations and distal edge of LAP narrow.

Inner side of LAP with well-defined, prominent, rather slender ridge; dorsal part of ridge slightly bent, oblique, with weakly widened dorsal tip; ventral part of ridge slightly narrower, less well defined and less strongly prominent than dorsal one, connected with the latter by a rounded kink and extending well onto the ventral portion of the LAP; ventral tip more strongly prominent than remaining ventral part of the ridge; no spurs on inner side of LAP; inner side of tentacle notch relatively small, poorly defined and shallow. Several small perforations loosely arranged in vertical row in very shallow, poorly defined furrow dorsally bordering tentacle notch.

Paratype supplements and variaton

GZG.INV.78809 is a dissociated median LAP; approximately 1.5 times higher than wide; proximal edge with very large, conspicuous, rounded protrusion; ventral portion of LAP wide but relatively short, accounting for less than one-quarter of the total LAP height, protruding ventro-proximalwards. Four spine articulations similar to those observed on holotype; very weak dorsalward increase in size of spine articulations; stronger dorsalward increase in size of gaps separating spine articulations.

Ridge on inner side of LAP similar to that of holotype but shorter and broader; kink between dorsal and ventral part of ridge conspicuously angular; inner side of tentacle notch relatively large, moderately well defined but shallow. Broad, moderately well-defined, short vertical furrow with four small, irregular perforations distally bordering kink of ridge.

GZG.INV.78810 is a dissociated median LAP; ventral portion missing; proximal edge with large but rather weak, inconspicuous rounded protrusion. Five spine articulations similar to those of holotype; very weak dorsalward increase in size of spine articulations and of gaps separating them.

Inner side of LAP fragmentary, with only dorsal part of ridge preserved. Two small perforations in shallow, narrow, poorly defined vertical furrow.

GZG.INV.78811 is a dissociated distal LAP; slightly higher than wide; dorsal edge weakly convex; proximal edge with large but rather weak and inconspicuous rounded protrusion; ventral portion of LAP wide and moderately long, accounting for slightly less than one-third of the total LAP height, strongly protruding ventro-proximalwards. Four spine articulations similar to those observed on holotype; dorsalward increase in size of spine articulations.

Ridge on inner side of LAP similar to that of holotype but shorter and with conspicuously angular kink between dorsal and ventral parts. Several small perforations closely grouped in vertical row distally bordering kink of ridge.

Remarks

These LAPs are unambiguously assignable to Ophiojagtus gen. nov. on account of the highly distinctive general morphology, the large, protruding ventral portion, the lack of outer surface ornament, the structure and position of spine articulations and the shape of the ridge on the inner side. Among the LAP types assignable to this genus, the present ones stand out in displaying up to seven spine articulations, a relatively short ventral portion and a conspicuously angular kink between the ventral and the dorsal part of the ridge on the inner side.

Occurrence

Early Kimmeridgian of France, late Kimmeridgian of Portugal.

Figures

Fig. 38. Fossil lateral arm plates (LAPs) of ophiacanthid brittle stars in external (a) and internal (b) views. 1-4. Ophiojagtus irimurai gen. et sp. nov. from the early Kimmeridgian (Late Jurassic) of the Pointe du Chay, France. 1. GZG.INV.78808 (holotype), proximal LAP. 2. GZG.INV.78809 (paratype), median LAP. 3. GZG.INV.78810 (paratype), median LAP. 4. GZG.INV.78811 (paratype), distal LAP. 5-7. Ophiojagtus acklesi gen. et sp. nov. from the late Aptian (Early Cretaceous) of Wizard Way, Texas. 5. GZG.INV.78814 (holotype), proximal LAP. 6. GZG.INV.78815 (paratype), median to distal LAP. 7. GZG.INV.78816 (paratype), proximal LAP. 8. Ophiojagtus sp. 1 from the early Cenomanian (Late Cretaceous) of Waco, Texas; proximal LAP. 9-10. Ophiojagtus alternatus (Kutscher & Jagt, 2000) comb. nov. from the early late Maastrichtian (Late Cretaceous) of Haccourt, Belgium. 9. NHMM 2012 060, proximal to median LAP. 10. NHMM 2012 061, proximal LAP. One common scale bar per species.