Fossil pushes back land-animal debut

Five-toed foot among new finds that help fill in evolutionary picture

A foot buried beneath Scottish soil for at least 345 million years pushes back the timeline for the appearance of the first four-legged creatures that spent their lives on dry land.

LAND TOE Five toes found near the Scottish fishing village Burnmouth belong to new caches of fossils helping to reveal how animals transitioned from fins to feet. J. Clack

“This is the earliest and smallest foot ever found with five digits,” says paleontologist Jennifer Clack of the University of Cambridge, England. “It tells us that terrestrialization occurred much earlier than we had a hint of before.” Feet with five toes tend to be good at bearing weight and rotating on land, she says.

The specimen, 20 million years older than any known five-toed fossil, is just 10 millimeters across and comes from an unknown species. It’s one of a slew of new finds described by Clack and colleagues online March 5 in the Proceedings of the National Academy of Sciences. New caches unearthed near rivers and coasts in Scotland and Canada are helping fill in a blank chapter in the evolution of life on land.

Before about 360 million years ago, fishlike creatures could drag themselves along the shores of their watery homes. Then a massive extinction wiped out half of all vertebrate groups on the planet. But by about 345 million years ago, the planet teemed with a variety of creatures that lived on dry land.

What happened to spur this exodus from water after the extinction has remained a mystery. Few terrestrial fossils have been found that date to between 360 million and 345 million years ago. Some have argued that little evidence has turned up because there is little to be found. A 2006 paper in the Proceedings of the National Academy of Sciences suggested that lower levels of oxygen during this time delayed the rise of air-breathing land creatures.

The new finds show that some terrestrial life had rebounded within 10 million years of the extinction. “This discovery clearly refutes the hypothesis of … low oxygen levels,” says paleontologist Philippe Janvier of the National Museum of Natural History in Paris.

At the same Scotland location that yielded the five-toed foot, researchers also found a jawbone from Crassigyrinus, a creature that grew up to 2 meters in length. Plants and small shelled invertebrates turned up at another new site called Willie’s Hole, which also held the remains of more than 100 four-limbed creatures.

“We’re seeing a different cast of characters showing up after the extinction,” says paleontologist Michael Coates of the University of Chicago.

Clack’s team needs to clean the fossils and take a closer look to chart the emergence of this diverse post-extinction ensemble. The researchers hope to identify which species ultimately gave rise to creatures that walk, crawl and slither the Earth today.

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