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

Abstract

Well preserved, articulated brittle star skeletons from the early Carnian (early Late Triassic) Halobia Shales 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 suggests L. gracilispina holds a basal, but not the basalmost, position within the family, forming a sister group with the extant Ophiomedea and Ophiopristis to the other ophiacanthids except for Ophiologimus, 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, only Inexpectacantha acrobatica from 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