The sudden extinction of the dinosaurs dramatically altered the North American landscape, according to a recent study published by a multi-institutional team including researchers from Penn State.
Just as large mammals like elephants and rhinoceroses shape their environment by consuming and damaging large plants in their vicinity, large herbivore dinosaurs like the triceratops or hadrosaurs kept the ground free of trees and brush, which destabilized riverbanks and caused rivers to regularly spill over large floodplains, according to the study published in Communications Earth & Environment. After the dinosaurs became extinct due to the Chicxulub asteroid impact 66 million years ago, dense forests thrived, leading to more stable riverbanks and more contained river channels.
The researchers made these discoveries by comparing rocks before and after the Cretaceous-Paleogene boundary, including at five newly discovered sites. The boundary is marked by a layer of sediment enriched in iridium - a super dense metal rare to Earth but commonly found in asteroids - that settled in a thin layer on Earth after the meteor impact eradicated the dinosaurs. However, there are some other key differences between the rocks above and below the boundary.
Below the Cretaceous-Paleogene boundary, explained co-author Isabel Fendley, assistant research professor in the Department of Geosciences at Penn State, silt and mud deposits are widespread, suggesting rivers regularly flooded. In contrast, after the extinction of the dinosaurs, large river-channel sandstone deposits are more common, and abundant coal seams indicate that swampland environments persisted. This stark difference has previously been attributed to gradual changes in climate or sea level. However, Fendley said, these new findings led the research team to this new study hypothesizes that the change was actually due to the extinction itself, indicating that the dinosaurs, and their demise, had a huge impact on landscapes.
"The change in rock composition before and after the boundary layer was initially discovered in one geographic region, but we've noticed that it's in more than one place, that it's not unique to that specific area," Fendley said. "The fact that it is so widespread, and such a sudden change, confirmed for us the hypothesis that the extinction itself was responsible for the abrupt shift in the ecosystem from rivers that frequently flooded to contained rivers and forests."
Though the boundary layer exists around the world, it is especially visible and accessible in areas in the western United States, Fendley said, including the studied area of Wyoming's Bighorn Basin, as well as other areas like the Williston Basin, which spans eastern Montana and western North and South Dakota.
Basin, which spans eastern Montana and western North and South Dakota, as well as other areas like Wyoming's Bighorn Basin.
"Very often when we're thinking about how life has changed through time and how environments change through time, it's usually that the climate changes and, therefore, it has a specific effect on life, or this mountain has grown and, therefore, it has a specific effect on life," said corresponding author Luke Weaver, assistant professor in the University of Michigan's Department of Earth and Environmental Sciences. "It's rarely thought that life itself could actually alter the climate and the landscape."
In addition to Fendley and Weaver, the co-authors include Mónica Carvalho of the University of Michigan; Thomas Tobin of the University of Alabama; Courtney Sprain of the University of Florida; Paige Wilson Deibel of the Burke Museum of Natural History and Culture; Vera Korasidis of the University of Melbourne; and Pim Kaskes of the Vrije Universiteit Brussel, Belgium.
The U.S. National Science Foundation and the David B. Jones Foundation supported this research.
