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Some 315 million years ago, a reptile about the size of house cat took a walk along a dried up river bed, leaving its small, splayed footprints behind.

A few small steps for an ancient tetrapod,聽one huge step for science.聽Turns out that was some walk -- providing evidence that early reptiles were the first vertebrates able to live on dry river plains far from the sea. Meanwhile, their amphibian cousins were still hanging out 鈥減oolside,鈥� needing the wetter environment to breed, lay their eggs and reproduce.

A stubby print in comparison to a penny.

Randy Miller with New Brunswick Museum with the fossilized trackways.

Trackways, detail.

It has long been suspected by scientists that reptiles were the first to make the continental interiors their home.聽The new discovery of trackways proves this theory.聽

鈥淭his is a major evolutionary development -- allowing our ancestors to live on land without having to beetle back to the water to reproduce,鈥� explains Martin Gibling, professor of Earth sciences and a co-author of a paper just published in journal Palaeogeography, Palaeoclimatology, Palaeoecology.

By 鈥渙ur ancestors鈥� Dr. Gibling clarifies: 鈥淢eaning vertebrates, from fish through amphibians to reptiles to birds, then through to us, mammals ... It鈥檚 a major step that allows larger animals to populate the land.鈥�

The聽reptile trackways preserved in rock were literally stumbled upon by the paper鈥檚 lead author, Howard Falcon-Lang of Royal Holloway, University of London, who grazed his knee as he scrambled over the fossilized slab. The rock -- 鈥渁bout the size of a filing cabinet,鈥� says Dr. Gibling -- had fallen out of a sea-cliff along the Bay of Fundy, near the town of St. Martins on New Brunswick鈥檚 southern coast.

The tracks are abundant and indicate a few different kinds of reptiles were there, including one with slender digits and a narrow splay and another with a much stubbier foot. Also recorded in the rock are the mud cracks of a dry riverbed, the pitter-patter of raindrops and various plants.

It was Arden Bashforth鈥檚 job to examine that plant. He determined there were big, seed-bearing trees nearby that would have lived in a dry landscape.

The paper鈥檚 publication caps off his time at H漫画 nicely; he just defended his PhD thesis on paleoecology (the study of fossil organisms and their relationship to ancient environments) and is returning to Denmark where he just landed a job at a geological museum in Copenhagen.

READ: "Diverse tetrapod trackways in the Lower Pennsylvanian Tynemouth Creek Formation, southern New Brunswick, Canada" (PDF) by Howard J. Falcon-Lang, Martin R. Gibling, Michael J. Benton, Randall Miller and Arden R. Bashforth. Palaeogeography, Palaeoclimatology, Palaeoecology, 2010.