Around 66 million years ago, at the end of the Cretaceous period, an asteroid the size of Mount Everest smote the Earth. It landed in Mexico’s Yucatan peninsula, punching a 20-mile deep crater into the ground. That impact, and the climatic upheaval that happened afterwards, ended the long reign of the dinosaurs. Of this dynasty of ruling reptiles, only the birds—a specialized group of feathered dinosaurs—survived.
But the birds didn’t escape unscathed.
Birds first appeared around 150 million years ago, during the late Jurassic period. They evolved from small predatory dinosaurs that were similar to Velociraptor. By the end of the Cretaceous, they were flourishing. But the same catastrophe that finished off their dinosaur cousins also killed most of them off. Even incredibly diverse and widespread groups, like the enantiornithines (eh-NAN-tee-OR-nih-theens), died out. The surviving birds were forced to re-evolve much of the diversity that once existed, and most groups of modern birds arose from those survivors, in the aftermath of the asteroid strike.
But which lineages survived, and why?
“A lot of people have focused quite intensively on trying to understand what went extinct [at the end of the Cretaceous],” says Daniel Field, from the University of Bath. “But we know very little about how or why birds managed to sneak across.” In a new study, Field and his colleagues have shown that the species that made it through the extinction event mostly lived on the ground, as modern chickens do today. They walked and strutted into the future, while their relatives that perched in branches and flew through trees largely died out—because many of those branches and trees were on fire.
Field’s team looked at the habits of modern birds, and worked backward in time to reconstruct the likely lifestyles of their shared ancestors. The ancestral species that gave rise to all living ones “was almost certainly a ground-dwelling bird,” Field says. That’s not to say it was flightless; it was probably something like today’s tinamous, small-bodied birds from Central and South America that can fly but mostly choose not to. It was only after a short period that many groups independently took to the trees once again, replacing the tree-dwelling species that had disappeared.
Pollen grains provide a clue as to why this was the case. They fossilize incredibly well, and scientists can easily recover them in the hundreds of thousands. By looking at these ancient grains at a site in North Dakota, the team showed that all over the world, tree pollen almost completely disappears in the immediate aftermath of the asteroid strike. During that time window, the pollen is replaced by the spores of ferns—pioneer plants that are usually the first to regrow in landscapes denuded by fires and other catastrophes.
This so-called fern spike also exists in almost every other continent. “This seems to be a really global signature,” says Field. “Within 1,500 kilometers of the impact site, forests would have been flattened.” But the heat that radiated from the impact also ignited wildfires on a global scale, scorching trees worldwide. Any trees that escaped the flames then had to deal with a curtain of acid rain, and a blanket of atmospheric soot and ash that blocked out much of the sun’s energy.
As the trees burned and withered, any birds that foraged, perched, nested, and courted among them would likely have died. Field’s team estimates that it took a thousand years for forests to recover, and for birds to start readapting to life within them.
There’s one possible problem with this interpretation, says Zhonghe Zhou from the Institute of Vertebrate Paleontology and Paleoanthropology, or IVPP, in Beijing. It certainly seems from the fossil record that tree-dwelling birds died out after the asteroid strike. Then again, those species are less likely to fossilize well than ground-dwelling birds. “We need more fossil evidence to support [Field’s] hypothesis,” Zhou says.
“It’s a great idea,” adds Jingmai O’Connor, also from the IVPP. “But we need to get over this one-answer-for-everything way of thinking. No one factor caused the end-Cretaceous extinction and similarly no one factor caused the extinctions within [the birds].”
Field actually agrees. Massive fires may well have kicked the tree-dwelling species out of their perches, but other factors were at work, too. His own team had previously shown that smaller species stood a better chance of survival, maybe because only they could find enough food in charred, sun-starved landscapes. O’Connor has suggested that the evolution of a more efficient digestive system allowed some groups to better thrive than others. And other researchers have suggested that reproductive traits, like laying bigger eggs, were important.
Living away from trees wasn’t a universal lifeline, either. Luis Chiappe, from the Natural History Museum of Los Angeles County, notes that not all enantiornithines lived in trees, and the ground-dwelling lineages also went extinct. Field’s hypothesis doesn’t explain why. Nor does it clarify why marine species, such as the cormorant-like Hesperornis and the tern-like Ichthyornis, also died out.
“Forest loss was only one of several factors working in combination that determined which bird lineages survived,” O’Connor adds.