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Primitive fish with 'butt fins' reveals evolution's quirky path

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Previously, researchers weren't sure exactly what to make of the creatures' weird fins. Some thought the anal appendages might actually have been displaced from another part of the animal's body in the fossilization process.

Early body plan

The researchers weren't convinced of the displacement explanation. By comparing the 3D surfaces of the fossil under a microscope, the team concluded that the fin pair was actually located below the anus on the living creature. Though it's not clear exactly how the fins were used, the fins most likely helped the fish get around, Sansom said.

The fossil dates to a critical period of vertebrate evolution: Jawed and jawless vertebrates diverged roughly around this time. Eventually jawed fish developed paired fins (but not anal fins) that evolved to become arms and legs. In fact, the same genes code for shark fins and human limbs.

The discovery changes the view of how fishes were evolving at this time, Sansom said.

"Rather than gradual acquisition of complex characteristics, maybe there was a bit more experimentation and odd acquisitions," he said.

Specialized trait?

The finding of paired anal fins is "beautifully illustrated," said Michael Coates, a vertebrate paleontologist at the University of Chicago, who was not involved in the study.

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