A "world-class" dinosaur track site discovered in Alaska's Denali National Park shows that herds of duck-billed dinosaurs thrived under the midnight sun.
"We had mom, dad, big brother, big sister and little babies all running around together," said paleontologist Anthony Fiorillo, who is studying the dinosaur tracks. "As I like to tell the park, Denali was a family destination for millions of years, and now we've got the fossil evidence for it."
The discovery adds to Fiorillo's growing conviction that dinosaurs lived at polar latitudes year-round during the Late Cretaceous Period, about 70 million years ago. [Images: Denali National Park's Amazing Dinosaur Tracks]
"Even back then the high latitudes were biologically productive and could support big herds of pretty big animals," said Fiorillo, curator of earth sciences at the Perot Museum of Nature and Science in Dallas.
The dinosaur track site, near Cabin Peak in the park's northeast corner, has thousands of tracks from hadrosaurs, or duck-billed dinosaurs. Many of the deep tracks contain preserved skin and "nail" impressions from the plant-eating hadrosaurs.
"This is definitely one of the great track sites of the world. We were so happy to find it," Fiorillo said.
Fiorillo and his collaborators also found traces from birds, clams, worms and bugs intermingled with the dinosaur tracks. Other dinosaur denizens who left behind footprints in Denali were ceratopsians, therizinosaurs and the flying reptiles called pterosaurs. Ferns and redwood cones complete the picture of a rich Cretaceous ecosystem.
The muddy ground is so rumpled by footprints that the researchers were hard-pressed to pull out tracks from individual hadrosaurs. Instead, they counted each print and grouped them by size. The results were published June 30 in the journal Geology.
There are four distinct size ranges, varying in length from 5 inches (about 12 centimeters) to 24 inches (about 60 cm). More than 80 percent of the tracks are from adults, and 13 percent are from young duckbills less than a year old. Just 3 percent are juvenile hadrosaurs. The researchers think the small number of juvenile prints suggest the young underwent a rapid growth spurt, spending only a short time as tiny, vulnerable dinosaurs (a theory also supported by studies of hadrosaur bones).
Duck-billed dinosaurs are the most common dinosaur fossils from the Late Cretaceous, with a wide range of species found on several continents. Call them the buffalo of the dinosaurs, grazing on the fern prairie and protecting their young in big herds.
The evidence emerging from the Denali hadrosaur tracks show babies and juvenile dinosaurs living in Alaska during the summer. The juveniles probably couldn't endure a long migration, so the tracks suggest the dinosaurs spent their entire lives in the Arctic, rather than migrating from somewhere else, the researchers said.