Ophiochondrus, Lyman, 1869

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 : 211-212

Ophiochondrus ? semirotundus ( Kutscher & Jagt, 2000) comb. nov. Fig. 37 : 6-7

Ophiothela? semirotunda Kutscher & Jagt, 2000: 74-75 , pl. 29 figs 11-13.

Diagnosis

Species tentatively assigned to Ophiochondrus with small, massive, rounded and thick LAPs displaying a coarsely granular outer surface; proximal edge slightly thickened, separated from the remaining outer surface by a shallow, moderately well-defined furrow; no spurs discernible on outer proximal and inner distal edges; up to six small, oval, horizontally elongate spine articulations on bulging distal portion of LAP; inner side of LAP with small, rather poorly defined ridge displaying weakly widened dorsal tip; large dorsal and ventral contact surfaces with opposite LAP facing midline of arm.

Material examined

GZG.INV.78800, GZG.INV.78801 and 32 dissociated LAPs in the Manfred Kutscher Collection in Sassnitz, Germany, the type material of Kutscher & Jagt (2000).

Description

Small, dissociated LAPs, proximal ones slightly higher than wide, distal ones slightly wider than high; of rounded, massive, thick aspect; dorsal edge slightly concave as a result of a constriction; distal edge convex; proximal edge nearly straight to slightly convex, with proximally protruding ventro-proximal tip; proximal edge slightly thickened, separated from remaining outer surface by shallow, moderately well-defined furrow; no spurs on proximal edge; outer surface with moderately coarsely granular stereom even between spine articulations and on thickened part of proximal edge; granules on outer surface with weak tendency to merge into short, knobby, irregular vertical ridges. Six (proximal LAPs) to five (distal LAPs) small, equal-sized and nearly equi-distant, ear-shaped, oval, horizontally elongate spine articulations freestanding on bulging distal portion of LAP; not bordered proximally by a ridge; dorsal and ventral lobes nearly horizontal, separated proximally; gap between spine articulations and distal edge of LAP as wide as half a spine articulation; row of spine articulations slightly oblique, dorsoproximalwards receding. Ventral edge of LAPs convex; tentacle notch invisible in external view.

Inner side of LAPs with small, short, poorly defined, prominent, inconspicuous ridge, with ventroproximalwards bent, slightly widened ventral part not merged with ventral portion of LAP; dorsal tip of ridge slightly widened, no extension discernible; inner side of distal edge of LAP devoid of spurs; inner side of tentacle notch very small, moderately well defined laterally, shallow; dorsal, ventral and ventro-proximal edges facing midline of arm with large, vertical contact surfaces. No perforations or furrow discernible.

Occurrence

Early Maastrichtian of Germany and Denmark.

Remarks

This LAP type from the Early Maastrichtian of Germany and Denmark was first described by Kutscher & Jagt (2000) as Ophiothela ? semirotunda. Assignment to the ophiotrichid genus Ophiothela Verrill, 1867 is untenable considering the fundamental differences in spine articulation morphology ( Martynov 2010). Interestingly, Kutscher & Jagt (2000) mentioned that their new species showed a certain resemblance to small-sized ophiacanthids, and later primarily compared it to LAP types originally assigned to hemieuryalids (here reinterpreted as species of the ophiacanthid Inexpectacantha ; see above). Indeed, the presence of a sigmoidal fold in the spine articulations unambiguously places the LAPs in question in the Ophiacanthidae .

A comparison with the LAPs of Recent ophiacanthids reveals that closest similarities are shared with the LAPs of Ophiochondrus , in particular on account of the generally round, massive and stout aspect, the size, shape and position of the spine articulations, and the outer surface ornament. Significant differences, however, can be found in the structure of the inner side. In fact, these fossil LAPs display large contact surfaces with the opposite LAP, facing the midline of the arm. Such contact surfaces are commonly found in species of the ophiolepidid genus Ophiomusium Lyman, 1869 (e.g. Kutscher & Jagt 2000), but are atypical of Ophiochondrus . This difference is very likely to warrant separation at the generic level. However, as long as the inner side of the LAPs of Ophiomoeris remains unknown, assignment to this genus cannot be ruled out. At the present state of knowledge, it thus seems best to tentatively assign the present LAPs to Ophiochondrus , stressing, however, that there are significant differences in LAP morphology between the fossil LAPs and those of Recent Ophiochondrus , in particular the type species of the genus.

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.