The Age of Dinosaurs wasn’t just a heyday of spiky and toothy giants. Birds were just as much a part of the dinosaurian saga as Stegosaurus, and a new find among the rocks of southern Australia indicates that one of the world’s longstanding avian traditions probably goes back more than 120 million years. The fossil footprints of hungry birds likely document one of Earth’s earliest avian migrations, with the feathery fliers timing their visit to what was then part of the southern pole to enjoy a warm-season glut of squishy invertebrate food.
Researchers described the fossilized tracks that help tell the long story of bird migration on Wednesday in PLOS One. The full significance of the tracks wasn’t fully clear when study co-author Melissa Lowery, of Australia’s Monash University, discovered them in 2020. She and her colleagues sent photos to study co-author Anthony Martin, a paleontologist at Emory University, who recalls, “my first impression of the smaller tracks was that they were from small theropod and ornithopod dinosaurs,” or non-avian dinosaurs. But when Australia lifted its Covid-19 travel ban and Martin could visit to see the tracks himself, it quickly became clear that the Cretaceous fossils were made by different creatures. The tracks were left by prehistoric birds, the oldest bird footprints yet found on Australia.
“The tracks examined by the authors are truly remarkable,” says paleontologist Carolina Acosta Hospitaleche of Argentina’s National University of La Plata, who was not involved in the new study. “It constitutes a significant discovery for trace fossil studies and provides valuable insights into the earliest stages of avian evolution.”
Telling bird tracks apart from those made by small, non-bird dinosaurs is a tricky task, Martin says. After all, birds are dinosaurs that usually walk on three toes, with a fourth toe higher on the foot, just as many of their relatives did back in the Cretaceous. Nevertheless, researchers can tell them apart in a few ways. Mesozoic bird tracks are often small, less than six inches long, and thin, with three weight-bearing toes that form wide angles that exceed ninety degrees. The tracks found in Australia meet these requirements and others, Martin says, “and I’m happy to say these Early Cretaceous tracks passed the test with flying colors.”
The 27 footprints at the site in southern Australia were left behind over 120 million years ago. Back then, Australia sat further to the south and was still attached to Antarctica. The ancient sediments represent a polar environment that would have been cold and dark for much of the year. And yet the tracks show that at least eight different forms of Cretaceous birds visited over the years. Something must have been drawing them back to this place.
“The bird tracks were not only plentiful, but also varied in size and form,” Martin says, “and they could be found on different rock levels.” Multiple Cretaceous bird species clearly came back to the same place over a span of years. Based upon the location on the Cretaceous Earth, Martin says, “a variety of birds were probably visiting river floodplains during polar summers, after dark, cold winters and spring thaws followed by flooding.” The cyclical nature of these seasonal changes helped create the shoreline conditions where the footsteps could be left in the sediment and then quickly covered over by more sediment, year after year.
Much like modern migrating birds, the Cretaceous avians were likely traveling for food. The bird tracks are not the only trace fossils in the rocks. The same layers show burrows created by small creatures that lived along the shore. “Abundant invertebrate burrows in the same rocks as the tracks suggest these early birds were getting the worms, insects or other invertebrates that were active during polar summers,” Martin says. The birds stopped by when the ancient buffet was well stocked.
Additional evidence will be needed to test the idea that birds were migrating seasonally to shores along the southern pole, Acosta Hospitaleche says, but the evidence so far is compelling. The Cretaceous birds likely behaved similarly to modern species that take advantage of the same seasonal shifts. “Migration in and out of such extreme habitats following the changing seasons is certainly plausible,” says paleontologist Benjamin Kear of Sweden’s Uppsala University, who was not involved in the new study.
Most notable of all is that the tracks are older than any fossils from birds’ bodies, like bones or feathers, found from Australia or the ancient Antarctic. Tracks can often help paleontologists identify what species were around in a particular area or when they began to move into a new region even when skeletal remains are scarce. “If nothing else, these tracks show that birds were able to thrive in polar environments during the Age of Dinosaurs,” says Kear.
Martin was not expecting to find a reflection of the modern world in rocks that were so old. The earliest birds evolved about 150 million years ago, near the end of the Jurassic, in the Northern Hemisphere. By about 30 million years later, the fossil tracks show, birds had spread throughout the planet. “These tracks tell us that birds had likely diversified and flew to widely separated landmasses through the world not too long after their origin,” Martin says, evolving to move with the sway of the ancient seasons.