Rethink on when animals left sea for land

SCIENTISTS WILL have to return to the drawing board on the theory of how and when creatures left the sea to walk on land.

The discovery of a set of 395 million-year-old footprints in the mud of a long-vanished seacoast has forced the change.

The original theory was so simple: Fish evolved fins that could work like feet, allowing them to leave the water and explore; eventually fins became feet, attached to animals that moved between water and land.

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The Acanthostega and Ichthyostega were early landwalkers, tetrapods, which could walk on land but also swim efficiently. The fossilised footprints discovered in Poland suggest this type of creature may have lived 18 million years before these animals.

The fossil record matched perfectly, with finwalker fossil discoveries sitting neatly in the record before those of the later landwalker amphibians.

This convenient theory has now been undermined after the discovery of footprints, trackways, found in stones extracted from the Zachelmie Quarry in the Holy Cross Mountains of southeastern Poland.

The footprints are those of a land walker, a tetrapod, with distinct front and back feet.

The problem is at 395 million years old they are 18 million years older than the oldest known tetrapod and also 10 million years older than the oldest finwalker fish.

“The discovery of the Zachelmie footprints substantially changes the context for future research on the origin of tetrapods,” said lead author Per Ahlberg and colleagues who describe their findings this morning in the journal, Nature.

For one thing the finwalkers, animals known as the elpistostegids, did not give way in an evolutionary manner to the later tetrapods. It turns out the animals coexisted for many millions of years.

The tetrapods might even have emerged first. But it’s not possible to tell because the fossils discovered so far do not support this theory.

Mr Ahlberg and his colleagues spoke on the fragmented nature of the fossil record. They now believe that the existing finwalker fossils probably date to the end of this animal’s evolutionary development, when they were on their last fins as it were.

They also believe that the discovery will alter our assumptions about where these animals lived. The prevailing theory holds that tetrapods likely evolved along rivers, where they could take to the land in search of food before returning to the river.

But the stones in which the footprints were found were formed from the mud flats along a sea coast.

The size of the footprints suggest a tetrapod perhaps 2.5m (8ft) long. These animals did not need to live along a quiet river. They were cavorting in “fully marine intertidal to lagoonal environments” 18 million years before the first tetrapod fossils turned up along former rivers.

This means tetrapod fossils might be more plentiful in marine rather than riverine environments, something that may help to fill in some of that missing fossil record.