A WOOD-FALL ASSOCIATION FROM LATE EOCENE DEEP-WATER SEDIMENTS OF WASHINGTON STATE, USA

Abstract

Fossil wood fragments and an associated species-rich invertebrate assemblage, analogous to those found on wood falls in the deep sea today, were found in late Eocene deep-water sediments of the Lincoln Creek Formation in Washington State, United States. This assemblage is the earliest known complex deep-sea biologic community based on decaying wood as its primary source of nutrients. The 495 recovered fossils (exclusive of foraminiferans) belong to 21 species; 7 species relied directly on the wood, either by ingesting it or by feeding on xylophagous microbes; these species are also the most abundant. Seven species were predators or scavengers that were most likely attracted by the wood-dependent species. The remaining seven species represent predators, detritus feeders, and suspension feeders that may or may not have had a relation to the wood fall or its fauna. All species had a benthic mode of life, and pseudoplanktonic taxa are absent, indicating that the colonization of the wood began only once it had arrived on the deep-sea floor. The wood-dependent species belong to taxa that fill the same ecologic niche in the deep sea today, indicating that the modern wood-fall ecosystem had evolved at least by late Eocene time. There is no uniformity or specialization of dispersal strategies among the recovered taxa; they rather reflect those of the phylogenetic group to which they belong. The wood-fall assemblage described here shares several families with fossil whale falls and cold seeps but very few species, a condition that can also be observed at modern examples of these ecosystems.