New ophiacanthid brittle stars (Echinodermata: Ophiuroidea) from the Upper Triassic of Japan: first insights into the origin and evolution of an extant deep-sea group.

Well preserved, articulated brittle star skeletons from the early Carnian (early Late Triassic)HalobiaShales of Yamaguchi, Japan, are described as a new genus and species of the family Ophiacanthidae:Leadagmara gracilispina. The new form constitutes the oldest unequivocal representative of this extant ophiuroid family, significantly extending its stratigraphical range. The first cladistic analysis of the Ophiacanthidae including all extant genera suggestsL. gracilispinaholds a basal, but not the basalmost, position within the family, forming a sister group with the extantOphiomedeaandOphioprististo the other ophiacanthids except forOphiologimus, which itself is sister to all other ophiacanthids. These results imply that ophiacanthids originated in the pre-Carnian. A critical re-evaluation of the ophiacanthid fossil record reveals that the vast majority of records consist of dissociated lateral arm plates, which provide no insights into divergence times of the lineages. Among the few records of articulated specimens, onlyInexpectacantha acrobaticafrom the Pliensbachian of France proved sufficiently well-known to yield phylogenetically relevant information. Its position within the ophiacanthid clade indicates that more than half of the extant ophiacanthid lineages, in particular the former ophiacanthinids and ophiotominids, must have diverged by the Pliensbachian. http://zoobank.org/urn:lsid:zoobank.org:pub:F64E44C3-7275-4EC8-B048-4C7B2C6D57EF [ABSTRACT FROM AUTHOR]