Unusual Anal Fish Fin Offers New Insight into Ancient Evolutionary History of Limbs

An ancient fish fossil may provide new insight into the way paired appendages--such as our legs--evolved. Researchers have uncovered the remains of Euphanerops, a jawless fish that swam the seas about 370 million years ago and possesses an unusual pair of anal fins.

What's so unusual about the fins? Up until now, anal fins have only been seen on jawed fish--and these have been unpaired. The period when Euphanerops existed was also a crucial time for evolution--a deep split between jawed and jawless fish occurred, creating the main divisions of vertebrates that are still alive today.

"Fins are the world's first limb-like appendages," said lead author Robert Sansom in an interview with Discovery News. "Paired limbs would subsequently develop from paired fins in the transition from sea to land, but the first evolution of paired appendages was a big, important step in the evolution and development and vertebrates."

Sansom first came across the paired fins as part of a study of Euphanerops fish fossils in Quebec, Canada. He employed 3D surface scans of the fossils and compared specimens preserved in different conditions. Eventually, he was able to determine that there were two fan-shaped anal fins on the left and right side of the fish's body.

"The unusual paired anal fin of Euphanerops lends support to the idea that there was some degree of development and evolutionary experimentation in some fish," said Sansom in a news release. "After the Devonian period and the extinction of a lot of species, the jawed vertebrate body exhibits fewer deviations from the formula of paired pectoral, paired pelvic, unpaired dorsal and unpaired anal appendages."

The research itself could shed light on the evolutionary history of fish and, in turn, the history of land animals. It could also allow researchers to better understand the timing and sequence of events underlying the origin and diversification of vertebrate appendages.

The study detailing the findings is published in the Royal Society's journal Biology Letters.