Fish Fossils Reveal How Tails Evolved, Penn Professor Finds

Despite their obvious physical differences, elephants, lizards and trout all have something in common. They possess elongated, flexible structures at the rear of their bodies that we call tails. But a new study by a paleobiologist reveals that the tails of fish and the tails of tetrapods, or four-limbed animals, are in fact entirely different structures, with different evolutionary histories.

With an analysis of 350-million-year-old fossil fish hatchlings, Lauren Sallan, an assistant professor of earth and environmental Science, showed that these ancient juvenile fish had both a scaly, fleshy tail and a flexible fin, one sitting atop the other. A similar dual tail structure is seen in the embryos of modern teleosts, a group of ray-finned fish that make up more than 95 percent of living fish species.

Over evolutionary time, to adapt to their environments, adult teleosts and tetrapods each lost one of these 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,” Sallan says. “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."

Sallan reported her findings in the journal Current Biology.

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