Three newly discovered groups of fossilized footprints show that dinosaurs and their early relatives were stalking the Earth some five million to nine million years earlier than scientists had previously estimated. This represents "a substantial extension of early dinosaur history," noted researchers in a description of the find, which was published online October 5 in Proceedings of The Royal Society B.
The diverse collection of tracks suggests that the dinosaur group contained some tenacious early members. The earliest traces found in the rocks show up relatively soon after the Permian–Triassic extinction, which occurred 252.3 million years ago.
"We see the closest dinosaur cousins immediately after the worst mass extinction," Stephen Brusatte, a graduate student at the American Museum of Natural History's Division of Paleontology and coauthor of the new study, said in a prepared statement.
The tracks were sampled from three areas in modern-day Poland and span about four million years and three different ancient ecosystems. The oldest protodinosaur prints were found near a village named Stryczowice and date to about 250 million years ago. (The oldest fossilized dinosaur bones date back some 244 million to 242 million years.)
But the animals that left these footprints were no Tyrannosaurus rex. These "stem dinosaurs," were likely smaller than a house cat and lived on the fringes of their environments. The largest tracks at Stryczowice, likely left by a protodinosaur called , are just 40 millimeters long.
Despite mostly being protodinos, the fossils of these creatures reveal some distinctively dinosaurian traits. The 120- to 140-millimeter-long imprints left near Baranów some 246 million years ago reveal an animal, named Sphingopus, that was bipedal and substantially larger than Prorotodactylus and other creatures that left tracks in the region. The find represents the earliest evidence of bipedality known in the dinosaur lineage and also raises the question of whether this trait arose more than once in the span of dinosaur evolution. Even some of the quadrupedal tracks show signs of short front limbs—an adaptation that can precede a more upright posture. The third group of tracks, found near Wióry, were made by early dinosaurs some time between those at Stryczowice and Baranów.
Dino tracks are an unusual find in the lineage's early history. Most early known fossilized footprints were left by members of the broader group archosaurs. "For the first 20 [million to] 50 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," Brusatte said. "The oldest dinosaurs were small and rare."
Footprint evidence is "often ignored or largely dismissed," the researchers noted in their paper. As a result, they wrote, the "footprint record has been marginalized and is yet to be fully integrated with the body fossil record."
But tracks might be an important place to look for new clues about early dinosaur evolution—especially with the fossil record for this group remaining thin. The three sets of ancient tracks in Poland were made some 40 million to 50 million years before dinosaurs came to dominate, the researchers pointed out in their study, underscoring the notion that, "the rise of dinosaurs was a drawn-out affair."
Images of Prorotodactylus and Prorotodactylus prints courtesy of Grzegorz Nied wiedzki