by Terri Cook Wednesday, January 10, 2018
“Paleontologists have long hypothesized that a land bridge between present-day Siberia and Alaska served as a gateway for fauna to migrate between Asia and North America during the Cretaceous, but they have unearthed little evidence that directly supports this idea. Now, researchers have found an Asian dinosaur track assemblage in North America, a discovery that backs the longstanding hypothesis.
At the annual meeting of the Geological Society of America in Seattle last October, Anthony Fiorillo, chief curator and vice president of research and collections at the Perot Museum of Nature and Science in Dallas, Texas, reported his team’s discovery of fossil therizinosaur footprints in Cretaceous rocks in a remote part of Alaska’s Denali National Park and Preserve. Therizinosaurs are an enigmatic group of plant-eating dinosaurs surprisingly related to Tyrannosaurus rex, and the rock layer in which the team found the trace fossils is also home to tracks made by duck-billed hadrosaurs, another group of herbivorous dinosaurs.
“In Asia, Cretaceous rock units contain both hadrosaur and therizinosaur remains, while in western North America, correlative rocks contain hadrosaur and ceratopsian remains,” Fiorillo says. The discovery of the typical Asian herbivore assemblage in Denali, he says, is the first time that this association has been found in North America. “The preservation of the [therizinosaur and hadrosaur] tracks is remarkably similar,” he notes. “It’s possible that members of both groups were in the same place at the same time, or passed through the area within hours of each other.”
The team discovered the footprints in the Lower Cantwell Formation, a well-known source of vertebrate trace fossils, including from both avian and nonavian dinosaurs. In Denali, the formation consists of a series of interbedded sandstone, siltstone, clay and shale layers that formed about 70 million years ago from sediments deposited in fluvial floodplains and alluvial fans.
Within the study area in Denali’s Big Creek drainage, the Lower Cantwell Formation hosts diverse paleoflora, including ferns, horsetails and a variety of flowering plants, as well as cones, seeds and shoots from deciduous conifers. In the same unit in other areas of the park, Fiorillo and colleagues have also discovered Alaska’s first pterosaur record as well as a site consisting of thousands of hadrosaur tracks made by a multigenerational herd.
The numerous types of tracks found in Denali highlight the remarkable diversity of vertebrates that thrived in this chilly, ancient ecosystem, Fiorillo says, which sat at latitudes as high as, or perhaps even slightly higher than, its current location, according to previous research. Denali’s therizinosaur tracks mark the group’s northernmost known occurrence, suggesting these creatures were well adapted to a demanding physical environment with very short growing seasons and large annual fluctuations in both temperature and light.
Although conifers, with an understory of horsetails and ferns, comprised the dominant vegetation amid the floodplain environment in this part of the Cretaceous Bering land bridge, fossilized water lily-like leaves also indicate the presence of small lakes and ponds — a setting characteristic of central Asian ecosystems during the Cretaceous. This similarity has led Fiorillo to propose that the presence of the Asian dinosaur track assemblage in North America may reflect the occurrence of a similar set of environmental parameters, including abundant standing water.
The occurrence of an Asian dinosaur track assemblage in North America appears to support the theory that ancient Alaska served as a gateway for the exchange of flora and fauna between the two continents. “The prediction was that dinosaurs from both continents would someday be found on the land bridge,” Fiorillo says. “Our study, for the first time, offers evidence that this prediction was indeed correct.”
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