Ophiacantha oceani, Numberger-Thuy & Thuy, 2020

Numberger-Thuy, Lea D. & Thuy, Ben, 2020, A new bathyal ophiacanthid brittle star (Ophiuroidea: Ophiacanthidae) with Caribbean affinities from the Plio-Pleistocene of the Mediterranean, Zootaxa 4820 (1), pp. 19-30 : 22-24

Ophiacantha oceani sp. nov.

Figure 2

urn:lsid:zoobank.org:act:F80B0B8E-7E60-4666-B682-E59E0B50A7A6

Etymology. Species named in honour of progressive metal band ‘The Ocean’. Musicians who so skillfully combine arts and science, composing albums like ‘Precambrian’ (with songs named after the periods of the Precambrian), ‘Pelagial’ (with songs named after the bathymetric subdivisions of the water column) and ‘Phanerozoic’ as well as the song ‘Turritopsis dohrnii’ referring to the immortal jellyfish from the Mediterranean, are more than deserving of being immortalized in the fossil record.

Holotype. MnhnL ( Musée national d’histoire naturelle, Luxembourg) OPH074, dissociated lateral arm plate.

Paratypes. MnhnL OPH075 and OPH076, two dissociated lateral arm plates.

Additional material. MnhnL OPH077, ten dissociated lateral arm plates.

Diagnosis. Species of Ophiacantha with lateral arm plates showing a very strong constriction; weak vertical striation close to the row of spine articulations; large, poorly defined, proximalwards protruding and slightly prominent spur on the outer proximal edge; up to seven dorso-ventrally compressed spine articulations composed of massive dorsal and ventral ridges fully separated at their proximal tips; inner side of lateral arm plates with vertebral articular structures composed of a well-defined prominent triangular central knob and a similarly well-defined oblique, slightly arched proximal ridge with a ventralward-pointing dorsal tip.

Locus typicus. Punta Mazza , Capo Milazzo, NE Sicily, Italy (38.270694° N, 15.239373° E) .

Stratum typicum. Yellowish marls of the MPL5–MPL6, Piacenzian to Gelasian, latest Pliocene to earliest Pleistocene, approximately 2.6 mya.

Description of the holotype. MnhnL OPH074 ( Fig. 2 A–B) is a dissociated proximal to median lateral arm plate approximately 1.3 times longer than high with deeply concave dorsal edge as a result of an extreme constriction; ventral edge weakly convex; outer proximal edge convex, with a large, smooth, protruding, slightly prominent and poorly defined spur proximally fading into the crest of the outer surface curvature; outer surface with moderately coarsely meshed stereom with very weak vertical striation in a narrow line along the proximal edge of a welldefined, slightly wavy ridge bordering the row of spine articulations; outer surface stereom slightly more densely meshed on crest of curvature in distal continuation of the spur on the outer proximal edge; seven spine articulations on strongly bulging distal portion of lateral arm plate, forming a row from the ventral to the dorsal edge of the lateral arm plate and probably forming a continuous to near-continuous fan of arm spines in the intact individual; size of spine articulations and of gaps separating them increasing towards the dorsal edge of the lateral arm plate; spine articulations dorso-ventrally compressed, consisting of a large, comma-shaped, near-horizontal dorsal lobe composed of compact stereom and a much shorter, straight, horizontal ventral lobe composed of compact to weakly perforate stereom; dorsal and ventral lobes proximally and distally completely separate, with weak sigmoidal fold distally; spine articulations distally bordered by brink composed of slightly more finely meshed stereom than on outer surface, with brink starting at second ventralmost spine articulation, widest at second dorsalmost spine articulation.

Inner side of lateral arm plate ( Fig. 2B ) with thick ventral edge showing a large contact surface with opposite lateral arm plate suggesting small ventral arm plates widely separated by lateral arm plates; dorsal edge much thinner but also showing extensive contact surface with opposite lateral arm plate, suggesting small dorsal arm plates restricted to the distal quarter of the arm segment; vertebral articular structures consisting of a well-defined, prominent central knob composed of slightly more finely meshed stereom than remaining inner side of lateral arm plate, proximally bordered and seemingly merged with a much smaller but similarly prominent and well-defined knob; additional vertebral articular structure consisting of a similarly well-defined, boomerang-shaped proximal ridge; weakly developed, narrow vertical ridge in distal third of the lateral arm plate, distally bordered by a very weak vertical furrow without clearly discernible perforations; inner distal edge with a large but poorly defined area of compact to very dense stereom corresponding to spur on outer proximal edge; tentacle notch small and inconspicuous.

Paratype supplements. MnhnL OPH075 ( Fig. 2 C–D) is a dissociated proximal to proximalmost lateral arm plate slightly higher than long; dorso-proximal tip missing, otherwise well preserved, with respect to finer stereom structures even better preserved than holotype specimen; outer proximal edge and outer surface ornamentation as in holotype; seven spine articulations as in holotype except for enlarged and distalwards displaced dorsalmost spine articulation.

Inner side with vertebral articular structures as in holotype except for more clearly separated large and small central knobs.

MnhnL OPH076 ( Fig. 2 E–F) is a dissociated distal lateral arm plate approximately 1.5 times longer than high; general plate outline, outer surface ornamentation and outer proximal edge as in holotype but with weaker constriction and thus less strongly concave dorsal edge; dorso-proximal tip of lateral arm plate missing; six spine articulations as in holotype; size of gaps separating spine articulations increasing dorsalwards but size of spine articulations unchanging.

Inner side as in holotype but with widely separated large and small central knobs; proximal vertebral articular structure roughly boomerang shaped but very poorly defined.

Figures

FIGURE 2. Ophiacantha oceani sp. nov. (A–F) from the Piacenzian to Gelasian, latest Pliocene to earliest Pleistocene (approximately 2.6 mya) of Punta Mazza, Capo Milazzo, NE Sicily, Italy, A–B: MnhnL OPH074, holotype, proximal lateral arm plate in external (A) and internal (B) views; C–D: MnhnL OPH075, paratype, proximal lateral arm plate in external (C) and internal (D) views; E–F: MnhnL OPH076, paratype, distal lateral arm plate in external (E) and internal (F) views. Recent Ophiacantha stellata (G–J) from the tropical NW-Atlantic as closest living relative of Ophiacantha oceani sp. nov., G: complete skeleton in ventral view; H: detail of arm in lateral view; I–J: lateral arm plate in external (I) and internal (J) views. Abbreviations: ckv: central knob of the vertebral articular structures; di: distal; do: dorsal; pr: proximal; prv: proximal ridge of the vertebral articular structures; sa: spine articulation; sp: spur; ve: ventral.