Ophiosternle crinitum ( Quenstedt, 1876 ) Thuy & Schulz, 2013

Thuy, Ben & Schulz, Hartmut, 2013, The oldest representative of a modern deep-sea ophiacanthid brittle-star clade from Jurassic shallow-water coral reef sediments, Acta Palaeontologica Polonica 58 (3), pp. 525-531 : 526-529

Ophiosternle crinitum ( Quenstedt, 1876) comb. nov.

Figs. 1 , 2 .

1876 Ophiurites crinitus Quenstedt, 1876: 170 .

Holotype: GPIT/69 /96−24, partial articulated sceleton.

Type locality: Buchenbrunnen near Steinenfeld, S−Germany.

Type horizon: Reef debris beds within the Mergelstetten Formation ( Hybonoticeras beckeri Zone , Lithacoceras ulmense Subzone ), latest Kimmeridgian, Upper Jurassic.

Material.— GPIT /69/96−23, GPIT / AS /56.

Diagnosis.—As for genus.

Description.—The holotype ( GPIT /69/96−24) is an articulated skeleton exposing both dorsal and ventral sides and preserving large portions of three arms; disc diameter 9.8 mm; disc interradii strongly incised; dorsal disc plating dominated by large, conspicuous radial shields, triangular in outline, contiguous on their entire length, longer than three quarters of the disc radius, distal edge with notch; remaining disc plates tiny, rounded, restricted to centre of the disc and narrow interradial areas; no enlarged plates distally lining radial shields; no disc granules or spines discernible, possibly worn away during preparation process; ventral interradial plates covered by matrix, thus not observable; oral shields relatively large, arrow−shaped to rhombic, with nearly right, rounded proximal angle; adoral shields broad and relatively short, not extending around lateral edges of oral shield, broadly abutting in front of oral shield; jaws not elongate; oral plates stout, beset with four to five spine−like lateral oral papillae, pointed, three to four times longer than broad; distalmost lateral papilla nearly two times wider than others, irregularly leaf−like and pointed, positioned in the corner formed by the oral plate and the adoral shield; ventral tip of dental plate beset with a cluster of three to four small, conical apical papillae; dorsally following teeth conical, in single row and slightly larger than apical papillae.

Arms broad, composed of numerous short segments and with “longitudinal furrow” on ventral side, formed by the ventrally protruding rows of arm spine articulations on the lateral arm plates (and probably causing Quenstedt [1876] to hesitate on the ophiuroid nature of the specimen, mistaking the “furrow” for an open ambulacral grove); proximal ventral arm plates nearly twice as wide as long, smooth, with gently convex proximal edge, obtuse distal angle and strongly incised lateral edges, separated by ventral protrusions of lateral arm plates on all observable arm segments; tentacle pores relatively small; at least one leaf−like tentacle scale; dorsal arm plates twice as wide as long, nearly rectangular to trapezoid in outline, with gently convex distal edge, broadly separating lateral arm plates on all observable arm segments; lateral arm plates very high and narrow, with conspicuous pointed ventral protrusion; ornamentation of outer surface not discernible; arm spine articulations large, ear−shaped with well−developed sigmoidal fold, positioned in continuous row on conspicuously elevated vertical ridge on distal portion of lateral arm plates, at least 10 spine articulations on each lateral arm plate; spines circular in section, very large, smooth to finely striated longitudinally, equalling at least the length of four arm segments; vertebrae and inner side of lateral arm plates unknown.

There are two additional specimens , one large, fully articulated arm fragment consisting of proximal and median arm segments ( GPIT/69 /96−23) and one articulated disc (10 mm in diameter) with five arms preserving proximal and median arm segments and exposing the dorsal side ( GPIT / AS /56). The morphological details of both specimens are very well in agreement with those of the holotype .

Remarks.—The long, erect spines attached to large, ear−shaped spine articulations with a well−developed sigmoidal fold, in combination with the single row of teeth unquestionably place the specimens described above in the extant family Ophiacanthidae . The specimens share superficial similarities with extant Ophiocomidae , assignment to which, however, is precluded by the presence of a single row of teeth rather than a cluster of tooth papillae. Within the Ophiacanthidae , greatest similarities are shared with species of the major, yet unnamed ophiacanthid clade uniting all genera of the former subfamily Ophioplinthacinae ( Thuy et al. 2012) , with respect to the well developed dorsal disc plates

http://dx.doi.org/10.4202/app.2011.0192

and the large, exposed radial shields. The combination of deeply incised interradii and very long arm spines, as observed in the above described specimens, is found in the extant former ophioplinthacinid genera Ophioplinthaca Verrill, 1899 and Ophiocamax Lyman, 1878 . The closely related genus Ophiomitra Lyman, 1869 generally lacks the incised interradii. Nevertheless, a number of species, including the type species Ophiomitra valida Lyman, 1869 display more or less well developed incisions, suggesting that the genus is rather polymorphic. As already pointed out by O’Hara and Stöhr (2006), it is far from clear that Ophiomitra as well as many other former ophioplinthacinid genera represent monophyletic taxa.

