From the emergence of the first of their kind about 228 million years ago to the modern abundance of birds (their living descendants), dinosaurs have been one of the most successful groups of organisms on the planet.
At the close of the Permian period, 252 million years ago, life on earth suffered the worst mass extinction of all time. More than 90 percent of known species in the seas disappeared, as did more than 70 percent of organisms known on land. In the wake of this catastrophe the surviving lineages proliferated and continued to evolve, and among them were the ancestors of the dinosaurs. We know this not from bones, but from tracks recently discovered in three approximately 251- to 249-million-year-old tracksites in Poland’s Holy Cross Mountains.
According to American Museum of Natural History paleontologist Stephen Brusatte and co-authors, the tracks were made by dinosauromorphs—forerunners of the dinosaurs which were more closely related to dinosaurs than to pterosaurs, crocodiles, or other archosaurs. Fossils of the bodies of these animals have been found dating back to about 243 million years ago—such as the recently described creature Asilisaurus from Tanzania—but the track fossils further close the gap between the emergence of the dinosauromorphs and the evolutionary recovery from the end-Permian mass extinction.
That the tracks were made by dinosauromorphs and not some other kind of creature was determined by comparing the limb skeletons with the footprints. “Footprints are notoriously difficult to identify,” Brusatte says, but the identity of the animals behind the Polish tracks can be narrowed down because of three factors: the two outer toes (the first and fifth) were reduced and the middle three were prominent; the middle toes were nearly parallel; and the back of the footprints are straight thanks to a simple, hinge-like arrangement of the ankle unique to dinosaurs and their closest relatives.
Small tracks made at one site, for example, closely matched the specific hand and foot anatomy of a dinosauromorph called Lagerpeton that walked on all fours, and a new larger type–given the designation Sphingopus–appeared to closely resemble the early predatory dinosaur Herrerasaurus. University of Utah paleontologist and expert on dinosaur origins Randall Irmis, who was not involved with the new study, generally agrees with this interpretation, saying “these do look like potential dinosauromorph tracks, and I think the authors make a good case for their identification.”
These tracks confirm what scientists have suspected on the basis of recently described body fossils: there was a “ghost lineage” of dinosauromorphs stretching back to the beginning of the Triassic. While Irmis stresses that the geologic timing of the Early Triassic is still being ironed out, the tracks are further evidence that the dinosaur stem lineage evolved shortly after the Permian mass extinction. “I don't think anyone is surprised by Early Triassic dinosauromorph fossils that are 5 million years older than those previously known,” Irmis says, but adds that “it’s definitely good to have some confirmation of these ghost lineages, and the authors are certainly right that the track record is under-utilized.” Both Brusatte and Irmis are confident that body fossils of the earliest dinosauromorphs will eventually be found, although due to the shifting of the continents over the past 250 million years, the deposits in which these fossils might be found are scattered from central Europe to Brazil to Africa.
What the tracks and other recent dinosauromorph discoveries mean for the big picture of dinosaur evolution is that the origins of these creatures must now be viewed in the context of the end-Permian mass extinction. The tracks from Poland were very rare—only 2 to 3 percent of those found at the tracksites–meaning that dinosauromorphs were marginal components of the ecosystem. It took a few million years for their diversity to increase, and the first dinosaurs did not branch off from their dinosauromorph relatives until about 228 million years ago. “Really, for most of the Triassic dinosaurs and their close relatives were completely overshadowed by the more common and diverse crocodile-line archosaurs,” Brusatte says, “and it wasn't until the Early Jurassic—some 50 million years after the first dinosauromorphs evolved—that dinosaurs were truly the pre-eminent terrestrial vertebrates in ecosystems across the globe.” These giants owed their origins to the earlier kinds of creatures that made the Polish tracks in the wake of the end-Permian extinction. While deadly for some, Brusatte casts the event as “a great opportunity for new groups to originate, diversify, and radiate in the barren and open landscapes of a post-apocalyptic world. Without the great contingency of the mass extinction, the Age of Dinosaurs probably never would have happened.”
Stephen L. Brusatte, Grzegorz Niedz´wiedzki, and, & Richard J. Butler (2010). Footprints pull origin and diversification of dinosaur stem-lineage deep into Early Triassic Proceedings of the Royal Society B : 10.1098/rspb.2010.1746