“These aren’t the right kind of rocks,” Tony Fiorillo said, pointing at the jagged pink and black stones along Alaska’s Yukon River. The sun blazed down on Fiorillo on the 14th day of a 16-day expedition. A paleontologist and the executive director of the New Mexico Museum of Natural History and Science, Fiorillo was looking for rocks twice as old as the ones he was standing on, alongside the wide, silty yet sparkling Yukon River. The rocks he aimed to find were from the Cretaceous Era, when dinosaurs roamed this part of Alaska in abundance.
Paleontologists like Fiorillo have long suspected that the area would be rich with fossil evidence, but this was the first time a team had set out to thoroughly survey the area. Fiorillo and his two colleagues, geologist Paul McCarthy and paleontologist Yoshitsugu Kobayashi, had spent the last two weeks snapping countless photos and penciling endless notes into field notebooks. A few days earlier, they’d stumbled upon a rock face the size of a living room end table that revealed dozens of footprints made by a bird the size of a willet or a curlew. Within the hour, they found 15 other blocks just like it.
The expedition set out to advance what little is known about the prehistoric Far North. Over 16 days, the team traveled more than 100 river miles looking for the “right kind of rocks”: sandstones, shale and siltstones layered like a cake and exposed in bluffs that tower over the river’s swift current. Armed with a geologic map of Alaska and an academic paper published on a survey of the area’s sedimentary geology almost 40 years ago, the team hoped to find evidence that dinosaurs once roamed this part of Alaska and did so in abundance. “Finding dinosaurs in Alaska challenges everything we think we know about dinosaurs,” Fiorillo said. “They’re described as warm-climate, swamp-going things. It’s clear they were way more adaptable than I think we appreciate.”
“It’s clear they were way more adaptable than I think we appreciate.”
One hundred million years ago, Alaska’s location on the globe wasn’t much different that it is now, but it was considerably warmer — similar to today’s climate in Portland or Seattle, thousands of miles south. McCarthy, a geologist at the University of Alaska Fairbanks, said they can nail down what the landscape — the dinosaurs’ habitat — was like, based on his work measuring hundreds of meters of exposed sediments. It’s likely similar to the Yukon River landscape of today: A deltaic system, with lots of braided channels, swamps, ponds and thick forests. “We don’t know how much precipitation there was quantitatively,” he said, “But there’s enough clues in the rocks that there was plenty of water around.”
Many rocks held giant fossil leaves and cones from coniferous trees. In one spot, enormous petrified logs lined the riverbank. Kobayashi, who is a professor of paleontology at Japan’s Hokkaido University, used a shovel to dig one out of the riverbank’s silty sand and gravel under an unseasonably hot sun. “I’m not a tree person, I’m a dinosaur person,” he joked. Kobayashi, an expert on dinosaur bones, said finds like this can help answer questions about the dinosaur species that lived here and the kinds of plants they may have eaten. “This was probably a dense forest,” he said, pointing to at least four other large petrified logs protruding from the riverbank. Eventually, Kobayashi’s shovel revealed a roughly 3-foot-by-3-foot length of petrified wood, its rings clearly defined. The team took a sample, hoping that a colleague who specializes in ancient plants — a paleobotanist — can identify this and other fossil species.
Fiorillo said the details along this section of the Yukon add to an understanding of dinosaurs all over the world. “It’s our opinion that Alaska is one of the most important places to work,” he said. “Because every dinosaur except one that lived in New Mexico, in the Cretaceous, came through the Bering Land Bridge from Asia. And so, if you know what’s going on in Alaska, you actually know a lot about the dinosaur faunas and interactions in two major landmasses, Asia and North America.”
Until this expedition, scientists hadn’t taken a close look at this stretch of the Yukon. “This is really the first time anyone has systematically looked at the sedimentology and the paleontology here,” said McCarthy. Based on a 1980s survey of the region’s geology, scientists knew there were likely to be dinosaur tracks in the region. Ten years ago, a research team reported finding dinosaur prints along the middle section of the Yukon River, and returned to the University of Alaska Fairbanks with a literal ton of rocks. Dozens of the preserved dinosaur footprints they collected are now housed in the basement of UAF’s Museum of the North. The find garnered plenty of media attention, but that team never returned to the area and its findings haven’t been published.
On their expedition, McCarthy, Fiorillo and Kobayashi built on those discoveries. Over roughly 130 river miles, the expedition found more than 90 sites where dinosaurs, ancient bird species and even fish left behind signs that they lived here 90 to 100 million years ago. In some places, ghosts of these creatures seemed to walk straight up to the scientists. “I keep saying it’s like going to the candy store. Someone opened the door and here they are,” said Fiorillo. In one spot, an enormous, table-sized block of sandstone lay haphazardly on the bank. It held three large footprints — one made by Magnoavipes, a giant crane-like bird, and two others made by an adult and a juvenile ornithopod, a plant-eating dinosaur that walked on two feet. Other tracks lay at the bottoms of eroding bluffs and in crumbling rocks falling from walls above. One print, left by the four-toed armored ankylosaur, hung from a layer of gray siltstone, more than a dozen feet above the river’s high-water mark.
This stretch of the Yukon is rich in tracks, especially compared to other parts of Alaska. The team averaged about six footprint discoveries per day, and on their final day of field work, they found 10. Fiorillo, who has spent nearly a quarter of a century scouring Alaska for signs of dinosaurs, said that farther east, in the Yukon Charley Rivers National Preserve, he found just two footprints over the course of six field seasons. Northwest of here, on the Kaukpowruk River, it took three field seasons to record 70 tracks. And 10 days of work in the Wrangell-St. Elias National Park and Preserve turned up only two tracks.
As the days progressed and clear, sunny skies gave way to thunderheads and then again to air thick with wildfire smoke, one question remained on everyone’s minds: Where are the bones? Kobayashi, who has made fossil discoveries in Japan, Uzbekistan, and Mongolia, said that bones can be hard to spot — they look different depending on the rock they’re preserved in. “You have to kind of know with your own eyes,” he said.
While bones didn’t appear during this trip, an impression of dinosaur skin did. The knobby, scaly impression was preserved in a softball-sized rock, and the researchers were overjoyed to find another breadcrumb that could help them identify not only which dinosaurs lived this far north so long ago, but what kind of habitat they preferred and how they interacted. In all, the team left the Yukon with notes on at least six ancient species and questions about two others, as yet unidentified. As for the bones, the team believes it’s only a matter of time until they reveal themselves — and the three scientists hope to return soon for another look.
Emily Schwing is a reporter based in Alaska. Follow @emilyschwing
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