Ophiotoma vadosa, 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 : 59-60

Ophiotoma vadosa sp. nov.

urn:lsid:zoobank.org:act:698A06E5-BC2D-4729-8C41-15F2EBBFEB3E

Fig. 12 : 5-7

Diagnosis

Species of Ophiotoma with relatively large LAPs displaying large, conspicuous, tongue-shaped and strongly protruding ventro-distal tip; two small, but comparatively well-developed spurs on the outer proximal and inner distal edges; up to six spine articulations displaying strong dorsalward increase in size and in the size of the gaps separating them; dorsal tip of the ridge on the inner side of the proximal LAPs only slightly enlarged.

Etymology

From vadosus, Latin for “shallow”, in reference to the relatively shallow (mid-shelf) water depth assumed for the palaeohabitat of the species.

Type material

Holotype GZG.INV.78541 .

Paratypes

GZG.INV.78542 and GZG.INV.78543.

Type locality and horizon

Blockley, near Cheltenham, Great Britain; shell lenses in clayey matrix, Davoei Zone, early Pliensbachian, Early Jurassic.

Additional material

GZG.INV.78544 (20 dissociated LAPs) from Blockley, Great Britain; 175 dissociated LAPs from the late Pliensbachian of Seewen, Switzerland, the original material of Hess (1962).

Description

Holotype

GZG.INV.78541 is a dissociated, large, proximal LAP; dorsal and distal edges slightly convex; proximal edge undulose, with small, well-defined, oval, prominent spur positioned in the centre of protruding part of proximal LAP edge; second slightly larger but much less well-defined spur in the middle of the ventral half of the proximal edge; ventral third to quarter of LAP strongly protruding ventro-proximalwards; ventro-distal tip of LAP large, strongly protruding, tongue shaped; distal portion of LAP neither bulging nor elevated; outer surface with well-developed, regular vertical striation composed of thin, distalwards slightly overlapping lamellae, replaced by finely meshed stereom on ventral portion of LAP and near proximal edge. Six ear-shaped spine articulations in notches of distal edge; strong dorsalward increase in size of spine articulations and of gaps separating them; no connecting ridge with striation of outer surface; dorsal and ventral lobes of spine articulations separated by very small incision proximally; large and ventrally strongly increasing gap between spine articulations and distal edge. Ventral edge with very large, deeply concave, conspicuous tentacle notch, exacerbated by strongly protruding ventro-distal tip of LAP.

Inner side of LAP with large, sharply defined, prominent, bent ridge; dorsal tip of ridge only very slightly widened, pointing dorsalwards; ventral tip of ridge merged with ventral portion of LAP; inner side of distal edge of LAP with two spurs, dorsal one of which small, round, poorly defined, very slightly prominent but not protruding; ventral spur much larger, better defined, prominent, lenticular; inner side of tentacle notch slightly wider than one-third of the total ventral LAP edge width. Irregular vertical row of perforations, ventralmost of which dorsally bordering tentacle notch, moderately large; remaining perforations minute; row of perforations distally bordered by poorly defined, slightly prominent ridge.

Paratype supplements and variation

GZG.INV.78542 is a dissociated median LAP, nearly as high as wide; of rectangular to trapezoid outline; dorsal and distal edges oblique, straight; proximal edge slightly undulose, with two small, poorly defined, slightly prominent and very slightly protruding spurs; ventral fifth of LAP protruding; outer surface with vertical striation becoming slightly irregular in ventral half of LAP; striation replaced by finely meshed stereom close to proximal edge of LAP. Four spine articulations in moderately deep notches of distal edge; very weak dorsalward increase in size of spine articulations and of gaps separating them; narrow but ventrally increasing gap between spine articulations and distal edge of LAP. Ventral edge of LAP convex with moderately large, weakly concave tentacle notch.

Inner side with large, sharply defined ridge; dorsal tip of ridge slightly widened, ventral tip separated from ventral portion of LAP; inner side of distal edge of LAP with two small, moderately well-defined, slightly prominent spurs; inner side of tentacle notch slightly smaller than one-quarter of the total ventral edge of the LAP. Poorly defined, irregular perforation halfway between ridge and distal edge of LAP.

GZG.INV.78543 is a dissociated distal LAP, approximately 1.5 times wider than high, of rectangular outline; well in agreement with other paratype; ventral portion of LAP hardly protruding.

Inner side of LAP with sharply defined, prominent, bone-shaped ridge; dorsal and ventral tips of ridge strongly widened.

Remarks

These LAPs show a certain resemblance with LAPs assigned to Dermocoma (see below), in particular on account of the relatively deep notches holding the spine articulations and the spurs on the outer proximal and inner distal edges. These similarities, however, are superficial at most. In fact, the very large tentacle notches clearly preclude assignment to Dermocoma . Among the large-pored ophiacanthid lineages, greatest similarities are shared with the LAPs of extant Ophiotoma and the fossil LAP types assigned to that genus. Thus, in spite of the rather atypically well-developed spurs on the outer proximal and inner distal edges, the above-described LAPs are assigned to Ophiotoma . Within this genus, the LAPs in question are highly distinctive on account of the spurs and their very large, conspicuous, tongue-shaped ventro-distal tip, which is why they are described as a new species.

Occurrence

Early Pliensbachian of Great Britain and Late Pliensbachian of Switzerland.

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.