Teeth of Small, Feathered Dinosaurs Tell a Story of Extinction and Survival
Today, my research group published a new paper in the journal Current Biology addressing one of the biggest mysteries of the end Cretaceous mass extinction event- why the did the ancestors of living birds survive, but contemporary small, feathered raptor dinosaurs and primitive toothed birds go extinct?
Our research provocatively suggests that at least some groups of living birds may have their ancestors’ beaks to thank for surviving the asteroid impact and resulting mass extinction that wiped out the dinosaurs. We show the closest relatives of modern birds, the small feathered raptor dinosaurs and primitive toothed birds, went extinct abruptly at the end of the Cretaceous Period, and that beaked birds may have benefited because of their ability to eat seeds. The study was led by University of Toronto PhD student Derek Larson (now at the Philip J. Currie Museum) and included Dr. Caleb Brown, who graduated a couple of years ago and is now at the Royal Tyrrell Museum– the study took almost five years to complete.
Several recent studies have suggested that large herbivorous dinosaurs were decreasing in diversity in the last few million years leading up to the end Cretaceous extinction event. We were interested in seeing if small, feathered dinosaurs and early birds followed this pattern. But their fossil record is extremely fragmentary in the last 20 million years of the Age of Dinosaurs. Therefore, it is difficult to quantify just how many species there were at any given time and how that changed. The only consistent and informative fossil indicator of their diversity is their teeth- which are abundantly preserved in microfossil bone beds. Interestingly, tooth shape is a key indicator of diet, so we reframed the question from how diverse they were in terms of species, to one that traced the breadth of feeding niches they occupied in the time leading up the extinction event. This would actually provide us with even more information because we could look at the ecology of their extinction.
So we analyzed more than 3,000 of these teeth to give the highest resolution picture of their changing shape and diets over the last 20 million years of the Age of Dinosaurs. The teeth of these bird-like dinosaurs and primitive toothed birds to show that these dinosaurs were a consistent and stable part of the ecosystem leading up to the end of the Cretaceous. There was no evidence of a long term decline- they went extinct very suddenly in geological terms.
Preserved bird and bird-like dinosaur teeth examined in this study were likely suited to eat a variety of animals- insects and small vertebrates. However, modern birds (Neornithes) are characterized by the shared presence of a toothless beak-so we hypothesized that diet might have been a key factor in their survival. And if the feathered bird-like dinosaurs that ate animals went extinct, perhaps birds survived because they could eat plants, or more specifically, seeds. In the conditions in the wake of the asteroid impact, global forest fires raged and the sun was blocked out by debris ejected into the atmosphere. The ecosystem would quickly lose foliage and animal life. The tooth maniraptoran and early birds were tied to photosynthetic food webs, and my have been more likely to perish. But seeds would be high-energy packets of food that would persist on the landscape, and any animal that could access them would have an ecological advantage. IF the ancestors of at least some groups of modern birds could access this resource in the critical time period immediately after the impact, that could have been vital.The bird fossil record at the end of the Cretaceous is very incomplete, so the ecologies of species that survived the mass extinction are still largely unknown. With little direct evidence of fossil species surviving the extinction, the reasons as to why some species were able to survive the extinction while their closest relatives went extinct have been unclear. So we looked to living birds to see if we could test our idea. First, we used the latest family tree of living birds to see if seed-eating was the primitive diet of the early ancestors of living birds. The data clearly infer that the earliest branches of the bird family tree were likely seed eaters that preferred terrestrial habitats, and were unlikely to be waterbirds- as has been previously suggested. So these groups would have been around when the asteroid hit, and could have benefited from eating seeds during this global crisis. We then looked to present day ecological studies to see what bird flourish in disturbed habitats. Interestingly, it turns out that seed-eating birds are very typically the first vertebrates back into disturbed habitats ravaged by forest fires. This makes sense, because a forest fire will burn all the foliage and decimate the animal population, but seeds can survive in their protective shell, and they can lie dormant for decades, providing food for seed-eating birds.
Its important to keep in mind that extinction patterns at the end of the Cretaceous are complex and involved multiple factors, and we are not proposing that the only reason crown birds survived was seeds. Body size, sensory and metabolic differences, and other factors may have also contributed, and conditions would have affected different bird lineages differently. But small teeth have given us big insights into the extinction patterns of bird-like dinosaurs and primitive birds, and a better understanding of how diet might have played a role in the survival of crown-group bird groups in the wake of the asteroid impact.
Read the paper:
Larson, D.W., C. M. Brown, and D. C. Evans. Dental Disparity and Ecological Stability in Bird-like Dinosaurs prior to the End-Cretaceous Mass Extinction. Current Biology: DOI: http://dx.doi.org/10.1016/j.cub.2016.03.039
Toronto Star story here.
Globe and Mail Story here.
CSM story here.
CBC story here.
Derek on Quirks & Quarks here.