Inexpectacantha lunaris ( Hess, 1962 ) 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 : 190-192

Inexpectacantha lunaris ( Hess, 1962) comb. nov.

Fig. 33 : 7-8

Hemieuryale? lunaris Hess, 1962: 627 , figs 29-33, 45-47.

Hemieuryale? lunaris – Hess 1964: 762, figs 4-10.

non Sigsbeia? lunaris – Kutscher 1996: 12, pl. 3 figs 6-8, pl. 4 figs 12-14. — Kutscher & Villier 2003: 189, pl. 6 figs 7-8, pl. 7 figs 1-2. — Thuy 2005: 41, pl. 4 figs 6-10. — Hess 2006: 66, pl. 29 figs 8-9.

Diagnosis

Species of Inexpectacantha with moderately large LAPs devoid of constriction but displaying a slightly depressed band of variable width along most of the proximal edge; outer surface with finely meshed stereom; up to six small, near-vertical spine articulations in shallow notches of bulging distal portion of LAP; ridge on inner side of LAP very broad and only slightly bent dorso-proximalwards.

Material examined

GZG.INV.78752, GZG.INV.78753 and GZG.INV.78754 (dissociated LAPs) from the Obtusum Zone, late Sinemurian of Bishop’s Cleeve, Great Britain; 17 dissociated LAPs from the late Pliensbachian of Seewen, Switzerland, the type material of Hess (1962); the original material of Hess (1964).

Description

Moderately large, dissociated LAPs of thick, massive and rounded aspect; proximal LAPs nearly twice higher than wide, distal ones slightly higher than wide; dorsal edge straight to slightly convex, oblique; no constriction; distal edge convex; proximal edge straight to slightly concave, devoid of spurs; slightly depressed band of varible width, from one-fifth to one-third of the total LAP width, along most of the proximal edge, widest in the middle, not reaching dorso- and ventro-proximal tips of LAP; depressed band not sharply separated from remaining outer surface distally and without conspicuous change in stereom mesh size. Six (proximal LAPs) to four (distal LAPs) small, ear-shaped, strongly oblique to near-vertical spine articulations in shallow notches of bulging distal edge; dorsalward increase in size of spine articulations and of gaps separating them; ventral lobe of spine articulations rugged, confluent with stereom of notch, merged proximally with almost equal-sized dorsal lobe into continuous volute; gap between spine articulations and distal edge of LAP nearly as wide as one spine articulation or slightly wider; spine articulations not sharply bordered proximally, not even by edge of notches. Ventral edge of LAPs oblique, straight; tentacle notch invisible in external view.

Inner side of LAP with very large, sharply defined, prominent, conspicuously broad, tongue-shaped ridge; ventral tip of ridge not merged with ventral portion of LAP; dorsal part of ridge wider than ventral one, weakly bent dorso-proximalwards; shallow, poorly defined furrow along distal edge of ridge; no spurs on inner side of distal edge of LAP; inner side of tentacle notch very small, weakly defined, almost indiscernible in many LAPs. Shallow, poorly defined, vertical furrow dorsally bordering tentacle notch. Articulated arm fragments from the Pliensbachian of Great Britain, assignable to this species, were described in detail and illustrated by Hess (1964).

Remarks

Hess (1962) originally described dissociated LAPs from the Pliensbachian of Switzerland as Hemieuryale ? lunaris , comparing them with Recent hemieuryalids such as Hemieuryale von Martens, 1867 and Sigsbeia Lyman, 1878 . He later ( Hess 1964) assigned dissociated LAPs and articulated arm fragments from the Pliensbachian of Great Britain to Hemieuryale ? lunaris , stressing that the minor morphological differences between the Swiss type material and the British specimens could be ascribed to the better preservation of the latter.

