Thousands of preserved footprints have been discovered in a floodplain in Denali National Park and Preserve in central Alaska. The most abundant tracks belong to hadrosaurids, also known as duck-billed dinosaurs, and they were all made between 69 million and 72 million years ago. This site, a 180-meter wedge in the Upper Cretaceous Cantwell Formation, is the largest tracksite known this far north. A new analysis of these high-latitude impressions reveals that duck-billed dinos lived in large, multigenerational herds, and they likely lived in the Arctic year-round.
Hadrosaur prints are pretty characteristic: three-toed, wider than they’re long, digits that end bluntly, and a wide heel with two lobes. Because tracks across the bedding surface at the new tracksite is consistent, that means the tracks were made during a short time period, likely during a warm month.
These footprints -- most of which contain skin impressions -- vary in width from 8 to 64 centimeters. Using statistical analyses, a trio of researchers led by Anthony Fiorillo of the Perot Museum found that the prints cluster within four different size ranges and specific age groups: adults, subadults, juveniles, and very young individual. This suggests that duck-billed dinos lived in extended family social groups, a behavioral pattern that hasn’t been recognized in hadrosaurs before, either in bone beds or other track assemblages. The work was published in Geology this week
Combined, tracks made by adults (stage 4) and subadults (stage 3) made up 84 percent of the footprints. Another 13 percent were made by youngsters less than a year old (stage 1). Just 3 percent of the tracks were made by juveniles (stage 2). Because the juvenile tracks seemed to appear so rarely, the researchers reasoned that individuals of this species stay at this vulnerable size for only a brief time before going through a growth spurt. Previous studies of fossilized bones have hinted at a rapid growth spurt early in life as well.
Furthermore, the researchers think these dinosaurs were year-round residents of the Arctic. Early work has suggested that because of the intense seasonality of high-latitude ecosystems, polar dinosaurs would travel thousands of kilometers to warmer climes during the colder months. But recent work with hadrosaur bones suggests that they didn’t migrate those distances, and these newly discovered tracks made by very young juveniles confirms this: The young animals would not have been capable of making the journey. The smallest femur had a circumference of 186 millimeters and could only have migrated 6,400 kilometers a year (that’s 3,200 kilometers one way); these distances would not have taken them out of the northern polar regions.
Images: Karen Carr via National Park Service