Dermocoma Hess 1964

Genus Dermocoma Hess, 1964

Type species

Dermocoma wrighti Hess, 1964, by original designation.

Diagnosis

Ophiacanthid with LAPs commonly displaying a fine vertical striation on their outer surface; welldeveloped spurs on the inner distal and, in some cases to a lesser extent, outer proximal edge; ventral portion of LAP strongly protruding ventro-proximalwards; up to six moderately large, ear-shaped spine articulations with continuous volute in notches of elevated distal portion of LAP; ridge on inner side of LAPs simple, continuous, generally slender, devoid of sharp kinks or conspicuously thickened parts; tentacle notch small to moderately large.

Remarks

Dermocoma was introduced by Hess (1964) on the basis of articulated specimens from the Middle Jurassic of Great Britain which were all assigned to the type, and at that time only known, species D. wrighti Hess, 1964. While the higher taxonomic position of Dermocoma was initially left open (Hess 1964), it was later used as type taxon for the introduction of the extinct family Dermocomidae Hess, 1972 (Hess 1972a). Thuy & Meyer (2013), however, showed it to be unambiguously assignable to the Ophiacanthidae.

Since the original description of Dermocoma wrighti provided only few data on LAP morphology, a reexamination of the type specimens was called for in order to work out a comprehensive LAP morphology including as many characters as possible. Dermocoma and its sister genus Alternacantha share basically similar LAP morphologies, as already noted above. While the median and distal LAPs of both genera may be almost indistinguishable, the proximal LAPs allow for an unambiguous distinction. In fact, in the proximal LAPs of Dermocoma the position of the dorsalmost spine articulation never alternates, the distal portion of the LAPs is less strongly elevated and the notches of the spine articulations are generally deeper. As in Alternacantha, however, distinction of species within Dermocoma on the basis of dissociated LAPs can be a major challenge with differences occasionally found in the finest details only.

The LAPs of Dermocoma can be easily distinguished from those of other ophiacanthids on account of the position of the spine articulations, the ventro-proximally protruding ventral part, and the simple ridge on the inner side devoid of sharp kinks and conspicuously widened parts. The LAPs assignable to Dermacantha gen. nov. (see below) share the greatest similarities with those of Dermocoma, differing only in having generally smaller spine articulations, a discontinuous volute of the spine articulations and a generally smaller height/width ratio. However, the similarities are so striking that Dermocoma and Dermacantha. gen. nov. are most probably closely related.

Many of the previously known Jurassic ophiacanthid records based exclusively on dissociated LAPs have turned out to be assignable to Dermocoma. Together with the new records described in the present study, Dermocoma is among the most speciose and most widely distributed ophiacanthids in Mesozoic shallow-water deposits. Scattered records from a deep-shelf to upper slope setting, however, suggests that the genus was also present in deeper settings. The closest extant relative Ophiocopa spatula, sister to the Dermocoma - Alternacantha clade (see above), inhabits upper to middle bathyal depths (279-965 m) of the Pacific (O’Hara & Stöhr 2006).