Following these two reports, however, confusion has arisen over the concept of Hemieuryale ? lunaris . First, Kutscher & Hary (1991) described dissociated LAPs from the Sinemurian of Luxembourg which they considered to be conspecific with Hess’s (1964) Brirish specimens but no with the type material from Switzerland. Then, Kutscher (1996) reported on dissociated LAPs from the Toarcian/Aalenian of Germany which he assigned to Hess’s (1962) species, tentatively transferring it to the extant hemieuryalid genus Sigsbeia . Finally, Thuy (2005) recorded dissociated LAPs from the Hettangian of Belgium and Luxembourg which were assigned to Sigsbeia ? lunaris . By that time, the concept of Sigsbeia ? lunaris had been inflated beyond meaningful limits as far as LAP morphology is concerned, including every type of thick, massive, rounded LAPs with small, more or less oblique spine articulations and a broad ridge on the inner side.

A re-examination of the original material of those three papers has allowed to disentangle the different LAP types previously assigned to S.? lunaris . Hess’s (1962) type material from Switzerland and the British specimens ( Hess 1964) are, indeed, conspecific and differ mainly in their state of preservation. They display the highly distinctive combination of characters typical of the ophiacanthid Inexpectacantha and are thus here transferred to that genus. The LAPs of Kutscher & Hary (1991) are here shown to be assignable to Inexpectacantha weisi sp. nov. Those of Kutscher (1996) and Kutscher &Villier (2003), in contrast, fundamentally differ from Inexpectacantha lunaris comb. nov., in particular with respect to the spine articulation morphology. They most probably even belong to a different family. Their exact systematic position requires more detailed investigations involving comparison with Recent non-ophiacanthid genera, which is beyond the scope of the present study.

Inexpectacantha lunaris comb. nov., as defined here, is unambiguously distinguishable from its congeners in displaying small, near-vertical spine articulations in shallow notches, a slightly depressed area along most of the proximal edge, and a very broad, weakly bent ridge on the inner side. Closest similarities are shared with the LAPs of Inexpectacantha ritae sp. nov., especially on account of the larger size, the larger general height/width ratio and the position of the spine articulations. Similarities with the LAPs of Ophioleviathan gen. nov. are more superficial and mainly pertain to the near-vertical spine articulations, the broad ridge on the inner side and the very narrow, slightly depressed band along the proximal edge. The LAPs of Ophioleviathan gen. nov., however, are considerably larger and display almost overturned spine articulations in deeper notches and with much thicker, more strongly prominent dorsal lobes.

Occurrence

Late Sinemurian to early Pliensbachian of Great Britain, late Pliensbachian of Switzerland.

Figures

Fig. 33. Fossil lateral arm plates (LAPs) of ophiacanthid brittle stars in external (a) and internal (b) views. 1-3. Inexpectacantha ritae sp. nov. from the Hettangian (Early Jurassic) of Bourglinster, Luxembourg. 1. MnhnL HE419 (holotype), proximal LAP. 2. MnhnL HE420 (paratype), median LAP. 3. MnhnL HE421 (paratype), distal LAP. 4-6. Inexpectacantha weisi sp. nov. from the Hettangian (Early Jurassic) of Fontenoille, Belgium. 4. MnhnL HE425 (holotype), proximal LAP. 5. MnhnL HE426 (paratype), median LAP. 6. MnhnL HE427 (paratype), distal LAP. 7-8. Inexpectacantha lunaris (Hess, 1962) comb. nov. from the late Sinemurian (Early Jurassic) of Bishop’s Cleeve, Great Britain. 7. GZG. INV.78752, proximal LAP. 8. GZG.INV.78753, proximal to median LAP. 9-10. Inexpectacantha acrobatica Thuy, 2011 from the early Pliensbachian (Early Jurassic) of Blockley, Great Britain (9) and the early Toarcian (Early Jurassic) of Aix-sur-Cloie, Belgium (10). 9. GZG.INV.78755, proximal LAP. 10. GZG.INV.78757, juvenile proximal LAP. One common scale bar per species.