Many small, feathered dinosaurs called maniraptorans went extinct along with the likes of Tyrannosaurus and Triceratops at the end of the Cretaceous. But those who managed to survive became ancestors to all the birds we have today. Now, researchers studying thousands of fossilized teeth reveal that bird-like dinosaurs with beaks were able to survive by eating seeds. The toothy, carnivorous maniraptorans died out abruptly when their food sources declined. The findings are published Current Biology this week.
Variations in teeth shape – what’s called dental disparity – is a proxy for ecological stability. Decreases in variation would indicate ecosystem decline, but if differences between teeth were maintained over time, that would suggest a rich, stable ecosystem.
To understand what bird-like dinosaurs were up to at the end-Cretaceous boundary, a team led by Derek Larson from the University of Toronto analyzed 3,104 maniraptoran teeth unearthed throughout western North America. These fossils represent four maniraptoran groups spanning 18 million years of the Cretaceous.
Representative teeth from the four groups of bird-like dinosaurs, with enlarged images of tooth serrations. Scale = 1 mm. Don Brinkman, modified from Larson et al., 2010. Can. J. Earth Sci.
Tooth shape disparity showed no major decline leading up to the mass extinction event within any of the groups studied, but this prolonged period of ecological stability was followed by the sudden extinction of many bird-like dinosaurs. "The maniraptoran dinosaurs maintained a very steady level of variation through the last 18 million years of the Cretaceous," Larson said in a statement. “There were bird-like dinosaurs with teeth up until the end of the Cretaceous, where they all died off very abruptly."
Then, by studying dietary information and the evolutionary relationships of modern-day birds, the team tried to figure out what their ancestors ate. They think that the last common ancestor of today's birds was a toothless, beaked seed eater. (The reconstruction of a hypothetical toothless bird is pictured to the right.)
An impact event – and the darkness, acid rain, and “nuclear winter” that came afterwards – would have had a huge negative effect on the production of fruits and leaves, which depend on photosynthesis and sunlight. As a result, the predators of plant-eaters would suffer. But seeds are not only hardy, they’re also common and rich in nutrients. Turning to granivory allowed the lineage to survive the mass extinction event and give rise to all the birds we have today.
Image in the text: A hypothetical toothless bird closely related to the earliest modern birds. Danielle Dufault