Ophiolimna tiamatia, 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 : 65-67

Ophiolimna tiamatia sp. nov.

urn:lsid:zoobank.org:act:89A5AD8F-A02B-47C1-8238-E21F29F1F526

Fig. 13 : 7-8

Diagnosis

Species of Ophiolimna with moderately large LAPs of rectangular outline; outer surface with relatively fine vertical striation; comparatively small spine articulations deeply sunken into notches of distal edge; inner side of proximal LAPs with large central ridge displaying slightly widened dorsally pointing dorsal tip.

Etymology

Species named after Tiamat, primordial goddess of the ocean and chaos monster in Babylonian mythology, in reference to the fact that this species is the oldest currently known occurrence of the Ophiolimna lineage and, on account of its deep-water origin, thus challenges the widely accepted concepts of the macroevolutionary significance of deep-water habitats.

Type material

Holotype

NHMW 2012/0137/0010 .

Paratypes

NHMW 2012/0137/0011.

Type locality and horizon

Glasenbach Gorge, Austria; Hauptknollenbrekzie, late Sinemurian to early Pliensbachian, Early Jurassic.

Additional material

NHMW 2012/0137/0012 (54 dissociated LAPs).

Description

Holotype

NHMW 2012/0137/0010 is a dissociated, medium-sized, proximal LAP of rectangular outline; approximately 1.5 times higher than wide; dorsal edge pointed, with strongly obtuse angle; distal edge convex; proximal edge unevenly concave, devoid of spurs; ventral portion of LAP slightly protruding ventro-proximalwards, accounting for less than a seventh of total LAP height; ventro-distal tip of LAP very slightly protruding, rounded; outer surface with very regular vertical striation composed of broadly overlapping lamellae, with strong proximalward decrease in size; lamellae replaced by finely meshed stereom in proximalmost third of outer surface and on ventral portion of LAP. Seven small, ear-shaped and nearly equal-sized spine articulations in deep notches of the distal edge and partly overlapped by the undulose distalmost lamella; ventral lobes of spine articulations strongly increasing in size dorsalwards, conspicuously tongue shaped and connected with the distalmost lamella in all but the two ventralmost spine articulations; dorsalward increase in size of the gaps separating spine articulations; narrow gap between the four ventral spine articulations and the distal edge of the LAP. Ventral edge of LAP with large, concave tentacle notch.

Inner side of LAP with large, sharply defined and prominent central ridge composed of an oblique, relatively narrow median part, a widened, dorsally pointing dorsal part, and a ventro-proximally bent, slightly less well-defined and relatively narrow ventral part; second small, inconspicuous, poorly defined and weakly prominent, proximally pointing ridge near ventro-proximal tip of LAP; inner side of distal edge of LAP devoid of spurs; inner side of tentacle notch distally bordered by slightly thickened ventrodistal edge of LAP. Very shallow vertical furrow dorsally bordering tentacle notch with single, very small perforation near ventralmost tip of furrow; no other perforations discernible.

Paratype supplements and variation

NHMW 2012/0137/0011 is a dissociated median LAP of trapezoid outline; nearly as high as wide; dorsal edge straight. Five equal-sized spine articulations; ventral lobe large, conspicuously tongue shaped and connected with distalmost lamella in all spine articulations; slight dorsalward increase in size of ventral lobes; strong dorsalward increase in size of gaps separating spine articulations. Ventral edge of LAP oblique, tentacle notch not discernible, possibly on account of insufficient preservation.

Inner side of LAP with central ridge well in agreement with that of holotype; dorsal tip only very slightly widened; no second, ventral ridge discernible; inner side of tentacle notch large but poorly defined. Vertical furrow dorsally bordering tentacle notch very faint, almost indiscernible.

Remarks

Although at first sight these LAPs appear rather atypical of the Ophiacanthidae , their spine articulations are overlapped by the distalmost lamella of the outer surface striation, and the ventral lobes of the spine articulations are large, tongue like and connected with the distalmost lamella, which, together with the well-developed ridges on their inner side, unambiguously places them in the Ophiolimna lineage. There is a superficial resemblance with the similarly rectangular LAPs of the extinct ophioleucinid Eirenura Thuy, 2011 . In the latter, however, the outer surface is only faintly striated, the dorsal tip of the ridge is not widened, and, most importantly, the spine articulations are rhombic rather than ear-shaped and lack the tongue-shaped ventral lobe.

The comparatively small and deeply sunken spine articulations and the slightly widened dorsal tip of the central ridge on the inner side are highly distinctive features of the above-described LAPs, precluding confusion with any other LAP type assigned to Ophiolimna . Thus, the LAP type in question is described here as a new species.

Occurrence

Late Sinemurian to early Pliensbachian of Austria.

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.