Sixty-six million years ago, when a large asteroid struck what is now Mexico鈥檚 Yucatan Peninsula, the world鈥檚 forests were devastated by fire and ash鈥攁nd the era of modern birds began.
It was the end of the Cretaceous, though the dinosaurs鈥T. rex, Triceratops, Velociraptor, and bird-like and Confuciusornis鈥攖hat ruled Earth wouldn鈥檛 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 , a paleobiologist at the Milner Center for Evolution at the University of Bath. 鈥淭rees would have been flattened from the force.鈥 Catastrophic earthquakes and 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. 鈥淧hotosynthesis on a large scale would have shut down,鈥 Field says. The result is that most of the world鈥檚 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 . It pieces together various evidence鈥攆rom fossil pollen, fossil birds, and evolutionary trees鈥攖hat 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," , 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."
, an expert in bird evolution and paleontology and the curator of science at the Bruce Museum in Connecticut, agrees. 鈥淭hey鈥檙e asking a really intriguing question鈥攚hy did some birds survive and others didn鈥檛 at the end of the Cretaceous?鈥攁nd they鈥檙e giving a really believable answer,鈥 he says. 鈥淚t鈥檚 a really good idea. I wish I would have thought of it.鈥
Field is on which have developed new avian trees of life recently, comparing fossil and genomic evidence to explain how today鈥檚 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. 鈥淭hat 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鈥檚 Nose. There, a stripe of clay and iridium鈥攁 rare element found on asteroids鈥攑reserves 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.
鈥淭he pollen fossil record is amazing鈥攖hough, if you鈥檙e a bird paleontologist, you have to get accustomed to small samples,鈥 Field says. 鈥淵ou can piece together not only the asteroid impact itself, but also the impact of the asteroid strike on global plant communities.鈥
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 鈥渇ern spike鈥 is further evidence that the world鈥檚 forests were absolutely devastated by the asteroid impact. 鈥淚n the modern world, we know that ferns are colonizers of rapidly denuded landscapes,鈥 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 鈥渙pposite birds,鈥 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鈥檚 birds. 鈥淭hey were very successful in their own right鈥攁nd then they all went extinct,鈥 Ksepka, who was not involved in the study, says. 鈥淎 lot of the things we see in the modern world would have been around back then鈥攏ot 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.鈥
Most bird-like dinosaurs, including 80 known opposite bird taxa, disappear from the fossil record after the asteroid strike. They simply couldn鈥檛 survive on the dark, deforested Earth, Field suggests. 鈥淲e think habitat for tree-dwelling birds essentially disappeared for a geologically rapid instant,鈥 he says, 鈥渂ut in terms of the lifespan of a living bird, that鈥檚 a sufficient amount of time to wipe out anything that was strongly associated with forests.鈥
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. 鈥淓arth was probably a horrible place to be, and my guess is that they were pretty skinny for a few years,鈥 Field says. These were likely small, ground-dwelling birds, like today's tinamous, that managed to eke out a living on grains and seeds. 鈥淎nything that made it through was probably pretty lucky to do so,鈥 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.
鈥淚f you love the birds that are around today, we鈥檇 probably have a completely different group if this event hadn鈥檛 happened,鈥 Ksepka says. We could still be living alongside opposite birds. 鈥淭his reset the course of bird evolution. This is the very beginning of the birds in your backyard,鈥 he says.
It鈥檚 sobering to consider another possibility, though: That no bird ancestors survived at all. 鈥淚t鈥檚 becoming increasingly obvious that we could have had no birds today; they鈥檙e pretty lucky to have snuck across the [Cretaceous-Paleogene] boundary in the first place,鈥 Field says. Dinosaurs didn鈥檛 make it, along with countless other animals that we鈥檒l 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.