The specimens described herein differ from Ophiocamax in lacking the multiple rows of spine−like oral papillae, as well as the highly characteristic ring of erect, spine−like tentacle scales surrounding the proximalmost pores. In addition, most extant species of Ophiocamax have strongly serrate arm spines and small thorns on the dorsal arm plates. In contrast to Ophiosternle crinitum , extant species of Ophioplinthaca are characterised by enlarged disc plates distally lining the interradial incisions. In addition, Ophioplinthaca generally has a single apical papilla rather than a cluster of papillae. Similarities are greatest with species of Ophiomitra , especially the species displaying deeply incised interradii. In these, however, dorsal arm plates are generally much narrower and separated by lateral arm plates at least from median arm segments onwards. Furthermore, Ophiomitra species tend to have smaller radial shields, thornier arm spines, shorter jaws and a larger disc in comparison to the width of the arms.

Evidently, the above described specimens are not satisfyingly compatible with any modern ophiacanthid genus. This is not surprising in the light of the considerable stratigraphic gap of some 155 Ma separating the specimens from their modern relatives. The genus name Ophiurites Schlotheim, 1820 , which Quenstedt (1876) assigned his new species O. crinitus to, is invalid since it falls into the category of names ending with −ites, introduced only to differentiate fossils from extant taxa (in this case Ophiura ), and explicitly banned by the ICZN (1999: article 20). Ironically, the first species included in Ophiurites ( O. filiformis octofilatus Schlotheim, 1820 , O. decafilatus Schlotheim, 1820 , and O. pennatus Schlotheim, 1820 ) turned out to be crinoids ( Boehm 1889), which is further reason not to use it for ophiuroids, had the name not been made invalid altogether. Other species originally assigned to Ophiurites include O. trunensis Böhm, 1891 , probably a synonym of Ophiomusium granulosum ( Roemer, 1840) ( Jagt 2000) , and O. eocaenus Leriche, 1931 , re−assigned by Jagt (1990) to Ophiozona (now Ophiolepis ).

Since the name Ophiurites is invalid, we thus introduce the new genus Ophiosternle to accommodate the specimens originally described as Ophiurites crinitus . The new genus is most probably a member of the clade formed by the extant Ophioplinthaca , Ophiocamax , and Ophiomitra ( Thuy et al. 2012) . The only known fossil record of this clade apart from Ophiosternle is the material from the Miocene of Japan described as Ophiocamax sp. by Ishida (2001). O. crinitum thus represents the oldest occurrence of the Ophioplinthaca – Ophiocamax – Ophiomitra clade. The assessment of its exact position within this group, however, requires further research.

Ophiosternle crinitum is the first ophiacanthid brittle star known from the Kimmeridgian. The fossil ophiacanthids which are stratigraphically nearest to O. crinitum are the species described by Hess (1965a, 1966, 1975a, b) from the Oxfordian of Switzerland and France on the basis of dislocated lateral arm plates. Among these, however, only Ophiacantha? constricta Hess, 1966 bears a certain resemblance with O. crinitum , especially in terms of number of arm spines and ventrally protruding ridge bearing the spine articulations. The lateral arm plates of O.? constricta , however, are considerably smaller and more fragile than those of O. crinitum , and furthermore lack the conspicuous pointed ventral extension, making a confusion unlikely.

Quenstedt (1876) described a specimen from the same locality as O. crinitum under the name Ophiura annulata Quenstedt, 1876 . These share a superficial similarity with the above−described material as far as the large, short arm segments, the long arm spines and the large radial shields are concerned. Preservation of the type specimen, however, precludes any further comparison: the disc exposes only the dorsal side, and the arms are so heavily worn as a result of the preparation process that only a few spines and the vertebrae remain visible (hence the species adjective annulata ). Ophiura annulata should therefore be considered a nomen dubium.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 1. Ophiacanthid brittle−star Ophiosternle crinitum (Quenstedt, 1876), GPIT/69/96−24 (holotype), from the Reef debris beds within the Mergelstetten Formation (Hybonoticeras beckeri Zone, Lithacoceras ulmense Subzone), latest Kimmeridgian, Late Jurassic of Buchenbrunnen near Steinenfeld, S−Germany. A. Complete specimen in ventral view. B. Detail of dorsal side showing arm base and distal tip of radial shields. C. Detail of disc in dorsal view. D, E. Detail of disc in ventral view; photograph (D) and explanatory drawing (E).

Fig. 2. Ophiacanthid brittle−star Ophiosternle crinitum (Quenstedt, 1876), from the Reef debris beds within the Mergelstetten Formation (Hybonoticeras beckeri Zone, Lithacoceras ulmense Subzone), latest Kimmeridgian, Late Jurassic of Buchenbrunnen near Steinenfeld, S−Germany. A. GPIT/69/96−24 (holotype). Detail of proximal arm segments in ventral view without arm spines (A1) and with arm spines (A2). B. 96/23 (paratype).Arm fragment in ventral view (B1), proximal arm segments in lateral view (B2). C. GPIT/AS/56 (paratype). Complete specimen (C1) and detail of proximal to median arm segments (C2) in dorsal view.