Distribution of biogenic structures in Paleozoic nonmarine and marine-margin sequences; an actualistic model

T-Many benthic invertebrates, especially insects (e.g., ephemeropterans and chironom- ids), oligochaetes and molluscs, burrow in and disrupt sediments accumulating in modem lakes and streams. However, the body-fossil record of most of the common nonmarine burrowers does not extend back before the Mesozoic, suggesting that aquatic sediments were not colonized by burrowers until or after the Permo-Triassic. Compilation of published descriptions of biogenic structures in Devonian through Triassic nonmarine rocks supports this suggestion. There is a marked increase in percent of studies reporting burrows in Permian and Triassic rocks and a corresponding decrease in the percent of studies with no reported biogenic structures. Benthic diversity generally decreases with decreasing salinity and in estuaries both abundance and species diversity are lowest where salinity fluctuations are greatest. Abundance and diversity increase with establishment of freshwater conditions. Given deposition in environments with similar trace-preservation potentials, trends similar to faunal trends of the present should be recognizable in Paleozoic marine to nonmarine transition ichnofaunas; bioturbation also should decrease with lower salinity. Within a Lower Pennsylvanian coal-bearing sequence in the northern Cumberland Plateau of Tennessee, sandstones and siltstones deposited under marine-dominated conditions are highly burrowed with a diverse ichnofauna; those deposited in a deltaic setting have zones with abundant traces; those deposited in coastal low-sinuousity fluvial environments have only scattered horizons with traces. Considering the apparent depauperate Pennsylvanian fresh- water infauna, zones with extensive burrowing or diverse ichnofaunas within Pennsylvanian marine to nonmarine sequences probably indicate colonization after a marine or brackish water incursion rather than establishment of a freshwater infaunal community.