Four-legged tracks in mud cause paleontologists re-think

Muddy tracks left 397 million years ago may reset the age when the first four-legged animals crawled onto land.

In the Nature journal study, reported Wednesday and led by Grzegorz Niedz´wiedzki of Poland's Warsaw University, a European team reports "numerous" tracks from the Zachelmie mountain quarry that reveal the era when clumsy-limbed fishes forsook the seas to live on land.

The tracks show that four-legged creatures, one of them more than 8 feet long, probably originated in lagoons, not swamps, as suggested by past fossil finds.

A beach "provides a ready food source of stranded marine animals," the study's authors say. That would have allowed marine

ancestors of four-legged creatures to evolve "terrestrial competence" while dining on "a new and essentially untouched resource."

The trackway discovery "is undoubtedly one of the most extraordinary and unexpected finds in paleontology for decades, perhaps a century," says paleontologist John Long of the Natural History Museum of Los Angeles County.

Until the discovery, paleontologists thought the evolutionary transition from fishes to land animals was tightly nailed down to fossils dated about 379 million years old, he says.

The tracks not only push back the age of the first four-legged animals by a "whopping 18 million years," he says, but also "show that quite advanced kinds of tetrapods (four-limbed creatures), with incipient wrist joints, had already appeared by this much earlier stage of animal evolution."

The tracks "lob a grenade" into fossil studies of the earliest land creatures, Philippe Janvier and Gael Clement of the Muséum National d'Histoire Naturelle in Paris say in a commentary accompanying the study.

Paleontologists had considered fish-like creatures such as "Tiktaalik," a discovery that made headlines in 2006, as the likely earliest land animals. Instead, the trackways suggest Tiktaalik was a relic species still adapting to land long after other species had made the transition.

Not so fast, Tiktaalik's discoverers say. "Trace fossils such as these presumed vertebrate tracks and trackways, however, are a notoriously difficult class of evidence to interpret with full confidence," says Ted Daeschler of the Academy of Natural Sciences in Philadelphia by e-mail. "With all respect to the scientists involved in this study, there may be other explanations for these suggestive tracks."

The study authors acknowledge they have only fossil tracks, not bones, from their four-legged creature, which perhaps resembled an oversized salamander. Based on its gait, the critter floated in the tide while making tracks. The authors recommend from their finding that other paleontologists look for fossils of four-legged creatures as far back as 420 million years ago.