Sixty-six million years ago, when a large asteroid struck what is now Mexico’s Yucatan Peninsula, the world’s forests were devastated by fire and ash—and the era of modern birds began.
It was the end of the Cretaceous, though the dinosaurs—T. rex, Triceratops, Velociraptor, and bird-like Ichthyornis and Confuciusornis—that ruled Earth wouldn’t have known that. The climate was warm and humid, the poles had no ice sheets, sea level was hundreds of feet higher than today, and an ocean filled what is now central North America.
Then, the asteroid struck, forming a crater 93 miles wide and 12 miles deep. The impact itself generated shockwaves emanating out in a 930-mile radius from the site, says Daniel Field, a paleobiologist at the Milner Center for Evolution at the University of Bath. “Trees would have been flattened from the force.” Catastrophic earthquakes and volcanic eruptions were triggered by the shockwaves, and widespread fires likely accompanied the impact, setting forests ablaze. Ash and debris filled the skies, blocking the sun for several years. “Photosynthesis on a large scale would have shut down,” Field says. The result is that most of the world’s forests likely died. Three quarters of plant and animal species went extinct.
The fossil record tells us that nearly all bird-like dinosaurs blinked out of existence, but a few birds managed to survive and eventually evolve into the diversity of birdlife we know today. The story of life after the asteroid impact is told in a new paper, led by Field, a birder as well as a paleobiologist, and published today in Current Biology. It pieces together various evidence—from fossil pollen, fossil birds, and evolutionary trees—that paint a vivid picture of life after the asteroid strike.
"This new paper is a superb piece of work that makes a convincing argument: the birds that died were mostly tree-living species, whereas a few hardy ground-living stragglers made it through," Steve Brusatte, a paleontologist at the University of Edinburgh who studies dinosaur and bird evolution during the end-Cretaceous mass extinction, wrote in an email. "It makes total sense that birds that lived and nested in trees would have felt the destruction hardest, whereas those that lived on the ground would have had more stable habitats."
Daniel Ksepka, an expert in bird evolution and paleontology and the curator of science at the Bruce Museum in Connecticut, agrees. “They’re asking a really intriguing question—why did some birds survive and others didn’t at the end of the Cretaceous?—and they’re giving a really believable answer,” he says. “It’s a really good idea. I wish I would have thought of it.”
Field is on one of several research teams which have developed new avian trees of life recently, comparing fossil and genomic evidence to explain how today’s birds are related to each other. Many of those trees disagree, he says, but they have something in common: They show a rapid evolution of birds right after the mass extinction event at the end of the Cretaceous. “That got me interested in trying to understand in better detail how the extinction event that wiped out the dinosaurs went on to influence the evolution of modern birds, which of course are dinosaurian descendants," Field says.
To find out what happened immediately after the asteroid impact, he went to North Dakota to a rock formation known as John’s Nose. There, a stripe of clay and iridium—a rare element found on asteroids—preserves the rock record of the impact and its aftermath, including fossilized pollen. In a single cubic centimeter of sediment, you can find 100,000 grains of pollen.
“The pollen fossil record is amazing—though, if you’re a bird paleontologist, you have to get accustomed to small samples,” Field says. “You can piece together not only the asteroid impact itself, but also the impact of the asteroid strike on global plant communities.”
Before the asteroid strike, forests were widespread throughout the world; the sediment contains an abundant and diverse array of plant pollen from flowering trees and cone-producing trees. After the impact, though, the sediment is dominated instead by fern spores. Field suggests that within 100 years of impact, ferns were widespread and dominated plant communities for probably 1,000 years.
This “fern spike” is further evidence that the world’s forests were absolutely devastated by the asteroid impact. “In the modern world, we know that ferns are colonizers of rapidly denuded landscapes,” Field says, like those wrecked by landslides, wildfires, and volcanic eruptions. In this denuded landscape, there would be little food or resources for bird-like dinosaurs that relied on forests.
These include the “opposite birds,” as Enantiornithes are known colloquially. These flying dinosaurs looked much like modern birds externally, except for their teeth and wing claws, and they lived diverse lifestyles like today’s birds. “They were very successful in their own right—and then they all went extinct,” Ksepka, who was not involved in the study, says. “A lot of the things we see in the modern world would have been around back then—not the same species, but doing the same things: walking along the beach probing for something, eating some seeds, being predatory. They filled a lot of the same roles.”
Most bird-like dinosaurs, including 80 known opposite bird taxa, disappear from the fossil record after the asteroid strike. They simply couldn’t survive on the dark, deforested Earth, Field suggests. “We think habitat for tree-dwelling birds essentially disappeared for a geologically rapid instant,” he says, “but in terms of the lifespan of a living bird, that’s a sufficient amount of time to wipe out anything that was strongly associated with forests.”
Because forests provide rich food and habitat, their devastation would have had widespread effects. But their destruction alone can't explain the survival of some birds through the mass extinction event. "There may have been other factors, too. It seems like the toothless, beaked birds that ate seeds also preferentially survived the extinction," Brusatte wrote. "The survivors may have had the perfect lottery ticket: lived on the ground, ate seeds, adaptable, fast-growing, and able to fly well."
Eventually the forests recovered. The world was a very different place, though, and there was very little competition. Somehow, a few birds managed to survive to that point. “Earth was probably a horrible place to be, and my guess is that they were pretty skinny for a few years,” Field says. These were likely small, ground-dwelling birds, like today's tinamous, that managed to eke out a living on grains and seeds. “Anything that made it through was probably pretty lucky to do so,” he says.
Those survivors hit the jackpot, evolutionarily speaking. The regenerated forests provided a wealth of habitats, and the ancestors of modern birds colonized the trees and evolved a variety of ways to survive. Within a few million years, the first ancestors of mousebirds (tree-dwelling birds currently confined to sub-Saharan Africa) appear in the fossil record. "It's likely other transitions to arboreality would have taken place at least by that time as well," Field says. They diversified rapidly, and as a result we have some 10,000 bird species living today.
“If you love the birds that are around today, we’d probably have a completely different group if this event hadn’t happened,” Ksepka says. We could still be living alongside opposite birds. “This reset the course of bird evolution. This is the very beginning of the birds in your backyard,” he says.
It’s sobering to consider another possibility, though: That no bird ancestors survived at all. “It’s becoming increasingly obvious that we could have had no birds today; they’re pretty lucky to have snuck across the [Cretaceous-Paleogene] boundary in the first place,” Field says. Dinosaurs didn’t make it, along with countless other animals that we’ll never know.
But a few small, ground-dwelling birds did, and a few small, ground-dwelling mammals, too. In time, those mammals evolved into humans. That's one thing we have in common: We exist only because our ancestors survived the asteroid impact that destroyed nearly all other life on Earth.