Credit: GEOL. SOC. AM.

The oldest fossils of footprints ever found on land suggest that animals may have emerged from the seas much earlier than had been thought. Lobster-sized, centipede-like animals made the prints shown here about 500 million years ago. Previous fossils had indicated that animals took this step around 40 million years later.

R. B. MacNaughton and colleagues analysed about 25 rows of footprints preserved in Cambrian–Ordovician aeolian sandstone in southeastern Ontario, Canada (Geology 30, 391–394; 2002). They concluded that the ripples and fine layering in the sandstone are characteristic of wind-blown sand compacted over millennia, rather than underwater sediments, as had previously been believed.

The animals that made the tracks were about 50 cm long, had 16–22 legs, and may have dragged a tail behind them. They were probably euthycarcinoids — relatives of modern centipedes, and similar in appearance to overgrown woodlice.

Given the fossils' age, it is unlikely that the creatures lived on land. They probably ventured ashore to mate and lay eggs, to escape predators or to scavenge for food.

But if animals did forsake the seas 500 million years ago, they may have found little vegetation to eat. The only known fossils of land plants that are as old as these footprints are remains of algal mats. So the tracks appear to contradict the prevailing hypothesis that animals colonized the land to exploit leafy resources. A single finding is insufficient evidence, however, and sandstone rocks of this age are notoriously difficult to date. More examples will be needed before palaeontologists can rewrite the textbooks.