Fish 'tails' result from outgrowth and reduction of two separate ancestral tails

The symmetrical, flexible teleost fish 'tail' has been a prime example of recapitulation - evolutionary change (phylogeny) mirrored in development (ontogeny). Paleozoic ray-finned fishes (Actinopterygii), relatives of teleosts, exhibited ancestral scale-covered tails curved over their caudal fins. For over 150 years, this arrangement was thought to be retained in teleost larva and overgrown, mirroring an ancestral transformation series. New ontogenetic data for the 350-million-year-old teleost relative Aetheretmon overturns this long-held hypothesis. The ancestral state consists of two outgrowths with distinct organizers and growth trajectories; a lower median fin turned caudal fin, and an upper vertebrae-bearing tail, equivalent to that of tetrapods. These two tails appear at a shared developmental stage in Aetheretmon, teleosts and all living actinopterygians. Ontogeny does not recapitulate phylogeny; instead, differential outgrowth determines final morphology. In Aetheretmon and other Paleozoic fishes, the vertebrae-bearing tail continues to grow beyond the caudal fin. In teleosts, and some others, a stunted tail is eclipsed by the upward-expanding caudal fin, rendering a once ventral body margin as the terminus. The double tail likely reflects the ancestral state for bony fishes. Many tetrapods and non-teleost actinopterygians have undergone body elongation through tail outgrowth extension, by mechanisms likely shared with distal limbs. Teleosts have gone to the other extreme; losing tail outgrowth for functional reasons. Recognition of the tail as a limb-like outgrowth has important implications for the evolution of vertebrate form.