Sixty-six million years ago, an asteroid struck the earth and wiped out non-avian dinosaurs. Now, researchers reporting in the journal Current Biology on May 24 have pieced together what that asteroid impact meant for birds. From multiple lines of evidence, including the plant fossil record and the ecology of ancient and modern birds, they show that the only birds to survive the Cretaceous-Paleogene (K-Pg) mass extinction event lived on the ground. That's apparently because the asteroid's impact destroyed forests worldwide, which took hundreds or even thousands of years to recover.
"We drew on a variety of approaches to stitch this story together," says lead author Daniel Field (@daniel_j_field) of the Milner Centre for Evolution at the University of Bath, UK. "We concluded that the devastation of forests in the aftermath of the asteroid impact explains why tree-dwelling birds failed to survive across this extinction event. The ancestors of modern tree-dwelling birds did not move into the trees until forests had recovered from the extinction-causing asteroid."
The researchers' analysis of the plant fossil record confirmed that global forests collapsed in the wake of the asteroid's impact. They then used the evolutionary relationships of living birds and their ecological habits to track how bird ecology has changed over the course of their evolutionary history. Those analyses showed that the most recent common ancestor of all living birds--and all bird lineages passing through the extinction event--most likely lived on the ground.
By contrast, many birds that lived at the end of the age of dinosaurs exhibited tree-dwelling habits. But those species didn't survive the K-Pg to give rise to any of the modern-day birds we know now.
"Today, birds are the most diverse and globally widespread group of terrestrial vertebrate animals--there are nearly 11,000 living species," Field says. "Only a handful of ancestral bird lineages succeeded in surviving the K-Pg mass extinction event 66 million years ago, and all of today's amazing living bird diversity can be traced to these ancient survivors."
The findings illustrate the fundamental influence of major events in Earth's history on the evolutionary trajectories of major groups of organisms, the researchers say. In the future, the team plans to continue to explore the precise timing of forest recovery and the early evolutionary radiation of birds.
"We are working hard to shed new light on this murky portion of the fossil record, which promises to tell us a lot about how birds and other animal groups survived--then thrived--following the extinction of T. rex and Triceratops," Field says.
Co-authors of the study are Antoine Bercovici of the Smithsonian National Museum of Natural History, Jacob Berv of Cornell University, Regan Dunn of the Field Museum of Natural History, Tyler Lyson of the Denver Museum of Nature and Science, David Fastovsky of the University of Rhode Island, Vivi Vajda of the Swedish Museum of Natural History, and Jacques Gauthier of Yale University.
The authors were supported by a 50th Anniversary Prize Fellowship at the University of Bath, a Smithsonian NMNH Deep Time Peter Buck Postdoctoral Fellowship, a Swedish Research Council VR grant, and the National Science Foundation.
Current Biology, Field et al.: "Early Evolution of Modern Birds Structured by Global Forest Collapse at the End-Cretaceous Mass Extinction" https://www.cell.com/current-biology/fulltext/S0960-9822(18)30534-7
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