Ever Since Owen: Changing Perspectives on the Early Evolution of Tetrapods

Abstract

The traditional notion of a gap between fishes and amphibians has been closed by a wealth of fish-like fossil tetrapods, many discovered since the mid 1980s. This review summarizes these discoveries and explores their significance relative to changing ideas about early tetrapod phylogeny, biogeography, and ecology. Research emphasis can now shift to broader-based questions, including the whole of the early tetrapod radiation, from the divergence from other lobed-finned fishes to the origins of modern amphibians and amniotes. The fish-to-tetrapod morphological transition occurred within the Upper Devonian; the divergence of modern tetrapod groups is an Early Carboniferous event. Modern tetrapods emerged in the aftermath of one of the five major extinction episodes in the fossil record, but the earlier Devonian tetrapod radiation is not well understood. Tetrapod limbs, paired fins, and comparative developmental data are reviewed; again, research emphasis needs to change to explore the origins of tetrapod diversity.