Ophiotoma incredibilis, 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 : 62-65

Ophiotoma incredibilis sp. nov.

urn:lsid:zoobank.org:act:768EE8E5-D636-4D23-9640-6D22A392265B

Fig. 13 : 1-3

Diagnosis

Species of Ophiotoma with large LAPs displaying a low height/width ratio; outer surface with welldeveloped vertical striation; up to four relatively small spine articulations in shallow notches of the non-elevated distal portion of the LAP; ridge on inner side devoid of thickened parts, composed of two separated parts: a long, oblique, nearly straight, dorso-proximally pointing dorsal one and a shorter ventro-proximally pointing ventral one.

Etymology

From incredibilis, Latin for “unbelievable”, referring to the surprisingly emotional reactions some colleagues showed when confronted with the discovery of the Blake Nose deep-sea ophiuroid assemblage.

Type material

Holotype GZG.INV.78549 .

Paratypes

GZG.INV.78550 and GZG.INV.78551.

Type locality and horizon

Blake Nose, borehole ODP 171B 1049A 20x5, tropical northeast Atlantic; latest Aptian to earliest Albian, Hedbergella trochoidea to Microhedbergella rischi planktonic foraminiferal zones, Early Cretaceous.

Additional material

GZG.INV.78552 (26 dissociated LAPs).

Description

Holotype

GZG.INV.78549 is a dissociated, very large, proximal LAP; slightly wider than high; dorsal edge straight; distal edge convex; proximal edge irregularly concave, devoid of spurs; ventral third to quarter strongly protruding ventro-proximalwards; ventro-distal tip of LAP weakly protruding, tongue shaped; outer surface with well-developed, rather regular vertical striation composed of coarse, overlapping lamellae replaced by finely meshed stereom in very narrow band along the proximal edge of the LAP. Four relatively small, ear-shaped spine articulations in shallow notches of the non-elevated distal edge of LAP; very weak dorsalward increase in size of spine articulations and of gaps separating them; notches of spine articulations deeply incising vertical striation of outer surface; dorsal and ventral lobes of spine articulations proximally separated by very small notch; gap between spine articulations and distal edge of LAP narrow, with ventralward increase in width. Ventral edge of LAP with large, deeply concave tentacle notch.

Inner side of LAP with two sharply defined, prominent, very slender ridges separated by poorly defined, very weakly prominent connection; dorsal ridge oblique, nearly straight except for ventrally pointing ventral tip; ventral ridge much shorter, nearly straight, ventro-proximally pointing; inner side of distal edge of LAP devoid of spurs; inner side of tentacle notch large, well defined, with coarsely meshed, horizontally elongate stereom, slightly narrower than one-third of the total ventral LAP edge width. No perforations or furrow discernible.

Paratype supplements and variation

GZG.INV.78550 is a dissociated median to proximal LAP; almost 1.5 times wider than high; dorsoproximal tip fragmentary; very well in agreement with holotype; vertical striation on outer surface slightly finer and slightly less regular. Three spine articulations similar to those observed on holotype but of equal size.

Inner side well in agreement with holotype; ridges slightly better preserved, completely separate. Single small perforation dorsally bordering tentacle notch.

GZG.INV.78551 is a dissociated distal LAP; twice higher than wide; dorsal edge slightly concave as a result of a very weak constriction; vertical striation on outer surface less well developed than in holotype. Three spine articulations similar to those observed on the holotype.

Inner side of LAP with small, slender ridge composed of slightly bent, oblique, dorso-proximally pointing dorsal part and shorter ventro-proximally pointing ventral part separated from the dorsal one by a gently rounded kink; sharply defined, prominent, round knob distally bordering the ridge.

Remarks

These LAPs are characterised by a well-developed outer surface vertical striation, a strongly ventroproximally protruding ventral portion, and the absence of a strongly elevated or bulging distal portion. The genera Ophiologimus and Lapidaster gen. nov. can be excluded since the spine articulations are in shallow notches rather than freestanding. The LAPs of Dermocoma display a continuous spine articulation volute rather than dorsal and ventral lobes separated proximally by a small notch.

Among the genera with LAPs displaying strongly protruding, generally large ventral portions, closets affinities are shared with Ophiotoma and Dermacantha gen. nov. The LAPs of the latter, although with similarly low height/width ratio, generally display spurs on the outer proximal and inner distal edges and a smaller tentacle notch. The above-described LAPs are thus assigned to Ophiotoma , acknowledging,

however, that the height/width ration is atypically low and the tentacle notch unusually small. Nevertheless, assignment to Ophiotoma is favoured for now.

Occurrence

Latest Aptian to earliest Albian of the tropical northeast Atlantic.

Figures

Fig. 13. Lateral arm plates (LAPs) of fossil and Recent ophiacanthid brittle stars in external (a) and internal (b) views. 1-3. Ophiotoma incredibilis sp. nov. from the latest Aptian to earliest Albian (Early Cretaceous) of Blake Nose, NE Atlantic. 1. GZG.INV.78549 (holotype), proximal LAP. 2. GZG. INV.78550 (paratype), median LAP. 3. GZG.INV.78551 (paratype), distal LAP. 4. Ophiolimna antarctica (Lyman, 1879), Recent, proximal LAP. 5-6. Ophiolimna perfida (Koehler, 1904), Recent. 5. Proximal LAP. 6. distal LAP. 7-8. Ophiolimna tiamatia sp. nov. from the late Sinemurian to early Pliensbachian (Early Jurassic) of the Glasenbach Gorge, Austria. 7. NHMW 2012/0137/0010 (holotype), proximal LAP. 8. NHMW 2012/0137/0011 (paratype), median LAP. 9-12. Ophiolimna malagasica sp. nov. from the Bathonian (Middle Jurassic) of Jumara, India. 9. GZG.INV.78553 (holotype), proximal LAP. 10. GZG.INV.78554 (paratype), median LAP. 11. GZG.INV.78555 (paratype), median LAP. 12. GZG. INV.78556 (paratype), distal LAP. One common scale bar per species is given.