Ishidacantha, 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 : 171

Genus Ishidacantha gen. nov.

urn:lsid:zoobank.org:act:0AA3C0DE-CB77-4AA9-847F-782B5B76F6CC

Type species

Ophiopholis? trispinosa Hess, 1965 , by present designation.

Other species included

Ishidacantha hirokoae sp. nov. and Ishidacantha fuersichi sp. nov.

Diagnosis

Ophiacanthid with large to moderately large LAPs displaying a very small height/width ratio, with even proximal LAPs almost twice wider than high; ventral portion of LAPs very large, wide, strongly protruding ventro-proximalwards; fine vertical striation on at least part of the outer surface; two variably well-defined, horizontally elongate and generally protruding spurs on the outer proximal and inner distal edges; three to four relatively large, ear-shaped spine articulations on elevated distal edge of LAP; inner side of LAP with two separate ridges, even in proximal LAPs; proximal ridge generally much longer than distal one; deeply incised but relatively small tentacle notches commonly developed as within-plate perforations in distal LAPs.

Etymology

Genus named in honour of my friend Yoshiaki Ishida, for his outstanding contributions to ophiuroid palaeontology and, most importantly, for his friendship and indefatigable support, and from Acantha , a nymph in Greek mythology whose name literally translates to “thorny”; gender feminine.

Remarks

The ophiacanthid fossil record inlcudes an unusual type of dissociated LAPs charactersied by a very small height/width ratio, coarsely meshed or vertically striated stereom on the outer surface, a strongly protruding, very large and wide ventral portion, three to four relatively large spine articulations, two separate ridges on the inner side, and relatively small but deeply incised tentacle notches. LAPS of this type were described by Hess (1965a) from the Oxfordian of France as Ophiopholis ? trispinosa Hess, 1965 . Very similar LAPs from the Toarcian/Aalenian of Germany and the Toarcian of France were described by Kutscher (1996) and Kutscher &Villier (2003), respectively, and in both cases assigned to Sinosura brodiei (Wright, 1866) , an extinct genus with probable ophioleucinid affinities (Thuy et al. 2011). Surprisingly, neither the close morphological similarities between these occurrences nor their ophiacanthid affinities have been commented on previously.

A careful, SEM-supported re-examination of this material has now confirmed the ophiacanthid affinites and enabled a reinterpretation of the material in question, along with news finds from the Toarcian of France and the Callovian of India, as a new morphologically consistent ophiacanthid LAP type. The only other ophiacanthid LAP type to display comparably small height/width ratios and two separated ridges on the inner side even in proximal LAPs is found in Geromura gen. nov. (see above). The two LAP types, however, differ fundamentally with respect to the size of the tentacle notch. In fact, while the LAPs of Geromura gen. nov. display a genuinely large tentacle notch, those of the new type in question have a deeply incised, but ultimately relatively small notch. The size of the ventral portion of the present LAPs, the oblique ventro-proximal tip and the relatively small semi-circular outline of the actual tentacle notch strongly suggest that the articulated arms were covered by large ventral plates separated by the LAPs and encompassing small tentacle pores as defined by Thuy et al. (2012). The present LAP type thus belongs to a small-pored ophiacanthid, here described as Ishidacantha gen. nov., whereas Geromura gen. nov. clearly is a large-pored genus. This implies that the striking morphological similarities, in fact, reflect convergent evolution, presumably as a result of paedomorphosis leading to elongated arm segments, a typically juvenile character ( Stöhr 2005). At least some of the shared characters, such as the presence of two separate ridges on the inner side, might be the result of biomechanical constraints due to the small height/width ratios.