Ophioculina hoybergia Rousseau & Thuy, 2018

Rousseau, Julie, Gale, Andrew Scott & Thuy, Ben, 2018, New articulated asteroids (Echinodermata, Asteroidea) and ophiuroids (Echinodermata, Ophiuroidea) from the Late Jurassic (Volgian / Tithonian) of central Spitsbergen, European Journal of Taxonomy 411, pp. 1-26 : 17-21

Ophioculina hoybergia Rousseau & Thuy gen. et sp. nov.

urn:lsid:zoobank.org:act:9832EAD6-F33E-4E37-9F38-0ADB944CBE4F Figs 6–7

Diagnosis

As for genus.

Etymology

In honor of Magne Høyberget, who greatly contributed to the collection of the echinoderm fossils from Spitsbergen and to Norwegian palaeontology.

Material examined

Holotype

NORWAY: central Spitsbergen , Konusdalen, 78°19′97.1″ N, 15°52′15.5″ E ( PMO 217.930 ).

Paratypes

NORWAY: same data as for holotpye ( PMO 218.010b, PMO 218.048); central Spitsbergen, Janusfjellet , 78°20′35.4″ N, 15°49′85.2″ E ( PMO 218.001c). More than 80 specimens each consisting of an articulated disc and at least the basal portion of one or more arms have been collected from the Janusfjellet and Konusdalen sites. A large amount of intermingled isolated arm fragments were also present in the samples.

Type stratum

Lower Middle Volgian (Middle Tithonian), Slottsmøya Member, Agardhfjellet Formation.

Description

PMO 217.930 ( Fig. 6 A–D) is a fragment of an articulated disc preserving the basal portions of three arms. The specimen is split close to the ventral side. The disc is round and has a diameter of 25.3 mm. The oral plates are long, stout and separated into two seemingly unfused halves ( Fig. 6D ). The adoral shields are insufficiently preserved on the holotype to provide an accurate description. The oral shields have a narrow rectangular distal portion and a much wider proximal portion which is, however, insufficiently preserved for an unambiguous description. Each dental plate bears a cluster of three small, conical apical oral papillae bordered by a seemingly continuous row of similarly small, uniform, pointed lateral oral papillae, most probably six or seven in number. The second oral tentacle pores are superficial and covered by at least three papillae that are slightly larger than the lateral oral papillae and seemingly form a separate row.

The interradial areas ( Fig. 6D ) are not indented and covered by small, thin, rounded scales rather uniform in size. The interradial disc plates are devoid of granules or other spines.

The genital slits are bordered on both sides by small (abradial side of slit) to very small (adradial side), block-like papillae which seem to have originally formed a continuous row. The adradial and abradial genital plates are nearly equal in length. The adradial ones are uniformly slender and bar-like while the abradial plates are flat, with a broadened distal portion reminiscent of a hockey stick, and with a longitudinal ridge. The distal edge of the abradial genital plate bears a continuous row of small, blocklike, blunt papillae similar in shape and size to those lining the abradial edge of the genital slit, and forming a well-developed arm comb ( Fig. 6C ).

The six most proximal arm segments are incorporated within the disc. The tentacle pores are enclosed by lateral and ventral arm plates and developed at least until the eighth segment counted from the first segment that is not incorporated into the disc. The pores rapidly decrease in size distalwards. There are at least three very small tentacle scales on the lateral arm plate part of the proximal tentacle pores. The lateral arm plates abut ventrally from the most proximal segments onwards. They are slightly bulging, but not constricted. The ornamentation of the outer surface is not discernible. There are two to three slender spines, the longest of which nearly equals the length of one segment, on each lateral arm plate ( Fig. 6C ). The outline of the ventral arm plates is not discernible from the holotype.

Paratype 218.001c ( Fig. 7 A–B) is an articulated disc preserving most of the five arms and exposing the ventral side. Disc diameter is 9.7 mm. Five arm segments are incorporated into the disc. The longest arm is 32 mm long and slowly tapering. It comprises 24 segments counted from the edge of the disc. Proximal segments are wider than long, distalmost segments are clearly elongated. The tentacle openings are developed as pores enclosed by the ventral and lateral arm plates until the eighth segment counted from the disc edge, and developed as perforations within the lateral arm plates in all the following segments, emerging below the ventralmost spine articulation. The lateral arm plates are ventrally abutting in all arm segments. The ventral arm plates are asymmetrically lozenge-shaped and nearly two times wider than long in all segments. Their proximal edge is pointed with an obtuse angle, while their distal edge is slightly convex. The spine articulations are small, sunken into a shallow, dorso-ventral incision of the lateral arm plate outer surface at some distance from the distal edge of the plate. The microstructure of the spine articulations is not discernible. There are at least two spines on each lateral arm plate, but the actual number is not discernible. The outer surface of lateral arm plate is finely meshed stereom devoid of constrictions or conspicuous ornamentation. The presence of spurs on the outer proximal edge of the lateral arm plates cannot be assessed.

