THE HUNSRÜCK SLATE (LOWER DEVONIAN, LOWER EMSIAN) IN THE CONTEXT OF THE PALEOZOIC HISTORY OF THE OPHIUROIDEA (ECHINODERMATA)

Revisionary work indicates the presence of 13 genera and 19 species of ophiuroids in the Hunsrück Slate, which makes it the most diverse Paleozoic ophiuroid fauna known. Nevertheless, the fauna has had little impact on our understanding of the evolutionary history of ophiuroids because adequate preparation techniques only became available with recent advances in airbrasive technology. For most species, hundreds of specimens of well-preserved, fully articulated individuals are available in both dorsal and ventral exposures.

Members of all 8 best-understood Devonian ophiuroid families are present in the Hunsrück Slate, and to that extent the fauna is plausibly representative of the mid-Paleozoic. Of the 13 genera in the Hunsrück, 6 have been reported elsewhere, and most of the endemic genera are assigned to families that are well-established beyond the Hunsrück. Notable exceptions are the family Eospondylidae, which has been reported from only one other Devonian locality, and the Palaeuridae, which is only known elsewhere from one Ordovician genus. Most families in the Hunsrück Slate originated in the Ordovician and only one is currently known from rocks younger than the Early Carboniferous. Two of three known multi-armed Paleozoic species are from the Hunsrück Slate. The genus with the most characters retained among surviving genera is Ophiurina lymani, but its family has not been reported from rocks younger than the Early Carboniferous.

Ecology is difficult because many fossils were transported prior to death and larger fossil associations were often broken up by roofslate processing before their discovery. Morphological diversity appears to exceed that recognized in modern associations of apparent equivalent extent. Some ophiuroids converge on asteroids, for example in tube foot size, thereby perhaps suggesting some similarities in life mode. Taxa with both large and small tube feet are present in abundance. Hunsrück adults are larger than representative congeneric taxa elsewhere. Monospecific mass occurrences allow documentation of incidence of sublethal arm damage, which is always minor. Furcaster, Bundenbachia, and Encrinaster are the most common genera in the Hunsrück Slate, and their families also have the longest and best documented Paleozoic ranges.