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Time tracks. Fossilized footprints preserved in ancient mud are evidence of an animal that used legs to crawl, as opposed to paired fins.

Per Ahlberg et al., Nature 463 (2010)

Ancient Four-Legged Beasts Leave Their Mark

Researchers have uncovered the earliest evidence of four-legged animals. Footprints and tracks preserved in the mud of an abandoned quarry in southeastern Poland date back 395 million years, upending accepted thinking about when and where land animals first emerged.

Scientists have long believed that four-limbed animals--also known as tetrapods--evolved from fish via transitional animals called elpistostegids. These creatures had paired fins, rather than true arms and legs, and were capable of very limited crawling. The oldest elpistostegid fossils, known as Tiktaalik and Panderichthys, are 386 million years old.

But the Polish tracks suggest that elpistostegids were an evolutionary dead-end. The imprints found in the quarry are clearly footprints: in some of the prints, individual digits can be made out. That means land animals already had feet 9 million years before the finlike structures of Tiktaalik and Panderichthys. In addition, some of the tracks show an animal walking with a diagonal, coordinated gait impossible for finned creatures. "This is an animal with four limbs, unique for true tetrapods," says team leader Grzegorz Niedźwiedzki, a paleobiologist at the University of Warsaw who has been digging at the site since 2002.

The surface around the tracks is amazingly well preserved, with visible cracks from drying mud and the impressions of raindrops. The tracks show no signs of dragging tails or bellies, which suggests that the animals' bodies and tails were floating in shallow water when the four limbs made the impressions. From the appearance of the surrounding surface and the discovery of invertebrate fossils nearby, Niedźwiedzki says, it looks as though the early tetrapods were walking across a saltwater lagoon of some sort. That's a surprise: Traditionally, researchers thought that the first animals to walk moved onto land from freshwater rivers. The team reports its findings tomorrow in Nature.

Other paleontologists are taken aback by the discovery of the tracks. "We thought we'd pinned down the origin of limbed tetrapods," says Jennifer Clack of the University of Cambridge in the United Kingdom. "We have to rethink the whole thing."

Niedźwiedzki's team is looking for body fossils to supplement the tracks, hoping to find more information on the creatures that made the footprints. Analyzing bone fossils would help explain the relationship between these early tetrapods and their finned forebears, he says.