Fossils From Scotland Reveal That Tails of Fish, Tetrapods Evolved Differently

A new study from the University of Pennsylvania reveals that tails of four-limbed animals and fish evolved separately and are two different structures. The new study breaks the long-held recapitulation theory.

According to the study published in the journal Current Biology, Lauren Sallan, an assistant professor in the School of Arts & Science's Department of Earth and Environmental Science, discovered that the tails of tetrapods (e.g., lizards, elephants, etc.) are different from that of fish by analyzing 350-million-year-old fossil of fish hatchlings.

The fossil specimens are from a fish species called Aetheretmon valentiacum, which were discovered in Scotland years ago. The fossils included a 3-centimer long specimen of the said ancient fish, which, as the study notes, is "the earliest known stage of development for such fishes."

"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 Sallan via Science Daily. "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."

The discovery breaks previous scientific claim, specifically the recapitulation theory, which states that "growth and development of organisms takes them through stages that mirror the evolutionary steps from simple to more complex organisms."

"What this shows is that ancient fish and modern fish had the same developmental starting point that has been shared over 350 million years," Salan said.

"It's not the ancestral tail showing up in modern teleost larvae; it's that all fish have two different structures to their tail that have been adjusted over time based on function and ecology for all of these species," she added.