Sabinacantha, Thuy, 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 : 206-207

Genus Sabinacantha gen. nov.

urn:lsid:zoobank.org:act:DBFBC938-27CE-42A1-932C-F397C289B2E7

Type and sole known species

Sabinacantha archetypa sp. nov.

Diagnosis

Ophiacanthid with moderately large LAPs of thick, massive and rounded aspect; single poorly to moderately well-defined, protruding spur on outer proximal edge paralleled by large, round, weakly prominent area of more densely meshed stereom on inner side of distal LAP edge; outer surface with coarse, irregular striation; three small, ear-shaped, oval, horizontally elongate spine articulations tightly grouped on weakly elevated ventral third to half of distal edge of LAP; dorsalward decrease in size of spine articulations; inner side of LAPs with well-defined, uniformly prominent, short, broad ridge with widened, round dorsal tip; inner side of tentacle notch small.

Etymology

Genus named in honour of my friend and colleague Sabine Stöhr, who has guided me when I took my very first steps in ophiuroid research; from Acantha , a nymph in Greek mythology whose name literally translates to “thorny”; gender feminine.

Remarks

Among the most unusual LAP types of extant ophiacanthids is that of Ophiohamus nanus O’Hara & Stöhr, 2006 ( Fig. 36 : 4-5). In this small-sized species, the LAPs appear atypically massive and rounded for an ophiacanthid. The most striking feature, however, are the four small, oval spine articulations with separated dorsal and ventral lobes, and grouped in the ventral half of weakly elevated distal edge. This combination of characters is highly distinctive and not found in any other currently known ophiacanthid LAP type, except for a remarkable extinct one from the Kimmeridgian of Germany, recorded herein.

These fossil LAPs are much larger and thicker that those of Ophiohamus nanus but nevertheless display the highly characteristic rounded aspect and the small, oval spine articulations with the discontinuous volute, tightly grouped in ventral half of weakly elevated distal edge. Other close similarities are seen in the shape of the ridge on the inner side, although in the fossil LAPs it is better defined and uniformly prominent. Other minor differences pertain to the presence of a single, well-developed spur on the outer proximal and inner distal edges in the fossil LAPs, and the outer surface ornament which, in the fossil LAPs, consists of a coarse, highly irregular vertical striation, generally best defined close to the spine articulations.

These close similarities in LAP morphology strongly suggest that the fossil LAP type in question and Ophiohamus nanus share strong phylogenetic ties, most probably forming a single lineage. The fossil LAPs probably are not congeneric with the latter, on account of the minor, yet significant, differences including the size of the LAPs, the number of the spine articulations, the development of the ridge on the inner side, and the outer surface ornament. A new genus, Sabinacantha gen. nov., is thus introduced here to accommodate these Ophiohamus -like fossil LAPs.

There is, indeed, a close resemblance with the LAPs of extant Ophiochondrus convolutus Lyman, 1869 , which is closely related to Ophiohamus nanus in the revised phylogenetic analysis presented herein, especially on account of the massive, rounded aspect of the LAPs, the weakly elevated distal edge of the LAPs, the small, oval, horizontally elongate spine articulations with separated dorsal and ventral lobes, the dorsalward decrease in size of the spine articulations, and even the shape of the ridge on the inner side. These astonishing similarities are perfectly in line with the observation by Thuy & Stöhr (2011) that the closest similarities in LAPs are to be found in more closely related ophiuroid taxa.

Figures

Fig. 36. Lateral armplates (LAPs) of fossil and Recent ophiacanthidbrittle stars in external (a) and internal (b) views. 1-3. Reitneracantha dissidens gen. et sp. nov. from the late Sinemurian to early Pliensbachian (Early Jurassic) of the Glasenbach Gorge, Austria. 1. NHMW 2012/0137/0029 (holotype), proximal LAP. 2. NHMW 2012/0137/0030 (paratype), median LAP. 3. NHMW 2012/0137/0031 (paratype), distal LAP. 4-5. Ophiohamus nanus O’Hara & Stöhr, 2006, Recent. 4. Proximal to median LAP. 5. Proximal to median LAP. 6-8. Sabinacantha archetypa gen. et sp. nov. from the late Oxfordian (Late Jurassic) of the Plettenberg, Germany. 6. GZG.INV.78771 (holotype), proximal LAP. 7. GZG.INV.78772 (paratype), median LAP. 8. GZG.INV.78773 (paratype), distal LAP. One common scale bar per species.