Aetheretmon (facing right), an early ray-finned fish, swims in a 348-million-year-old river in Foulden, Scotland. Aetheretmon exhibits the ancestral state of two distinct 'tails', fleshy tail above and caudal fin below. The 32,000 species of living teleost fishes (the pufferfish, center facing left) have lost the upper tail, while early tetrapods (upper left) lost the lower caudal fin. Thus, living fish and tetrapod tails are entirely distinct structures. CREDIT John Megahan
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