Ophiotoma 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 : 56-57

Genus Ophiotoma Verrill, 1899

Type species

Ophiotoma coriacea Lyman, 1883 , by monotypy.

Diagnosis

LAPs devoid of constriction or strongly elevated or bulging distal portion; spine articulations free standing or in shallow notches of the slightly elevated distal edge; ventral portion of LAP very large, protruding ventro-proximalwards; tentacle notch large, inner side of tentacle notch in proximal LAPs at least as wide as one-third of the total ventral LAP edge width; outer surface commonly with vertical striation; no conspicuous spurs on outer proximal and inner distal edges; ridge on inner side of LAP generally with widened dorsal tip.

Remarks

The LAPs of extant species of Ophiotoma ( Fig. 12 : 1-3) are characterised by the lack of a constriction and of a bulging or strongly elevated distal portion, as well as a large, strongly protruding ventral portion bordering a large tentacle notch. The spine articulations are generally in shallow notches or depressions of the slightly elevated distal LAP edge, which differentiates them from the LAPs of Ophiologimus (see above). Superficial similarities exist to the LAPs of extant species of Ophiolimna Verrill, 1899 (see below). In these, however, the spine articulations are proximally slightly overlain by the coarse vertical striation of the outer surface and merged with the latter by a connecting ridge.

The ophiacanthid fossil record includes only very few dissociated LAP types which convincingly match the diagnosis of Ophiotoma . Given their scarcity and limited currently known diversity, these fossil LAP types are here assigned to the extant genus Ophiotoma . The discovery of articulated specimens with the LAP types in question, however, are very likely to reveal differences in general skeletal morphology which imply distinction at the generic level. Irrespective of generic placement issues, the striking morphological similarities shared by the fossil LAPs and those of Ophiotoma strongly suggest close phylogenetic ties, which is the relevant aspect for the scope of the present study.

The LAPs of Ophiotoma , especially the fossil ones, share an intriguing similarity with those assigned to extinct Dermocoma Hess, 1964 (see below), in particular on account of the strongly protruding ventral portion of the LAPs, the absence of a constriction and a bulging distal portion, the position of the spine articulations and the shape of the ridge on the inner side of the LAPs. The LAPs of Dermocoma tend to differ from those of Ophiotoma in displaying large and strongly protruding spurs on their outer proximal and inner distal edges. However, since both genera include species which deviate from this pattern, the latter cannot be taken as a rule. The only unambiguous character to distinguish the LAPs of Ophiotoma and Dermocoma is the size of the tentacle notch.

Figures

Fig. 12. Lateral armplates (LAPs) of fossil and Recent ophiacanthidbrittle stars in external (a) and internal (b) views. 1. Ophiotoma assimilis Koehler, 1904, Recent, proximal LAP. 2-3. Ophiotoma megatreta (H.L. Clark, 1911), Recent. 2. Proximal LAP. 3. Median LAP. 4. Ophiotoma sp. nov. innom. from the late Sinemurian to early Pliensbachian (Early Jurassic) of the Glasenbach Gorge, Austria; NHMW 2012/0137/0009, median LAP. 5-7. Ophiotoma vadosa sp. nov. from the early Pliensbachian (Early Jurassic) of Blockley, Great Britain. 5. GZG.INV.78541 (holotype), proximal LAP. 6. GZG.INV.78542 (paratype), median LAP. 7. GZG.INV.78543 (paratype), distal LAP. 8-10. Ophiotoma charlottae sp. nov. from the early Kimmeridgian of the Pointe du Chay, France. 8. GZG.INV.78545 (holotype), proximal LAP. 9. GZG.INV.78546 (paratype), proximal to median LAP. 10. GZG.INV.78547 (paratype), distal LAP. One common scale bar per species is given.