Paratype PMO 218.048 ( Fig. 7C ) is an articulated disc with the basal portion of two arms preserved. It is split close to the ventral side and its morphology is in agreement with that of the holotype. The oral plates, the second oral tentacle pores and the rows of papillae bordering the genital slit are better preserved and complement or unambiguously corroborate the observations made on the holotype. The oral shield is arrow-shaped with a right to acute proximal angle and a much narrower rectangular distal portion. The adoral shields are narrow, L-shaped and with pointed, abutting proximal tips.

Paratype PMO 218.010b ( Fig. 7 D–E) is an articulated, complete dorsal disc preserving basal portion of five arms. The disc diameter is 21.8 mm. The specimen is split closer to the dorsal side than the holotype. The dorsal disc is covered by small, rounded, thin and uniform scales. The radial shields are thicker than the dorsal disc scales, shaped as rounded, isosceles triangles about one and a half times longer than wide. They are about a quarter of the disc radius in length. The radial shields are distally separated from the arm base by the distal portion of the abradial genital plate that bears a continuous row of small, round to square-like papillae. The dorsal arm plates are not visible. The section of the lateral arm plates suggests a rather thin plate architecture.

Remarks

The combination of an arm comb and superficial second oral tentacle pores unambiguously places the Spitsbergen specimens in the suborder Ophiurina as recently circumscribed by O’Hara et al. (2017). The suborder currently includes the Astrophiuridae with the highly modified, asteroid-like Astrophiura Sladen, 1879 and its relatives, the Ophiuridae with the type taxon Ophiura ophiura Linnaeus, 1758 and the genera Ophiocrossota Clark, 1928 , Ophiocten Lütken, 1855 , Ophioctenella Tyler et al., 1995 and Ophionotus Bell, 1902 , and the Ophiopyrgidae comprising most of the remaining genera of the former subfamily Ophiurinae ( Smith et al. 1995) .

Assignment to the Astrophiuridae can be ruled out as the specimens from Spitsbergen lack the highly diagnostic characters of the group. With the other two families of the suborder, however, matters are more complicated. While the specimens from Spitsbergen resemble recent Ophiura ophiura with respect to disc morphology and arm skeleton, in particular the orientation of the tentacle notch on the inner side of the lateral arm plate, the development of the tentacle pores in median to distal arm segments rule out most members of the Ophiuridae . Ophiura ooplax (H.L. Clark, 1911) is an exception within the Ophiuridae in having tentacle pores developed as within-plate perforations on almost all arm segments. With respect to lateral arm plate morphology, in particular the outer surface ornamentation and the development of the spine articulations, however, it differs significantly from the specimens described herein.

The Ophiopyrgidae include the genera Aspidophiura Matsumoto, 1915 , Euvondrea Fell, 1961 and Ophioplinthus Lyman, 1878 which combine within-plate tentacle pores and an overall lateral arm plate morphology comparable to that seen in the Spitsbergen specimens. Assignment to any of these genera is nevertheless precluded since they have rudimentary arm combs and within-plate tentacle pores on almost all arm segments outside the disc rather than on median and distal ones only. We therefore propose the new genus Ophioculina gen. nov. to accommodate the here-described material.

A definite family-level assignment is hampered by the absence of diagnoses for the clades of O’Hara et al. (2017). Since Ophioculina gen. nov. shares greatest similarities with members of the Ophiopyrgidae , we tentatively place it in this family.

Ophioculina hoybergia gen. et sp. nov. likely used its relatively long, flexible arms for locomotion, while the short arm spines might have been used to gather carrion, detritus or prey (see Hess & Meyer 2008). At Janusfjellet, specimens of Ophioculina hoybergia gen. et sp. nov. are found stacked up in multiple monospecific layers which may have originated from gregarious behavior of living individuals as commonly seen in modern members of the Ophiurina (e.g., Fujita & Ohta 1990).

Figures

Fig. 6. Ophioculina hoybergia Rousseau & Thuy gen. et sp. nov., holotype PMO 217.930. A. Specimen preserving the ventral side of about half of the disc area and the proximal section of three arms. B. Drawing detail of the oral region, from D. C. Detail view of the base of an arm showing the arm comb. D. Detail view of the oral region. Abbreviations: abgp = abradial genital plate; adgp = adradial genital plate; aopa = apical oral papilla; as = arm spine; gpa = genital papilla; lap = lateral arm plate; lopa = lateral oral papilla; op = oral plate; os = oral shield; v = vertebra; 2opa = papilla of the second oral tentacle pore.

Fig. 7. Ophioculina hoybergia Rousseau & Thuy gen. et sp. nov., paratypes. A–B. Paratype PMO 218.001c. A. Nearly complete specimen (outlined) preserving five arms and the ventral side of the disc, on a concretion slab with an accumulation of specimens. B. Detail view of arm section. C. Paratype PMO 218.048. Articulated ventral disc preserving good details of the oral plates, the second oral tentacle pores and the rows of papillae bordering the genital slit. D–E. Paratype PMO 218.010b. D. Articulated specimen preserving the basal portion of five arms and showing details of the dorsal disc. E. Detail view of arm section. Abbreviations: aos = adoral shield; lap = lateral arm plate; os = oral shield; rs = radial shield; sart = spine articulation; tp = tentacle pore; vap = ventral arm plate; 2otp = second oral tentacle pore.