An unusual fossil of Euphanerops, a fossil jawless fish that swam in the seas around 370 million years ago, has fins behind its anus - a pair of them. The find makes the fish one of the first vertebrate to develop paired appendages such as fins, legs or arms.
However, their positioning is the interesting part.
"Euphanerops is unique because its anal fin is paired meaning there is one fin on each side of the fish. Up until now anal fins have only been seen on jawed fish where they are unpaired and this is true of both extinct and modern fish. The age of Euphanerops is important as it dates from the time of a deep evolutionary split between jawed and jawless fish, the two main divisions of vertebrates alive today. As such, it represents an important stage in the evolution of paired appendages," explains Dr. Robert Sansom from The University of Manchester."It's not clear why the fins are positioned so far back on the fish, or what advantage they might have provided. However, they do show that our early vertebrate ancestors tried out lots of different body plans before settling on two arms and two legs. If they hadn't then our bodies would have looked very different!"
The paired fins were found as part of a study of Euphanerops fossils in Quebec, Canada. 3D surface scans of fossils and comparison of specimens preserved in different conditions revealed that there were two fan-shaped fins, a left and a right. Sansom's research on the paired fins followed on from a 2009 study of early vertebrate evolution and fossil preservation with colleagues from The University of Leicester.
"The unusual paired anal fin of Euphanerops lends support to the idea that there was some degree of developmental and evolutionary experimentation in some fish. 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 discovery of new anatomical conditions will hopefully shed more light on the timing and sequence of the events underlying the origin and diversification of vertebrate appendages," says Sansom.
Published in Biology Letters.