Inexpectacantha Thuy 2011

Genus Inexpectacantha Thuy, 2011 Type species

Inexpectacantha acrobatica Thuy, 2011, by original designation.

Ophiacanthid with small to moderately large LAPs of thick, massive and generally bulging aspect; outer surface devoid of conspicuous ornament; no spurs on outer proximal and inner distal edges; small, earshaped spine articulations freestanding or in shallow depressions of bulging distal portion of LAP; spine articulations commonly tilted; ventral lobe rugged to irregularly corrugate, commonly confluent with outer surface stereom; spine articulations not sharply bordered proximally and with dorsalward increase in size; short, broad, dorso-proximalwards bent, tongue-shaped ridge devoid of kinks or widened parts; tentacle notch very small, poorly defined.

Inexpectacantha acrobatica is probably one of the best-known fossil ophiuroids. In fact, the original description was based on outstandingly well-preserved material from the Pliensbachian of France, comprising both articulated skeletons and dissociated skeletal parts, including LAPs, allowing for a morphological analysis of exceptional detail (Thuy 2011).As a result, fossil dissociated LAPs compatible with the LAP morphological diagnosis of Inexpectacantha are unambiguously identifiable and thus provide evidence for bracketing the divergence time of the genus.

The LAPs of Inexpectacantha are among the most characteristic of all ophiacanthid LAP types. In fact, the a massive, thick, round and bulging general aspect, the lack of conspicuous outer surface ornament and spurs on the outer proximal and inner distal edges, small spine articulations with dorsalward increase in size and not bordered proximally by a ridge, a large, broad, tongue-shaped ridge devoid of kinks, extensions or strongly widened parts on the inner side, and very small, poorly defined tentacle notches are a combination of characters not found in any other ophiacanthid genus. Superficial similarities are shared with the LAPs of Ophiochondrus Lyman, 1869, in which, however, the spine articulations are generally not oblique and not confluent with the outer surface stereom. In addition, the LAPs of Ophiochondrus are less massive, and the ridge on the inner side displays a ventro-proximalwards projecting extension.

Greatest similarities are shared with the LAPs of Ophioleviathan gen. nov. (see below), particularly with respect to the massive and thick general aspect, and the shape of the spine articulations and of the ridge on the inner side. The LAPs of Ophioleviathan gen. nov., however, are significantly larger than those of Inexpectacantha and display larger, near-vertical spine articulations in deeper notches and composed of distal and proximal rather than dorsal and ventral lobes, with a strongly thickened distal lobe. Nevertheless, as discussed below, the LAP morphologies of both genera as so similar that they most likely represent sister taxa.

Interestingly, the currently known species of Inexpectacantha fall into two groups on the basis of LAP morphology: I. ritae sp. nov. and I. lunaris (Hess, 1962), plus I. weisi sp. nov. and I. acrobatica. The two last named are characterised by generally smaller LAPs with smaller height/width ratios, spine articulations freestanding rather than in notches, a more coarsely meshed stereom on the outer surface, and lip-shaped distal knobs in the spine articulations of the distal LAPs. Most of these characters are typically indicative of a paedomorphic condition (Stöhr 2005, 2011). Indeed, the articulated specimens of Inexpectacantha acrobatica display a strikingly paedomorphic general morphology (Thuy 2011).