Alternacantha, Thuy & Meyer, 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 : 128-129

Alternacantha sp. nov. innom. 2

Fig. 22 : 9-10

Material examined

GZG.INV.78652 and GZG.INV.78653 from sample 95, lower Chari Formation, Callovian of Jumara, India.

Description

GZG.INV.78652 is a dissociated, large, median LAP; slightly longer than wide; dorsal edge concave as a result of a well-developed constriction; distal edge nearly straight; proximal edge gently convex, with two very poorly defined, almost indiscernible, weakly prominent, non-protruding spurs; outer surface with weakly developed, irregular vertical striation close to spine articulations composed of slender, widely separate, vertically merged trabeculae of outer surface stereom; moderately finely to finely meshed stereom on proximal three-quarters of outer surface. Three large, ear-shaped spine articulations in shallow notches of strongly elevated distal portion of LAP; dorsalward increase in size of spine articulations and of gaps separating them; gap between spine articulations and distal edge of LAP almost as wide as one spine articulation. Ventral edge of LAP nearly straight, deeply incised but small tentacle notch.

Inner side of LAP with small, sharply defined, very slender, oblique ridge with slightly widened ventral tip merged with ventral portion of LAP; inner side of distal edge of LAP with two small, horizontally elongate, moderately well-defined spurs composed of slightly more densely meshed stereom; inner side of tentacle notch deeply incised, laterally sharply defined. No perforations or furrow discernible.

GZG.INV.78653 is a dissociated proximal to median LAP; ventro-proximal portion missing; dorsal spur on proximal edge similarly poorly defined and weakly prominent as in the LAP described above but protruding; vertical striation less regular and restricted to a smaller area of outer surface. Three spine articulations, better preserved than in the previous specimen; dorsal and ventral lobes composed of finely meshed stereom and merged into continuous volute; spine articulations not sharply bordered proximally; dorsal gap between spine articulations more than twice larger than ventral one.

Inner side partly obscured by sediment but visible part well in agreement with the previous specimen.

Remarks

These specimens are unambiguously assignable to Alternacantha on account of the unusually large gap which separates the dorsalmost spine articulation from the remaining ones. Superficial similarities are shared with the median and distal LAPs of Ophiomalleus gen. nov., which, however, are much more robust, display freestanding spine articulations and a broader ridge on the inner side. Within Alternacantha , greatest similarities are shared with the single, fragmentary LAP from the Toarcian of France described above, with respect to the outer surface ornament and the height/width ratio of the LAPs displaying the alternating position of the dorsalmost spine articulation. In the Toarcian specimen, however, the lobes of the spine articulation are composed of densely rather than finely meshed stereom. The Callovian material described above most probably represents a new species, the formal description of which, however, requires more complete material.

Figures

Fig. 22. Lateral arm plates (LAPs) of fossil and Recent ophiacanthid brittle stars in external (a) and internal (b) views. 1-2. Hanshessia sp. from the Callovian (Middle Jurassic) of Jumara, India. 1. GZG. INV.78641, proximal to median LAP. 2. GZG.INV.78642, proximal LAP. 3-4. Ophiocopa spatula Lyman, 1883, Recent. 3. Proximal LAP. 4. Distal LAP. 5. Alternacantha? sp. from the late Pliensbachian (Early Jurassic) of Amellago, Morocco; GZG.INV.78645, proximal LAP. 6. Alternacantha sp. nov. innom. 1 from the middle Toarcian (Early Jurassic) of Le Clapier, France; GZG.INV.78647, proximal to median LAP. 7-8. Alternacantha occulta Thuy & Meyer, 2013, from the early Bajocian (Middle Jurassic) of Longwy, France. 7. GZG.INV.78648, proximal LAP. 8. GZG.INV.78649, distal LAP. 9-10. Alternacantha sp. nov. innom. 2 from the Callovian (Middle Jurassic) of Jumara, India. 9. GZG.INV.78652, proximal to median LAP. 10. GZG.INV.78653, median LAP. One common scale bar per species.