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 : 126

Alternacantha sp. nov. innom. 1

Fig. 22 : 6

Material examined

GZG.INV.78647 (dissociated LAP) from the middle Toarcian of Le Clapier, France.

Description

The sole known specimen is a dissociated, moderately large, median LAP; ventro-proximal portion missing; LAP originally slightly wider than high; dorsal edge concave as a result of a well-developed constriction; distal edge convex; proximal edge largely missing; ventro-distal tip of LAP protruding, tongue shaped; outer surface with coarsely meshed stereom displaying thickened trabeculae merged into irregular vertical striation close to spine articulations. Three large, ear-shaped spine articulations freestanding in very shallow notches of the slightly elevated distal portion of the LAP; ventral and dorsal lobes composed of densely meshed stereom and merged into continuous, near-circular volute; spine articulations not sharply bordered proximally by a well-defined ridge; strong dorsalward increase in size of spine articulations; dorsalmost spine articulations widely separated from ventral two.

Inner side of LAP with dorsal tip of ridge preserved, narrow, sharply defined, prominent, oblique and not reaching dorsal edge of LAP; inner side of distal edge of LAP with two moderately large, well-defined, prominent, oval spurs composed of densely meshed stereom. No perforations or furrow discernible.

Remarks

In spite of its fragmentary condition, this LAP is unambiguously assignable to Alternacantha on account of the highly distinctive gap separating the dorsalmost spine articulation from the remaining ones. All other characters preserved are in agreement with this assignment. It is very unfortunate that the present specimen is the only one of this type available at the time. In fact, the outer surface ornament, the unusually low height/width ratio of the LAP (or, alternatively, the occurrence of an alternating position of the dorsalmost spine articulation even in elongated distal LAPs) and the spine articulations composed of densely rather than finely meshed stereom is a unique combination of characters. The specimen in question clearly belongs to a new species, but its formal description 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.