Scientists have long wondered just how many birds survived the asteroid impact that wiped out the rest of the dinosaurs some 66 million years ago. Now, they may have their answer: very few, mostly small ones. A new study suggests that widespread forest fires made it impossible for tree-dependent birds to survive, meaning the vast avian diversity of today likely arose from just a few ground-dwelling survivors.
Not everyone agrees with this scenario, but Daniel Ksepka, a paleontologist at the Bruce Museum in Greenwich, Connecticut, calls it an “elegant” hypothesis. “It’s nice, kind of a eureka moment.”
Molecular biologists and paleontologists have long debated the origins of bird diversity. Some argued, based on molecular data, that many groups of birds emerged unscathed from the asteroid impact in Chicxulub, Mexico, that killed off so many other species. However, more recent molecular evidence, combined with fossil finds, has convinced others that most bird groups did perish. Those researchers argue that the vast array of bird shapes, sizes, and lifestyles emerged quickly, after the great extinction event.
To understand what happened to those early birds, Daniel Field, a vertebrate paleontologist at the University of Bath in the United Kingdom, and his colleagues looked at three lines of evidence. First, they determined which branches of the avian family tree contain modern species that depend on trees for food or shelter, and which branches do not. They then did the same for species from North and South America, Europe, and Asia known only by their fossils, based on leg proportions and other skeletal traits. Finally, they collected pollen and spores from above and below the asteroid’s impact line, in southwestern North Dakota.
As they marked up the avian family tree, they noticed that tree-dwelling species—which today vastly outnumber ground-dwelling species—had ground-based ancestors. And the skeletons of the preimpact and postimpact fossils were different as well. Before the impact, there seemed to be many tree-loving species. But those species were missing in the postimpact fossils, Field and colleagues report today in Current Biology.
The spore and pollen data suggested why so many of the tree-dwellers died: Forests thrived before the impact but not afterward, most likely because the asteroid set off a firestorm, they report. After the impact, fossil evidence from North America suggests ferns were the major flora for about 1000 years. “When we started this study, we didn’t know where it would lead,” Field says. But the impact of the global deforestation was clearly devastating.
Among modern birds, it appears that only five groups predate the impact, including species like today’s ostriches, ducks, and chickens. Their impact-surviving ancestors were probably small ground-dwellers, like quail, Field says. They likely survived on seeds banked in the soil, what Ksepka calls “a food source that’s prepackaged for preservation.”
But not everyone is convinced. “It’s difficult to conclude all forests disappeared globally based on [just] evidence from northern areas,” says Alan Cooper, a molecular evolutionist at the University of Adelaide in Australia. Joel Cracraft, an evolutionary biologist at the American Museum of Natural History in New York City, thinks forests may have disappeared in North America, but he has doubts about the rest of the world. “They are trying to claim too much,” he says. Nonetheless the paper “does need to be taken seriously,” says David Penny, an evolutionary biologist at Massey University in Palmerston North, New Zealand, who was not involved with the work. He, Cracraft, and others would like to see more work done on what birds did survive and where, and when. “It’s a debate that’s been going on for decades,” Cracraft concludes. “I don’t think it’s going to end any time soon.”