Prehistoric Aetheretmon fish larvae had two tails—one fleshy and, the other, more fin-like. Now, an analysis of 350-million-year-old Aetheretmon fossils, published in Current Biology this week (December 5), suggests that fish kept their fins and lost their fleshy tails over evolutionary time, while tetrapods did the opposite, ditching their fins in favor of fleshy, vertebrae-bearing tails.
“The tetrapod tail likely started as a limb-like outgrowth in the first vertebrates, while the fish caudal fin started as a co-opted median fin, like the dorsal fin,” said coauthor Lauren Sallan of the University of Pennsylvania, in a statement. “All vertebrate tail diversity might be explained by the relative growth and loss of these two tails, with the remaining fleshy tail stunted in humans as in fishes.”
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