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 : 123-125

Genus Alternacantha Thuy & Meyer, 2013

Type species

Alternacantha occulta Thuy & Meyer, 2013 , by original designation.

Diagnosis

Ophiacanthid with large LAPs displaying a relatively large, ventro-proximalwards protruding ventral portion; small tentacle notch; large, ear-shaped spine articulations with the position of the dorsalmost spine articulation alternating between close to the remaining ones and widely separated from the latter.

Remarks

In terms of LAP morphology, Alternacantha is probably among the most distinctive and unambiguously identifiable ophiacanthids. In fact, the alternating position of the dorsalmost spine articulation, varying from close to the remaining spine articulations to widely separated from the latter, is a feature not found in any other LAP type. Since Alternacantha is known from well-preserved, articulated specimens ( Thuy & Meyer 2013), its phylogenetic position is well understood. Together with its extinct sister taxon Dermocoma , it holds a basal position within the large-pored ophiacanthids displaying well-developed disc plates rather than translucent scales obscured by skin.

Indeed, the close phylogenetic relationship with Dermocoma is reflected by LAP morphology. Both genera share ear-shaped spine articulations sunken in shallow notches of the gently elevated distal edge, a strongly ventro-proximally protruding ventral portion of the LAPs, strongly protruding spurs on the outer proximal edge, and a relatively simple ridge on the inner side devoid of sharp kinks or an exceedingly widened dorsal part. While proximal LAPs of the two genera can be readily differentiated, median and, in particular, distal LAPs can be so closely similar that distinction is very difficult, if possible at all. Thus, it is crucial for records based exclusively on dissociated LAPs to focus on well-preserved proximal LAPs in these genera. It should be stressed that it is strongly advisable in general to choose adult proximal LAPs as holotypes of new ophiuroid species known solely from dissociated LAPs, as clearly stressed by Thuy & Stöhr (2011). Unfortunately, the holotype of Ophiacantha ? suprajurassica Hess, 1965 is a median LAP from strata which have yielded both Dermocoma and Alternacantha . In view of the near-identical morphologies of median and distal LAPs in these two genera, the holotype in question cannot be unambiguously assigned. Ophiacantha ? suprajurassica should therefore be considered as a nomen dubium, even though non-type specimens assigned to Ophiacantha ? suprajurassica by Hess (1965a, 1966) can be assigned beyond doubt.

The closest extant relative of Alternacantha is Ophiocopa Lyman, 1883 , which is sister to the clade formed by Alternacantha and Dermocoma . Again, the close phylogenetic ties are reflected by LAP morphology. In fact, the LAPs of extant Ophiocopa spatula Lyman, 1883 ( Fig. 22 : 3-4) closely resemble those of the two fossil genera, in particular on account of the ventro-proximalwards protruding ventral portion of the LAP, the shape, number and position of the spine articulations, the outer surface ornament and the conspicuous, strongly protruding spur on the outer proximal edge. Differences pertain mainly to the shape of the ridge on the inner side of the LAPs, which in Ophiocopa is composed of a slightly bent, oblique dorsal part with a triangular tip, widely separated from a short knob-like part on the ventral portion of the inner side.

All currently known species of Alternacantha share strongly similar, near-identical LAP morphologies, making a distinction on the basis of dissociated LAPs a major challenge. Similarly indistinguishable LAP morphologies have been reported from closely related extant taxa ( Thuy & Stöhr 2011). Fortunately, most species of Alternacantha are known from articulated arm fragments, in addition to the dissociated LAPs, providing additional features for species-level distinction.

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.