They included two unique footprints: one made by an ankylosaur, an armored plant-eating dinosaur, and the other by a carnivorous theropod. Theropods are three-toed predators that include tyrannosaurs. Only two such tracks from a theropod have been recorded by Fiorillo’s team here.
“I’m very excited because it allows us to do a statistical analysis with the robust data,” said a member of Fiorillo’s team, Yoshitsugu Kobayashi, a paleontology professor at Japan’s Hokkaido University Museum. “With a few [prints], it’s like you’re sharing a whisper from dinosaurs, but if you have a huge number, it’s like screaming. The dinosaurs are telling us something.”
The team is collecting data to explain how enormous reptiles were able to survive 75 million years ago in a climate that more closely resembled present-day Seattle or Portland, Ore. A wet and rainy climate and relatively temperate weather doesn’t seem ideal for multi-ton reptiles, but dinosaurs thrived here, Fiorillo said.
Fiorillo, the executive director of the New Mexico Museum of Natural History & Science, has spent 10 field seasons at Aniakchak. The area has intrigued him since he first discovered a dinosaur footprint here in 2002. “Walking these couple of miles, there’s just a truly remarkable frequency of tracks on the beach and in the cliffs,” Fiorillo said, “And I would be a little hard pressed to think of that kind of density in track abundance” elsewhere.
Most of the footprints from roughly 75 million years ago in the late Cretaceous were made by hadrosaurs, which were duck-billed herbivores. Dinosaur remains and ancient soil samples informed a study published in April in the journal Geosciences. It showed that mean annual precipitation had more to do with structuring habitat selection than mean annual temperature among dinosaurs that roamed in Alaska. The study compared the team’s findings not only from Aniakchak Bay but also from work on Alaska’s North Slope and in Denali National Park and Preserve.
During the late Cretaceous, Aniakchak wasn’t much farther south than it is today, so this team has returned here nearly every year since 2016 to piece together a more comprehensive picture of how dinosaurs were able to survive here.
“We don’t have a lot of high-resolution dating throughout this section” of rocks, said Paul McCarthy, head of the Geosciences Department at the University of Alaska at Fairbanks, who specializes in ancient soils, or paleosols.
At Aniakchak, the geologist is focusing on a 300-meter-thick (328-yard-thick) section of layered sedimentary rock. “So we know the age of the 300-meter section,” he said, but what’s missing is a layer, or several, within that section that can help provide more details about how and when the climate changed here.
It “is really impossible without knowing exactly how much time each individual segment represents,” McCarthy said.
This outcrop, however, offers other details that allow the team to get specific about dinosaurs and their habitat preferences.
“As you go through time within this section, we can compare who’s walking around to the plants, the soils and whether they’re on a river flood plain or in an estuary” or some other place, McCarthy said.
One end of a roughly three-mile-long stretch of coastline is littered with tracks made by juvenile hadrosaurs. The rocks indicate that area was once an estuary, where a river dumped into a tidal flat. On the other end, the majority of tracks were laid down in an exposed intertidal zone by fully grown adult hadrosaurs.
Nicknamed “the cradle of storms,” Aniakchak Bay offers something new to Fiorillo on each visit. Where piles of kelp had washed up to rot on the beach last year, swaths of black sand took over this summer. Storms here are dramatic enough to move vehicle-size boulders, and the rocks seem to shape shift as thick sheets of rain give way to intermittent sunshine.
Fiorillo and his team will present some of their findings at the annual meeting of the Geological Society of America in Denver. The four-day event begins Oct. 9.
“And then we’ll see what new questions come up as we really start to analyze the data and then think about next year,” Fiorillo said.