A NEW SOURCE OF EXCEPTIONALLY-PRESERVED ARTHROPODS

Current reconstructions of the paleobiology of Paleozoic terrestrial ecosystems are based on a limited fossil record, reflecting the low preservation potential of most non-biomineralized terrestrial organisms. A purely uniformitarian approach, comparing the faunas of Recent and Paleozoic ecosystems, is considered inadequate. Exceptional faunas in which terrestrial elements occur are thus of fundamental importance to our understanding of the evolution of early terrestrial ecosystems, but few in number; the fossils themselves are frequently allochthonous. Further, the precise depositional contexts in which such faunas occur has been little studied, impacting on the search for other examples. Identifying primarily autochthonous faunas and elucidating their sedimentary context remains essential.

To refine and develop further predictive models of terrestrial faunal occurrence, Lower Carboniferous lithologies at Ballycastle, Co. Antrim, N. Ireland have been systematically processed for organically-preserved terrestrial arthropods; the presence/absence and relative abundance of different faunal elements has been correlated with specific lithofacies from identifiable continental paleoenvironments. The study has also identified a new, recurrent, source of exceptionally-preserved terrestrial arthropods. Stigmarian root casts contain within them rich assemblages of centipede and scorpion cuticle, but the most significant element is the articulated remains of three-dimensional mites. Their high degree of articulation implies these mites are autochthonous, entombed in situ by episodic infilling of voids created by the decay of root tissue. As a result of limited compaction (<15%) of the roots relative to the surrounding lithologies during burial the mites can remain three-dimensional. Stigmarian roots and tree trunks previously seen as occasional pitfall traps for terrestrial vertebrates should now also be examined as a new source of autochthonous terrestrial arthropod faunas.