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Fins, not feet, may have made fossil ‘footprints’

Scientists studying fossil ‘footprints’ may have followed fish where they thought they tracked tetrapods.

This morphological mix-up is suggested by a paper demonstrating ‘walking’ behaviour in the African lungfish (Protopterus annectens).

Heather King, Neil ‘Tiktaalik’ Shubin and their colleagues at the University of Chicago set out to confirm claims that this animal indeed walked on four legs. As you can see in the video below, they did. Using video evidence they report in the Proceedings of the National Academy of Sciences (PNAS) that lungfish use their pelvic fins to lift themselves off the bottom of rivers and streams and walk and even bound, despite lacking feet.

The reason this is important is the species is one of the few remaining members of the sarcopterygian class. Sarcopterygian fishes were the mothers and fathers of the animals that later became the four-legged land animals researchers call tetrapods.

The only other known sarcopterygian is the ‘living fossil’ coelacanth, which can also move in a way reminiscent of terrestrial tetrapods. That lungfish use their pelvic fins to ‘walk’ on the bottom suggests that this skill could have appeared before tetrapods roamed dry land. (As their name suggests, lungfish have lungs that allow them to survive out of water, making them useful models for how fish might have migrated out of the world’s waters.)

This all means that distinctive fossil tracks that have been attributed to tetrapods in the past could actually have been made by fins rather than feet.

“Trackways lacking unambiguous evidence of digits, particularly those from the Devonian, when sarcopterygian fishes were diverse and abundant, are now open to question,” write the researchers in PNAS.

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