Paleontologists have examined a large assemblage of dinosaur tracks and fossilized plants from the Nanushuk Formation, which crops out over much of the central and western North Slope of Alaska, varying from 1,500 to 250 m (4,921 to 820 feet) thick from west to northeast.
A theropod dinosaur footprint in the Nanushuk Formation, Alaska, the United States. Note the sinusoidal curve of the middle toe impression. Scale bar – 10 cm. Image credit: Fiorillo et al., doi: 10.3390/geosciences14020036.
“We’ve had projects for the last 20 years in Alaska trying to integrate sedimentology, dinosaur paleontology and the paleoclimate indicators,” said University of Alaska Fairbanks Professor Paul McCarthy.
“We’ve done work in three other formations — in Denali, on the North Slope and in Southwest Alaska — and they’re about 70 million years old.”
“This new one is in a formation that’s about 90 to 100 million years old.”
“What interested us about looking at rocks of this age is this is roughly the time that people think of as the beginning of the Bering Land Bridge — the connection between Asia and North America.”
“We want to know who was using it, how they were using it and what the conditions were like.”
“The mid-Cretaceous was the hottest point in the Cretaceous period.”
“The Nanushuk Formation gives us a snapshot of what a high-latitude ecosystem looks like on a warmer Earth.”
The Nanushuk Formation dates to roughly 94 million to 113 million years ago in the mid-Cretaceous period and about when the Bering Land Bridge began.
The fieldwork occurred between 2015 and 2017 and centered on Coke Basin, a circular geologic feature of the formation.
The basin is in the DeLong Mountains foothills along the Kukpowruk River, about 100 km (60 miles) south of Point Lay and 32 km (20 miles) inland from the Chukchi Sea.
In the area, the paleontologists found approximately 75 fossil tracks and other indicators attributed to dinosaurs living in a riverine or delta setting.
“This place was just crazy rich with dinosaur footprints. One site stands out,” said Dr. Anthony Fiorillo, a researcher at the New Mexico Museum of Natural History and Science.
“We were at a spot where we eventually realized that for at least 366 m (400 yards) we were walking on an ancient landscape.”
“On that landscape we found large upright trees with little trees in between and leaves on the ground. We had tracks on the ground and fossilized feces.”
“We found numerous fossilized tree stumps, some 60 cm (2 feet) in diameter. It was just like we were walking through the woods of millions of years ago.”
The Nanushuk Formation encompasses rock of marine and non-marine characteristics and composition, but the new research focuses primarily on the non-marine sediments exposed along the upper Kukpowruk River.
“One of the things we did in our paper was look at the relative frequencies of the different kinds of dinosaurs,” Dr. Fiorillo said.
“What was interesting to us was that the bipedal plant eaters were clearly the most abundant.”
Two-legged plant eaters accounted for 59% of the total tracks discovered. Four-legged plant eaters accounted for 17%, with birds accounting for 15% and non-avian, mostly carnivorous, bipedal dinosaurs at 9%.
“One of the things that was interesting is the relative frequency of bird tracks,” Dr. Fiorillo said.
Carbon isotope analysis of wood samples led to a determination that the region received about 178 cm (70 inches) of precipitation annually.
This record of increased precipitation during the mid-Cretaceous provides new data that supports global precipitation patterns associated with the Cretaceous Thermal Maximum.
The Cretaceous Thermal Maximum was a long-term trend approximately 90 million years ago in which average global temperatures were significantly higher than those of today.
“The temperature was much warmer than it is today, and what’s possibly more interesting is that it rained a lot,” Dr. Fiorillo said.
A paper on the findings appears in the journal Geosciences.
_____
Anthony R. Fiorillo et al. 2024. New Dinosaur Ichnological, Sedimentological, and Geochemical Data from a Cretaceous High-Latitude Terrestrial Greenhouse Ecosystem, Nanushuk Formation, North Slope, Alaska. Geosciences 14 (2): 36; doi: 10.3390/geosciences14020036