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Anal fins hold key to evolutionary history

10 April 2013, by Harriet Jarlett

A pair of anal fins on a fossilised fish may give scientists a deeper insight into the evolution of our ancestors.


The study, published in Biology Letters shows Euphanerops, a 400 million year old jawless fish, had the unique characteristic of a pair of fins located behind its anus.

'The normal plan for a standard fish body is to have two pectoral fins, two pelvic fins and an anal fin.' explains Dr Robert Sansom from the University of Leicester, who led the study. ' Euphanerops doesn't have the paired fins in its shoulder or pelvic region but it does have them behind the anus. To the best of my knowledge it's the only animal that has this characteristic. It's unique.'

Euphanerops lived during the Devonian – an era which, due to the prolific evolution of fish during it, has been dubbed the Age of Fishes.

Jawless fish from this time tended to have only single fins, and resembled something a bit like an eel. So when Sansom and his colleagues noticed this fossil, from a collection in Quebec, Canada, had a pair of fins they were understandably surprised.

Whilst the fins would have had some impact on how the fish moved, Sansom says it is unknown whether or not these paired fins had a specific function.

During the Devonian our early ancestors, jawed vertebrates, began to evolve and eventually created the standard body plan of two arms, or fins, or wings that we know today.

'Jawless fish didn't have anything like that, they just had tail and dorsal fins.' explained Sansom. 'To develop a paired structure requires a completely different developmental set up.'

Euphanerops reconstruction

Euphanerops reconstruction.

Euphanerops is placed as a key character for reconstructing our own evolutionary history.

'This particular characteristic occurs right at the split where our ancestors, the jawed vertebrates, started evolving so it's very important to help us understand how we ended up the way we are,' says Sansom.

To establish that there were indeed two fins the scientists looked at the fossil under a microscope which takes 3D surface scans. This enabled them to analyse the bones whilst they were still embedded in the surrounding rock.

'It's more likely that before evolution settled on the body plan of having two arms and two legs our ancestors were trying out different body plans and various unusual things occurred that no longer exist today,' he explains.

Robert S. Sansom, Sarah E. Gabbott, and M. A. Purnell (2013) Unusual anal fin in a Devonian jawless vertebrate reveals complex origins of paired appendages Biol Lett doi:10.1098/rsbl.2013.0002

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