Ophiacanthidae (Martynov, 2010)

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 : 213-214

Ophiacanthidae gen. et sp. nov. innom. Fig. 37 : 8-9

Material examined

GZG.INV.78802, GZG.INV.78803 and GZG.INV.78804 (dissociated LAPs) from the Bimammatum Zone, late Oxfordian, Late Jurassic of the Plettenberg, Germany.

Description

GZG.INV.78802 is a dissociated, tiny, proximal LAP; very fragile, dorso- and ventro-proximal tips missing; originally at least twice higher than wide; distal edge convex; outer surface with finely and moderately densely meshed stereom. At least six very large, densely spaced spine articulations freestanding on elevated distal portion of LAP; shape of spine articulations very unusual, complex; dorsal lobe very large, distally encompassing several openings of variable size; sigmoidal fold well developed but very short, proximally bordering very small nerve opening; ventral lobe very slender, proximally merged with dorsal lobe; spine articulations slightly oval, horizontally elongate; middle spine articulations largest, with dorsalward and ventralward decrease in size; spine articulations proximally sharply bordered by straight, well-defined but weakly prominent edge of elevated distal portion of LAP; gap between spine articulations and distal edge of LAP generally narrow, widest near middle spine articulations. No tentacle notch discernible in external view.

Inner side of LAP with very slender, moderately well-defined, weakly prominent, oblique and nearstraight ridge with widened, vertically elongate dorsal tip composed of more finely meshed stereom; ventral tip of ridge indiscernible or poorly preserved; single, well-defined, prominent, oval spur composed of densely meshed stereom in ventral half of inner side of distal LAP edge; inner side of tentacle notch not preserved or indiscernible. No perforations or furrow discernible.

GZG.INV.78803 is a dissociated median or distal LAP, strongly broken; outer surface with very densely meshed stereom. At least four very closely spaced spine articulations similar to those observed on the proximal specimen described above, displaying dorsalward increase in size; row of spine articulations ventrally bordered by very large, oblique, lenticular, round, strongly prominent ridge-like structure composed of dense stereom.

Inner side of LAP with very slender, moderately well-defined, prominent ridge similar to the main part of the ridge observed on the previous specimen; ventral tip of ridge merged with large, well-defined, strongly prominent, oblique, elongate knob composed of densely meshed stereom; spur on inner side of distal edge of LAP much larger than in previous specimen; very large, round, lenticular, strongly prominent structure ventrally bordering row of spine articulations almost completely exposed in internal view.

Remarks

These specimens probably are among the most unusual ophiuroid LAPs ever recorded. The very weird, complex spine articulations display a sigmoidal fold, which suggests that the LAPs in question are assignable to the Ophiacanthidae . Affinities within this family, however, are purely speculative. These LAPs appear so highly derived that it is difficult, if not impossible, to deduce any phylogenetic relationships with other ophiacanthids on the basis of LAP morphology. It is even unfeasible to determine with certainty if this LAP type belongs to a large-pored or a small-pored ophiacanthid. To make matters worse, the material available is very limited and fragmentary, which hampers a proper systematic assessment of this highly intriguing LAP type.

Striking similarities in the structure of the spine articulations are shared with the LAPs of the ophiocomid Ophiopteris papillosa Lyman, 1875 . It cannot be ruled out that these similarities reflect close phylogenetic ties, which would place the present LAP type in the Ophiocomidae Ljungman, 1867 . However, most of the other characters, in particular the size and fragility of the LAPs, the shape of the ridge on the inner side, the enigmatic structure ventrally bordering the row of spine articulations and the absence of perforations on the inner side, differ fundamentally. It thus seems more probable that the similarly shaped spine articulations were acquired independently.

In the absence of more material, this LAP type cannot be formally described. It is clear, however, that it represents a new genus and species most probably assignable to the Ophiacanthidae . It is hoped that more material of this intriguing LAP type will be discovered in order to investigate its phylogenetic position and speculate on the function of the unusual spine articulation morphology and the enigmatic structure ventrally bordering the row of spine articulations.

Figures

Fig. 37. Lateral arm plates (LAPs) of fossil and Recent ophiacanthid brittle stars in external (a) and internal (b) views. 1-2. Ophiochondrus convolutus Lyman, 1869, Recent. 1. Proximal LAP. 2. Median LAP. 3. Ophiochondrus stelliger Lyman, 1879, Recent; proximal LAP. 4-5. Ophiochondrus punctatus (Kutscher & Jagt, 2000) comb. nov. from the early Maastrichtian (Early Cretaceous) of Rügen, Germany. 4. GZG.INV.78775, proximal LAP. 5. GZG.INV.78776, distal LAP. 6-7. Ophiochondrus? semirotundus (Kutscher & Jagt, 2000) comb. nov. from the early Maastrichtian of Rügen, Germany. 6. GZG.INV.78800, proximal LAP. 7. GZG.INV.78801, median LAP. 8-9. Ophiacanthidae gen. et sp. nov. innom. from the late Oxfordian (Late Jurassic) of the Plettenberg, Germany. 8. GZG.INV.78802, proximal LAP. 9. GZG.INV.78803, median or distal LAP. 10-11. Ophiojagtus argoviensis (Hess, 1966) comb. nov. from the late Oxfordian (Late Jurassic) of Savigna, France. 10. NHMB M1124, proximal LAP. 11. NHMB M11225, proximal to median LAP. One common scale bar per species.