A trove of fossilized footprints found in Alaska’s Denali National Park and Preserve in 2007 is providing a wealth of information about dinosaur behavior, researchers say. The thousands of impressions, created on a 180-meter-long portion of near-coastal flood plain, today pepper a steep mountainside. Most of the tracks, made somewhere between 69 million and 72 million years ago, were left by hadrosaurs, commonly known as duck-billed dinosaurs (the crested creatures in this artist’s representation). The consistent and excellent preservation of tracks suggests all the footprints were created within a short time period. Varying in width from 8 to 64 centimeters, the footprints cluster within four distinct size ranges, which researchers suggest represent specific age groups within a multigenerational herd. About 84% of the tracks were made by adult and near-adult hadrosaurs and 13% by young presumed to be less than 1 year old. A mere 3% of the tracks represent juvenile hadrosaurs, a rarity that strongly suggests the young of this species experienced a rapid growth spurt and therefore spent only a short time at this vulnerable size, the researchers report online this week in Geology. (Previous analyses of fossilized bones also hinted at a growth spurt among hadrosaur youngsters, the researchers note.) The presence of juveniles in the herd also strongly hints that these creatures spent their entire lives in the Arctic, the team says; hadrosaurs of that size wouldn’t have had the size or stamina to migrate to and from warmer climates during wintertime, as some scientists have proposed.