The earliest evidence for the existence of reptiles has been found in Canada.
A block of sandstone covered with fossil reptile tracks
The 315 million-year-old fossilised tracks give an insight into a key milestone in the history of life, when animals left water to live on dry land.
The footprints suggest reptiles evolved between one and three million years earlier than previously thought.
They were found by UK scientist Dr Howard Falcon-Lang in fossil-rich sea cliffs at New Brunswick. "The discovery was pure luck," he said.
"As I walked along remote sea-cliffs at the end of a long day in the field, I passed a recent rock fall.
"One large slab of rock was covered with hundreds of fossil footprints! The Sun was low in the sky and I probably wouldn't have seen them if it hadn't been for the shadows," the University of Bristol researcher explained.
The ancient trackway gives an insight into a time when vertebrates were evolving through amphibians to reptiles.
The origin of reptiles, in particular the appearance of eggs protected by a shell, allowed four-legged animals to avoid having to go back into water to lay eggs, heralding life on dry land.
How the reptile might have looked
"The evolution of reptiles was one of the most important events in the whole history of life," Dr Falcon-Lang told BBC News.
"That paved the way for the diverse ecology that we have on our planet today."
Scientists believe the tracks preserved in sandstone were left by reptiles gathering around a watering hole on river plains that were dry for at least part of the year.
List of suspects
Using a bit of biological detective work, Dr Falcon-Lang and colleagues in the UK and Canada tracked down the likely maker of the footprints.
"There were only a few species capable of making prints like this around at the time so we came up with a shortlist of suspects," said Professor Mike Benton, also of the University of Bristol, who co-authored the study.
"However, the prints showed that the hands had five fingers and scales, sure evidence they were made by reptiles and not amphibians."
The most likely contender was a lizard-like reptile named Hylonomus lyelli after the 19th Century geologist Sir Charles Lyell.
Until now, the oldest evidence for reptiles was thought to be skeletal fossils of the creature found in 1859 by William Dawson.
Dr Falcon-Lang said the new material was found in the same general region of an area of rock formation known as Joggins but at a level almost a kilometre below Dawson's discovery.
Print shows five fingers - characteristic of reptiles
"Consequently we can be confident the footprints are older than the skeletons," he said.
"The most likely track-maker was the Hylonomus lyelli reptile we know from the slightly later remains at Joggins," he added.
The results of the study are published in the Journal of the Geological Society of London.