THE PALEOENVIRONMENTAL HISTORY OF STEM-TETRAPODS

Interest in the environmental origin of the first tetrapods has been reinvigorated in recent years following the discovery of digit-bearing marine tracks from the early Middle Devonian of Poland. This has reopened old questions about whether the first tetrapods arose in marine or freshwater environments.

To test the paleoenvironmental origins question, I trace the ecological history of Devonian stem-tetrapods in a combined phylogenetic and paleoenvironmental analysis. Sedimentological, assemblage, and isotopic data are analyzed to elucidate how the Devonian rock record informs the evolution of stem-tetrapod habitats, and are used to reconstruct the ancestral environments of the first digit-bearing taxa.

Data suggest that tetrapodomorphs (total-group tetrapods) took a freshwater or marginal marine origin; that the first near-tetrapods likely inhabited coastal environments and later moved shoreward (possibly including the Polish trackmaker); and that the first digited tetrapod body fossils appear following a recent history where extramontane freshwater systems were the ancestral paleoenvironment.

These transitions were unfolding during a time in the Devonian when nearshore habitats were under significant stress. Thus, the selective survival of freshwater stem-tetrapods may yield important clues as to how continental systems worked as safe havens for the first terrestrial vertebrates on the road to the Carboniferous.