Ecological changes following the dinosaur-ravaging Chicxulub impact 65 million years ago may have been more detrimental to meat-eating bird-like dinosaurs, and early birds with toothless beaks were able to survive on seeds when other food sources declined, according to a team of paleontologists writing in the journal Current Biology this week.

“The small bird-like dinosaurs in the Cretaceous, the maniraptoran dinosaurs, are not a well-understood group. 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,” said team member Dr. Derek Larson, of the Philip J. Currie Dinosaur Museum.

“Why did that difference occur when these groups were so similar?”

Dr. Larson and co-authors began by investigating whether the extinction at the end of the Cretaceous was an abrupt event or a progressive decline simply capped off by the Chicxulub impact.

The fossil record holds evidence to support both scenarios, depending on which dinosaurs are being examined.

Delving into the bird-like dinosaurs, the scientists collected data describing 3,104 fossilized teeth from four different maniraptoran families.

“We examined tooth morphology, an ecological indicator in fossil reptiles, from over 3,100 maniraptoran teeth from four groups (Troodontidae, Dromaeosauridae, Richardoestesia, and Aves) across the last 18 million years of the Cretaceous,” they explained.

If the variation between teeth decreased over time, this loss of diversity would indicate that the ecosystem was declining and may have paralleled a long-term species loss.

If the teeth maintained their differences over time, however, that would indicate a rich and stable ecosystem over millions of years and suggest that these bird-like dinosaurs were abruptly killed off by an event at the end of the Cretaceous.

In the end, the tooth data favored the latter interpretation.

“The maniraptoran dinosaurs maintained a very steady level of variation through the last 18 million years of the Cretaceous. They abruptly became extinct just at the boundary,” Dr. Larson said.

The paleontologists suspected that diet might have played a part in the survival of the lineage that produced modern-day birds, and they used dietary information and previously published group relationships from birds to infer what their ancestors might have eaten.

They hypothesized that the last common ancestor of modern birds was a toothless seed eater with a beak.

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Derek W. Larson et al. Dental Disparity and Ecological Stability in Bird-like Dinosaurs prior to the End-Cretaceous Mass Extinction. Current Biology, published online April 21, 2016; doi: 10.1016/j.cub.2016.03.039