Tetrapod Evolution

Tetrapods include all extant limbed vertebrates (and those that lost limbs, such as gymnophionans and snakes) and many or all extinct ones (depending on the adopted definition). This large clade is represented by over 21,000 extant species: more than 7,000 species of extant amphibians and more than 13,000 species of amniotes, which include mammals and reptiles, the latter also including birds in many recent taxonomies. The origin of tetrapods can be followed back into the Early Devonian (419–393 million years ago), shortly after tetrapodomorphs (as stem-tetrapods are called) diverged from dipnomorphs (the largest clade that includes lungfishes but not tetrapods). The first tetrapodomorphs retained paired fins, but by the Middle Devonian (393–383 million years ago), the first limbed vertebrates had appeared. The tetrapod crown (the smallest clade that includes lissamphibians and amniotes) first appears in the fossil record in the Early Carboniferous. The group subsequently diversified fairly quickly and occupied a diversity of habitats (saltwater, freshwater, and terrestrial) by the Early Carboniferous. Amniotes appeared no later than about 317 million years ago (Ma), although they remained a minor component of the terrestrial biota until the Early Permian. The origin of turtles remains contentious, but the first undisputed stem-turtle and lepidosauromorphs (squamates, Sphenodon, and extinct relatives) are known from the Triassic, whereas archosauromorphs (which include birds and crocodiles) can be traced back to the Late Permian. Much of what we know about early tetrapod evolution is based on the fossil record, but our understanding of the subsequent evolution of the group has progressed tremendously in recent years through molecular phylogenetics. This article is organized into thematic sections, such as tetrapod origins, biodiversity evolution, and mass extinction events, as well as systematic sections that deal with various clades, especially (but not only) extant ones. Throughout, the emphasis has been on recent papers because their bibliographies usually include references to older, influential publications, although some older, very important papers are covered too.