Dinosaurs ruled the Earth during the late Triassic through the late Cretaceous periods, a span of some 160 million years. Their evolutionary rise and their ultimate reign was made possible thanks to the largest mass extinction the planet had ever seen, which occurred at the boundary of the Permian and Triassic periods. Newly uncovered tracks in Poland have revealed what are now considered to be the earliest known evidence of dinosaurs in existence.

For the first 20 million or so years after the Permian-Triassic extinction, dinosaurs' direct relatives were small and quite rare; larger crocodile-like reptiles ruled the planet, and finding early dinosaur remains has been difficult. A new paper in the Proceedings of the Royal Society B discusses the identification of fossilized footprints found in the Holy Cross Mountains of central Poland.

The paper focuses on three sets of tracks all found within a 25 mile radius that were created between 250 and 246 million years ago. The oldest of the tracks, the Stryczowice trackway, contains evidence of the earliest predecessors to dinosaurs (called stem dinosaurs) ever discovered—a full five to nine million years older than the earliest skeletal remains.

The most recent track, the Baranów trackway, contains the 15cm long footprints of Sphingopus and are the oldest evidence of a bipedal and large-bodied dinosaur. The researchers determined that a dinosaur made these tracks because of the distinctive gait that dinosaurs had, which is easily distinguished from tracks made by their crocodile and lizard evolutionary cousins. The morphology of the toes and evidence of a dinosaur-like hinged ankle also provided evidence.

The discovery is unique due to the sparsity of dinosaurs and stem dinosaurs during this period of the Earth's history. Less than two percent of fossilized footprints that date from this time period are from these early dinosaurs; most come from crocodile-like archosaurs.

According to Stephen Brusatte, lead author of the paper, "For the first 20 million years of dinosaur history, dinosaurs and their closest relatives were living in the shadow of their much more diverse, successful, and abundant crocodile-like cousins. The oldest dinosaurs were small and rare."

Proceedings of the Royal Society B, 2010. DOI: 10.1098/rspb.2010.1746