Ecospace utilization, paleoenvironmental trends, and the evolution of early nonmarine biotas

Paleozoic nonmarine trace fossils evidence paleoenvironmental trends that are linked directly to progressive ecospace utilization (spatial and functional). These trends resulted in a considerable increase of ichnodiversity throughout the Paleozoic. Colonization of nonmarine settings began as early as the Late Ordovician, as recorded by the presence of terrestrial burrow systems associated with ancient soils. In the other cases, the presence of "nonmarine" trace fossils in Cambrian and Ordovician sedimentary rocks was the result of temporary colonization by marginal marine organisms and does not reflect establishment of true fresh-water faunas. Devonian trace fossils have been reported from alluvial and alluvial-lacustrine transitional environments. Surface arthropod trackways are dominant in marginal lake settings, representing the development of true nonmarine biotas. The presence of vertical burrows in fluvial deposits records the establishment of deep suspension-feeding infaunal organisms. Trace fossils from the Carboniferous reflect migration farther into the lakes. Ichnofossils have been reported from shallow and deep lake deposits and from alluvial and alluvial-lacustrine transitional environments. Lake assemblages are dominated by surface grazing trails. The Permian was characterized by the establishment of a deep deposit-feeding infauna, commonly recorded by the Scovenia ichnofacies represented by backfilled meniscate burrows.