While most small bird-like dinosaurs disappeared along with the rest of them during the extinction event 66 million years ago, the reason for the remaining survivors has baffled scientists. Now, researchers claim that sudden changes in ecology following the meteor impact was most damaging to carnivorous bird-like dinosaurs, leaving early modern birds with toothless beaks relatively unharmed due to their ability to survive on seeds.
"The small bird-like dinosaurs in the Cretaceous, the maniraptoran dinosaurs, are not a well-understood group," said Derek Larson, a Ph.D. candidate at the University of Toronto and first author on the study. "They're some of the closest relatives to modern birds, and at the end of the Cretaceous, many went extinct, including the toothed birds - but modern crown-group birds managed to survive the extinction. The question is: why did that difference occur when these groups were so similar?"
Larson and his team examined the extinction at the end of the Cretaceous period in order to determine if the event was a gradual decline or an abrupt one. Fossil records show support for both hypotheses, depending on which dinosaurs are examined.
The team examined 3,104 fossilized teeth from different maniraptoran families and examined the patterns of diversity in the teeth, including shape and size. They believed that if variation between the teeth decreased over time, the diversity loss would point to the decline of the ecosystem and possibly a progressive, long-term species loss. However, if they maintained their differences, it would point to a rich, stable ecosystem and the abrupt killing of the bird-like dinosaurs at the end of the Cretaceous.
The results pointed to the latter explanation - they observed a steady level of variation over the last 18 million years of the Cretaceous before abruptly going extinct.
Using these findings, the team suspects that diet likely played a role in survival and examined dietary information and previous research from modern-day birds to determine that the last common ancestor of today's birds is a toothless seed-eater that possessed a beak.
The results suggest that the lineages that eventually evolved into today's birds survived due to the ability to eat seeds following the meteor impact that destroyed most other food sources.
"There were bird-like dinosaurs with teeth up until the end of the Cretaceous, where they all died off very abruptly," said Larson. "Some groups of beaked birds may have been able to survive the extinction event because they were able to eat seeds."
The findings were published in the April 21 issue of Current Biology.
