Vertebrate preservation in fluvial channels
Abstract Two taphonomic modes for attritional vertebrate assemblages in channels are proposed, based on the sedimentary context of the vertebrate remains and taphonomic features of the bones themselves. The channel-lag mode includes bones that are buried with coarse lithologies near the bases of active channels. The channel-fill mode occurs in fine-grained to mixed fills of abandoned channels. The extreme for a channel-lag assemblage would be a cluster of allochthonous, abraded, unidentifiable fragments, and the extreme for a channel-fill assemblage would be a cluster of autochthonous, unbraded, complete skeletons. Between these extremes there is a broad spectrum of possible taphonomic histories for bones in channels, but distinct channel-lag vs. channel-fill modes can be recognized in fluvial deposits in different tectonic and climatic settings throughout the Phanerozoic. Physical and biological processes that affect the different modes produce different samples of vertebrate paleocommunities, with bones in the channel-lag mode representing transported remains from a variety of habitats, whereas channel-fill assemblages are more autochthonous and habitat-specific. Channel facies, channel pattern, and alluvial architecture are used to develop hypotheses concerning how the taphonomic modes relate to different scales of fluvial processes. Fluvial systems with numerous abandoned channels provide more sites for preservation of relatively complete fossil vertebrates in channel-fills, while systems that continually rework sediments by lateral migration preserve more vertebrate remains as channel-lags. Large-scale physical controls on channel pattern and fluvial architecture probably have had significant effects on the quality and quantity of the vertebrate record throughout the history of land vertebrates. Taphonomic modes provide a basis for comparing faunas with similar preservational histories throughout the geologic record, and they can help to minimize biases in important paleobiological parameters such as diversity estimates and the timing of appearance and extinction events.