BIOTURBATION AS A PROXY FOR INFAUNAL ANIMAL ACTIVITY IN PERMIAN – JURASSIC FRESHWATER DEPOSITS

"When were freshwater benthic habitats colonized by invertebrates? When did the substrate ecospace in freshwater habitats become fully used?” Given the establishment of a marine infauna during the Cambrian, the extension of burrowing activity into the substrate to a depth of a decimenter or more by the Ordovician, and the spatial connection between marine and freshwater systems, one would predict that freshwater benthic communities would have had deep infaunal components by the early Paleozoic. Substrate ecospace use in freshwater deposits of Permian through Jurassic age was evaluated using semi-quantitative assessment of bioturbation on vertical and bedding plane surfaces with five categories of bioturbation (Droser and Bottjer, 1986; Miller and Smail, 1997). The dataset consists of >28,000 observations of bioturbation in fluvial channel, floodplain, and lacustrine deposits exposed in the Transantarctic Mountains, Antarctica (ANT), Colorado Plateau (CO), and the northeastern (NJ, MA) United States (NE). In all facies, on both horizontal and vertical surfaces, category #1 (no bioturbation) is the category most commonly observed, and there is a monotonic decrease in observations of categories 2-5 representing increasing intensity of bioturbation. There are significant differences between facies, with the fluvial channel facies the least bioturbated and the lacustrine facies the most. Within facies, NE lake deposits are more bioturbated than ANT lake deposits on both horizontal and vertical surfaces. This probably reflects colonization by air-breathing organisms of the gently sloping margin of the asymmetrical rift lakes during low water stands. Similarly, higher levels of bioturbation in Triassic fluvial channel deposits in ANT may well have resulted from settling of arthropods, some of which have been semi-terrestrial rather than fully aquatic, during periods of low flow. In all facies bioturbation on bedding plane surfaces is greater than on vertical surfaces, indicating that infaunal animal activity was confined to surficial sediment. The bioturbation data indicate that Paleozoic lake and stream substrates, in contrast to co-eval marine substrates, did not teem with active infaunal animals, and that freshwater substrate ecospace was not fully used until the mid Mesozoic at the earliest.