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 : 125-126

Alternacantha ? sp.

Fig. 22 : 5

Material examined

GZG.INV.78645, GZG.INV.78646 (2 dissociated LAPs) from the late Pliensbachian of Amellago, Morocco.

Description

Dorsal tips of moderately large, dissociated proximal LAPs; originally more than twice higher than wide; dorsal tip rounded, tongue shaped; distal edge convex; preserved part of proximal edge gently concave, devoid of spurs; outer surface with finely meshed stereom. Two large, widely separated, earshaped spine articulations in shallow notches of the moderately elevated distal portion of the LAP, not sharply bordered proximally by a ridge; remains of a third spine articulation suggests much smaller gap between the fragmentary third and the middle spine articulations than between the dorsalmost and the middle ones.

Inner side of LAP with dorsal tip of a narrow, sharply defined, prominent and originally oblique ridge. No perforations or furrow discernible.

Remarks

Assignment of the present record has posed problems due to its highly fragmentary condition. The unusually wide gap between the two dorsalmost spine articulations, the size and position of the latter, and the shape of the ridge on the inner side are strongly suggestive of Alternacantha , in particular A. occulta , because of the rounded, tongue-shaped dorsal tip of the LAPs. Superficial similarities are also shared with the LAPs of Hanshessia . Assignment to the latter genus is less likely, however, on account of the oblique, rather than near-vertical, dorsal part of the ridge on the inner side and the large gap between the two dorsalmost spine articulations. The present specimens are thus described here as a questionable record of Alternacantha , indeterminate at the specific level.

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.