Ophiomitrella Verrill, 1899

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 : 178

Genus Ophiomitrella Verrill, 1899

Type species

Ophiacantha laevipellis Lyman, 1883 , by monotypy.

Diagnosis

Ophiacanthid with moderately large, ear-shaped spine articulations proximally sharply separated by a prominent, generally strongly thickened distalmost lamella or ridge; row of spine articulations extended tentacle notch very small, inconspicuous; ridge on inner side oblique, with ventro-proximalwards bent ventral part; dorsal tip of ridge widened and with ventralwards projecting extension.

Remarks

Many extant ophiacanthid lineages, in particular the small-pored ones, share highly similar LAP morphologies, as already pointed out by Thuy & Stöhr (2011). As a result, it can be a major challenge to work out comprehensive LAP morphological diagnoses on the basis of which dissociated LAPs can be unambiguously assigned on genus level. Ophiomitrella certainly is one of the more challenging cases, being one of the genera which lack any striking, distinctive feature such as the lip-shaped ventral lobe of the spine articulations in Ophiolimna or the shape of the ridge in Ophiopristis and Ophiotreta . Although Ophiomitrella is a relatively common and diverse genus ( O’Hara & Stöhr 2006), only four extant species [ O. clavigera (Ljungman, 1865) , O. conferta ( Koehler, 1922) , O. granulosa (Lyman, 1878) and O. tenuis (Koehler, 1904) ] of the genus were assessed with respect to the morphology of their LAPs ( Fig. 31 : 7-9). On the basis of this admittedly limited evidence, the LAPs of Ophiomitrella are probably best characterised by the relatively large, ear-shaped spine articulations that are proximally sharply bordered by a near-straight, prominent, well-defined and strongly thickened distalmost lamella or ridge, and the oblique ridge with a ventro-proximalwards bent ventral part and a widened dorsal tip with a ventralwards projecting extension.

It is true that this combination of characters is not among the most distinctive, considering how similar the LAPs of some species of Ophiacantha are. To make matters worse, the LAP morphology of the type species Ophiomitrella laevipellis is largely unknown, which poses problems considering that numerous species of Ophiomitrella are probably not congeneric with O. laevipellis (compare O’Hara & Stöhr 2006). Therefore, in order not to add to the confusion but at the same time confer a maximum of taxonomic information, the diagnosis above is here taken as a reference to assign fossil dissociated LAPs to the genus Ophiomitrella , stressing, however, that the diagnosis is not among the most robust and only valid with certainty for the four extant species of the genus that have been examined personally.

Figures

Fig. 31. Lateral armplates (LAPs) of fossil and Recent ophiacanthidbrittle stars in external (a) and internal (b) views. 1-4. Ishidacantha fuersichi gen. et sp. nov. from the Callovian (Middle Jurassic) of Jumara, India. 1. GZG.INV.78728 (holotype), proximal LAP. 2. GZG.INV.78729 (paratype), proximal LAP. 3. GZG.INV.78730 (paratype), median LAP. 4. GZG.INV.78731 (paratype), distal LAP.5-6. Ishidacantha trispinosa (Hess, 1965) comb. nov. from the early Oxfordian (Late Jurassic) of Longecombe, France. 5. NHMB M11222, proximal LAP. 6. NHMB M11223, distal LAP. 7-9. Ophiomitrella conferta (Koehler, 1922), Recent. 7. Proximal LAP. 8. median LAP. 9. distal LAP. 10-12. Ophiomitrella? sp. 1 from the Callovian of Bauer-Wehrland, Germany. 10. GZG.INV.78735, proximal LAP. 11. GZG.INV.78736, median LAP. 12. GZG.INV.78737, distal LAP. One common scale bar per species.