Chordate phylogeny of major extinct (boxed) and extant (unboxed) taxa (modified3435) with maximum estimated total lengths (length data36).
Divergence times are based on best estimates of combined molecular and fossil data3537. The star represents the approximate appearance of gill filaments in the fossil record, which we propose were predominantly associated with acid-base regulation. Major increases in body size and/or reductions in body surface area to volume ratio occurred after this event and likely signify the transition of acid/base ion regulation from the skin to the gills.