Ophiojagtus, 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 : 214-215

Genus Ophiojagtus gen. nov.

urn:lsid:zoobank.org:act:0B5FB6EE-42F6-43D5-BC67-41AE35B82EE3

Type species

Ophiojagtus acklesi sp. nov. by present designation.

Other species included

Hemieuryale? argoviensis Hess, 1966 ; Ophiojagtus irimurai sp. nov. and Ophiosmilax? alternatus Kutscher & Jagt, 2000 .

Diagnosis

Ophiacanthid with stout, thick, strongly curved LAPs generally with a large height/width ratio; outer surface devoid of ornament elements; no spurs on outer proximal and inner distal edges; ventral portion of LAPs long to extremely long, strongly protruding ventralwards and often with widened ventral tip; proximal edge commonly with proximally pointing protrudion; large, ear-shaped spine articulations composed of thick, continuous volute, freestanding on bulging distal portion of LAP and not bordered proximally by a ridge-like structure; broad, well-defined ridge on the inner side of the LAPs, generally with dorso-proximally pointing dorsal part; tentacle notch large but poorly defined and generally shallow.

Etymology

Genus named in honour of my friend and colleague John W.M. Jagt for his generous support in the course of the present project and for his outstanding contributions to ophiuroid palaeontology; gender masculine.

Remarks

The Mesozoic ophiuroid fossil record includes a highly distinctive type of dissociated LAPs characterised by a thick and massive general aspect, very long, conspicuous ventral portion, large, ear-shaped spine articulations freestanding on the bulging distal portion of the LAP and composed of a thick, continuous volute, and the absence of outer surface ornament and spurs. This combination of characters is not found in any other ophiuroid LAP type. A new genus, Ophiojagtus gen. nov., is thus introduced here to accommodate the LAP type in question.

Previous records which can be assigned to this new genus include Hemieuryale ? argoviensis Hess, 1966 from the Oxfordian of Switzerland and France ( Hess 1966), Ophiosmilax ? alternatus Kutscher & Jagt, 2000 from the Campanian and Maastrichtian of Germany, Denmark and Belgium ( Kutscher & Jagt 2000) and records of the latter form from the Turonian of the Czech Republic and Tunisia ( Štorc 2004). Astonishingly, the striking similarities between these two species have not been noted previously. As a consequence, they were assigned to completely different genera belonging to unrelated families ( Smith et al. 1995). On account of fundamental differences in spine articulation structure, assignment either to the Hemieuryalidae (Hemieuryale) or the Ophiomyxidae ( Ophiosmilax Matsumoto, 1915 ) is tenable. Rather, the presence of large, ear-shaped spine articulations with a sigmoidal fold in combination with a series of small perforations on the inner side rather than a single large one near the ventral part of the ridge is suggestive of strong ophiacanthid affinities, however atypical this LAP type might appear for an ophiacanthid.

Affinities within the Ophiacanthidae , in contrast, are much more difficult to resolve. In spite of the relatively large, poorly defined tentacle notch in the LAPs of Ophiojagtus gen. nov., the tentacle pore in the context of the articulated arm was presumably small, considering the extremely long ventral portion of the LAPs. Thus, it seems probable that Ophiojagtus gen. nov. belongs to the small-pored ophiacanthids. Closest similarities seem to be shared with the LAPs of Inexpectacantha , mainly on account of the massive, thick general aspect, the position of the spine articulations, and the lack of outer surface ornament and of spurs. However, it is likely that these similarities are superficial, considering the significantly different spine articulation morphologies and ridge shapes. The LAPs of Ophiojagtus gen. nov. appear to be so highly derived that the phylogenetic relationships of the genus cannot be further explored on the basis of LAP morphology.

As already hinted at by Štorc (2004), the LAPs here assigned to Ophiojagtus gen. nov. remained morphologically conservative over considerable time intervals. Although it seems very unlikely that a single species spanned some 20 Ma, as suggested by Štorc (2004), it can be challenging, if not impossible, to differentiate stratigraphically close LAP types of Ophiojagtus gen. nov., in particular when only few LAPs are available. In the present study, a total of four LAP types of Ophiojagtus gen. nov. are characterised and identified at the species level. A few additional occurrences of the genus, including those recorded by Štorc (2004) and Štorc & Žítt (2008), could not be assigned with certainty and are thus treated here as indeterminate records of the genus.