Ophiotreta dendrophyllicola, 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 : 90-92

Ophiotreta dendrophyllicola sp. nov.

urn:lsid:zoobank.org:act:87010E71-C329-48FF-ADEB-F2A939732039

Fig. 17 : 3-11

Diagnosis

Species of Ophiotreta with large LAPs displaying a poorly developed vertical striation on the outer surface; poorly defined, thickened area in the middle of the proximal edge of the LAP; up to seven large spine articulations with a very well-developed connecting ridge with the distalmost lamella of the outer surface stereom.

Etymology

The species name is composed of Dendrophyllia Blainville, 1830 , the prevailing deep-water scleractinian genus at the type locality of this ophiuroid species, and the Latin suffix - cola, translating to “residing on”, “inhabitant of”.

Type material

Holotype

MGUH 30236 .

Paratypes

NHMM 2012 050-2012 057.

Type locality and horizon

Fakse, Denmark; pockets of unconsolidated sediment within azooxanthellate coral bioherms, Tylocidaris bruennichi Zone , middle Danian, Paleocene.

Additional material

NHMM 2012 058 (114 dissociated LAPs), NHMM 2012 059 (22 dissociated vertebrae, 17 arm spine fragments, 1 oral plate and 2 ventral arm plates).

Description

Holotype

MGUH 30236 is a dissociated, large, proximal LAP; slightly more than twice higher than wide; ventral portion of LAP slightly protruding ventro-proximalwards; dorsal edge concave as a result of a welldeveloped constriction; distal edge gently convex; proximal edge irregularly undulose to nearly straight, with very poorly defined, slightly thickened, weakly prominent and protruding central area; outer surface almost entirely covered by finely meshed stereom; very few broad vertical lamellae on proximal edge of strongly elevated distal portion of LAP. Seven large, ear-shaped, nearly equal-sized spine articulations freestanding on strongly elevated distal portion of LAP, proximally bordered by strongly undulose distalmost lamella of outer surface striation; large dorsal lobe and smaller ventral separated by small, sharp, moderately deep notch; ventral lobe slightly thickened into small knob, with moderately welldeveloped, short connecting ridge with distalmost lamella; very weak dorsalward increase in size of gaps separating spine articulations. Ventral edge of LAP with almost indiscernible, moderately large tentacle notch.

Inner side of LAP with large, sharply defined, prominent, conspicuous ridge composed of long, straight, near-vertical dorsal portion very close to proximal edge of LAP, and shorter, near-straight, oblique ventral portion with rounded, sharply defined and slightly more prominent ventral tip; dorsal and ventral parts of ridge separated by broad kink with ventro-proximally pointing angle; round, sharply defined, prominent knob on ventral portion of inner side of LAP, widely separated from ventral tip of ridge; inner side of tentacle scale moderately large, not sharply defined. Shallow, moderately well-defined vertical furrow with small perforations dorsally bordering inner side of tentacle notch.

Paratype supplements and variation

NHMM 2012 050 is a dissociated median LAP; slightly wider than high; proximal edge concave with a small, moderately well-defined, prominent and protruding spur composed of densely meshed stereom; outer surface with poorly developed vertical striation composed of broad lamellae replaced by finely meshed stereom in the proximal half of the outer surface. Five nearly equal-sized spine articulations similar to those observed on the holotype. Ventral edge of LAP convex with relatively small, shallowly concave tentacle notch.

Inner side of LAP with well-defined ridge with straight, oblique dorsal part with widened dorsal tip with ventro-proximally pointing angle strongly reminiscent of kink between dorsal and ventral parts of ridge in holotype; inner side of distal edge of LAP with small, moderately well-defined, weakly prominent spur composed of densely meshed stereom; ventral part of ridge shorter, ventro-proximally pointing; no knob on ventral portion of inner side. Poorly defined, shallow furrow with single perforation near its ventral tip dorsally bordering inner side of tentacle notch.

NHMM 2012 051 is a dissociated distal LAP; nearly 1.5 times wider than high; proximal edge with small, moderately well-defined, prominent and protruding, pointed spur; distal half of outer surface with weakly developed vertical striation. Five spine articulations similar to those observed on holotype, on strongly elevated distal portion of LAP, proximally sharply bordered by undulose distalmost lamella; well-developed connecting ridge between ventral lobe of spine articulations and distalmost lamella; ventral lobe of spine articulations slightly thickened; dorsalmost spine articulation smaller than remaining four. Ventral edge of LAP convex; tentacle notch obscured by syntaxial calcite crystals.

Inner side of LAP well in agreement with that of paratype. No perforations discernible.

Additional material

Vertebrae of rounded aspect, with nearly equal-sized proximal muscular fossae, large dorso-distal and smaller ventro-distal muscular fossae; large and strongly protruding distal articular knobs, zygospondyline articulation.

Oral plate short, approximately as high as wide; deeply incised notch for ring nerve; adradial articular area with conspicuous dorso-distalwards projection.

Spines flattened, hollow, with slight kink near base; coarse longitudinal ribs with short thorns.

Remarks

These LAPs are unambiguously assignable to the Ophiotreta - Ophiopristis group on account of the highly distinctive spine articulation morphology and the shape of the ridge on the inner side. For reasons outlined above, Ophiotreta is preferred here. Closest similarities are shared with the LAPs of Ophiotreta striata comb. nov., which, however, differ in completely lacking spurs on the outer proximal and inner distal edges, and in displaying a better-developed vertical striation and more poorly developed connecting ridges between the spine articulations and the outer surface throughout.

Since the present LAPs were the only ones found in the original sample, all other ophiuroid skeletal parts can be assumed to have belonged to the same species. The flattened arm spines found in the sample substantiate assignment to the Ophiotreta - Ophiopristis group, modern species of which commonly display similarly flattened arm spines.

Occurrence

Early Paleocene of Denmark.

Figures

Fig. 17. Fossil skeletal plates of ophiacanthid brittle stars; lateral arm plates (LAPs) in external (a) and internal (b) views. 1-2. Ophiotreta striata (Kutscher & Jagt, 2000) comb. nov. from the early Maastrichtian of Rügen, Germany. 1. GZG.INV.78588, proximal LAP. 2. GZG.INV.78589, distal LAP. 3-11. Ophiotreta dendrophyllicola sp. nov. from the middle Danian (Paleocene) of Fakse, Denmark. 3. MGUH 30236 (holotype), proximal LAP. 4. NHMM 2012 050 (paratype), median LAP. 5. NHMM 2012 051 (paratype), distal LAP. 6. NHMM 2012 052 (paratype), vertebra in distal view. 7. NHMM 2012 053 (paratype), vertebra in proximal view. 8. NHMM 2012 054 (paratype), vertebra in dorsal view. 9. NHMM 2012 055 (paratype), oral plate in adradial (a) and abradial (b) view. 10. NHMM 2012 056 (paratype), arm spine fragment. 11. NHMM 2012 057 (paratype), arm spine fragment. 12-15. Ophiotreta hedone sp. nov. from the middle Lutetian (Eocene) of Grignon, France. 12. GZG.INV.78590 (holotype), proximal LAP. 13. GZG.INV.78591 (paratype), median LAP. 14. GZG.INV.78592 (paratype), distal LAP. 15. GZG.INV.78593 (paratype), median LAP. One common scale bar per species.