The earliest known fossils associated with dinosaurs have just been identified in 250-million-year-old rocks from Poland, according to a new Proceedings of the Royal Society B paper.
The fossils -- footprints made by dinosaur relatives known as dinosauromorphs -- suggest that dinosaurs evolved from small, four-legged animals that lived during the Early Triassic just a few million years after the "Great Dying," Earth's most severe extinction event to date.
"For some reason, the major dinosaur lineages survived this extinction -- we don't know exactly why, and it may have been little more than random fortune -- and they probably then had the freedom to flower in a post-apocalyptic world," lead author Stephen Brusatte told Discovery News.
"(Dinosauromorphs) are the very closest relatives to dinosaurs, animals that were right on the cusp of becoming dinosaurs, shared many features with dinosaurs, probably looked and behaved like dinosaurs, but are not bona fide dinosaurs by definition," Brusatte, a paleontologist at the American Museum of Natural History, explained.
Brusatte and colleagues Grzegorz Niedzwiedzki and Richard Butler analyzed multiple fossilized animal tracks dating from the Early and Middle Triassic at the Holy Cross Mountains in southern Poland, an exciting new frontier for early dinosaur research.
The oldest tracks from this site are only about a half an inch in length, so the scientists conclude they belonged to an animal that was approximately the same size as a modern housecat, weighing at most around four pounds. Its hind legs were also longer than its forelimbs, since the footprints overstep the handprints.
The best way to securely identify a track-maker is to tie features of the footprint with those known to be present in the skeleton of the suspected animal's foot. Often this is a difficult process, since many footprints only preserve vague outlines and little other anatomical information.
Three primary attributes of the dinosauromorph tracks allowed the research team to connect the fossils to this group of animals.
First, he said the three central digits of the track-maker are dominant and the outer "toes" are reduced. This arrangement, he explained, is unique to dinosaurs and their closest relatives versus the larger digits of other animals from the time, such as lizards and crocodile-like archosaurs.
"Second," he continued, "the digits are essentially parallel, and this reflects the unique condition of dinosaurs and close relatives in which the foot is a tightly bunched structure."
The third characteristic is that the tracks reflect the unique ankle development of dinosaurs and their closest relatives. "(It's) a simple hinge and not a more complex rotary joint like the ankles of crocodile-line archosaurs, lizards and even humans," he said.
Martin Ezcurra of the Bernardino Rivadavia Natural Sciences Museum told Discovery News that he agrees with the study.
"This discovery forces us to think on an older evolutionary history for this group and, for the first time, in a probable relation with the biggest massive extinction in the history of the Earth, the Permo-Triassic extinction around 250 million years ago," Ezcurra said.
Michael Benton, professor of vertebrate paleontology at the University of Bristol, thinks the conclusions of the study are "undeniable," including the new theory that dinosaurs and their close relatives date to the Early Triassic rather than the Late Triassic.
"The Earth had been devastated by massive climactic changes at the end of the Permian, and pulses of flash warming continued through the Early Triassic, some five million years, continually destabilizing the environment and delaying full recovery of ecosystems," Benton said.
"This was the rather grim world the first dinosaurs may have seen," he added.