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© Grzegorz Niedzwiedzki/University of WarsawThe footprints are thought to be about 250 million years old
Evidence of the oldest dinosaurs yet discovered suggests the reptiles may have been walking the earth far earlier than was previously thought.

A study of footprints found in Poland from the early Triassic age found they dated from just a few million years after what scientists describe as the greatest mass extinction of all time, the ''Permo-Triassic mass extinction''.

The footprints, thought to be about 250 million years old, are ''the indisputably oldest fossils of the dinosaur lineage'', according to the researchers who carried out the study.

The scientists, from Poland, Germany and the US, said the prints, along with those from two other slightly younger sites, provided important insight into the origin and early evolutionary history of dinosaurs.

As well as suggesting that the origin of the animals occurred in the immediate aftermath of the Permo-Triassic event, they indicate that the earliest dinosaur relatives were very small animals.

They had feet only a few centimeters long, walked on four legs and were very rare compared to contemporary reptiles, the findings suggested.

The scientists, Stephen L Brusatte, Grzegorz Niedzwiedzki and Richard J Butler, wrote: ''The Polish footprints prompt a substantial extension of early dinosaur history.

''The dinosauromorph lineage originated by at least the Early Olenekian, within a few million years of the devastating Permo-Triassic mass extinction (PTE).

''The narrowing gap between the extinction and the oldest stem dinosaurs raises the intriguing possibility that the dinosauromorph radiation may have been part of the general recovery from the PTE, not an unrelated event that occurred 10 to 20 million years later as previously considered.''

The scientists concluded that the dinosaur radiation was a drawn-out affair, ''unexplainable by broad platitudes''.

The research was published in Proceedings of the Royal Society B.