One of the most intriguing and enduring aspects of dinosaurs is their extinction at the end of the Cretaceous Period. After decades of research into this topic, most palaeontologists can agree on several details regarding the dinosaur mass-extinction. First, the extinction was due, at least in part, to an asteroid impact with the Earth at the end of the Cretaceous. Second, not all dinosaurs went extinct at the end of the Cretaceous. A group of small, feathered, and very specialized dinosaurs survived, and actually thrived – today we just call these birds.
Despite the relative agreement on these areas, there is still ongoing debate between palaeontologists regarding other aspects of the end-Cretaceous mass extinction. Two of these hotly debated questions include:
- Was the dinosaur extinction a sudden, catastrophic event, or were dinosaurs already on the decline prior to the impact?
- Why did birds survive the extinction when so many closely related, and very similar, dinosaur groups died out completely?
A new scientific article published today in the journal Current Biology investigates these questions. The scientific team was led by Derek Larson, Assistant Curator at the Philip J. Currie Dinosaur Museum, Wembley, Alberta, who completed the research as a PhD student at the University of Toronto. Also on the team are co-authors Dr. Caleb Brown of the Royal Tyrrell Museum of Palaeontology, Drumheller, and Dr. David Evans of the Royal Ontario Museum, Toronto.
The team concentrated on a group of small meat-eating dinosaurs known as maniraptorans – a group that includes modern birds, and dinosaurs like Velociraptor and Dromaeosaurus. Because these dinosaurs are smaller, rare, and generally more incomplete than their larger counterparts, their response to the end-Cretaceous extinction has been less well-studied. Due to the rarity of well-preserved skeletons, the team decided to use teeth; specifically, they measured the teeth and tracked how they changed through time. These dinosaurs (like sharks today) constantly shed teeth throughout their lifetime, so as a result, one animal could contribute hundreds of teeth to fossil record. Teeth are also very useful, because their shape is related to the diet of the animal. Look at an animal’s teeth, and you get a good idea of what it eats. Despite the rarity of complete skeletons, the team had many teeth to sample, and in the end they were able to measure more than 3,000 teeth from four different groups of maniraptorans. These teeth did not come from one fossil deposit, but actually spanned rocks for the 18 million years preceding the Cretaceous mass-extinction.
The ultimate goal of the project was to assign all of these teeth to successive time bins, and then track how their shape changed through time right up to the extinction event. If these dinosaurs were in steady decline, we would expect the variety of tooth shapes to decrease up to the extinction event, but if the extinction was sudden, the tooth shapes would be relatively constant through time. After crunching the numbers, the end result is that the disparity of the teeth (a fancy way of saying how different they are from each other in terms of shape) shows no decline leading up the extinction event. This means that, at least for the small meat-eating dinosaurs, the extinction was sudden.
But what does this research say about why birds survived the extinction? After looking at so many teeth, the team realized that the difference might be that those birds that survived the extinction did not have teeth, but had toothless beaks. This means that while most of these small dinosaurs with sharp teeth needed to eat meat regularly, the beaked birds might have been able to eat seeds.
This is important because seeds are very good at lying dormant for long periods of time. If the ecosystems collapsed following the impact, and most resources were limited, there would still have been lots of seeds to eat. The same thing is seen today—when a forest fire clears out a section of forest, the first birds to return to the area are seed-eaters. To test this idea, the team mapped seed-eating diets onto a family tree of birds and showed that many of the bird groups that survived the extinction would likely have had ancestors that ate seeds. Whether or not this idea holds up over time will depend on future scientists finding more fossil birds and testing these ideas.
A link to the press release is available here: https://dinomuseum.ca/wp-content/uploads/2016/04/Larson-2016-Bird-Dinosaur-Press-Release-PJCDM.pdf
A link to the scientific paper is available here: http://www.cell.com/current-biology/fulltext/S0960-9822(16)30249-4
Reference: Larson, D.W., Brown, C.M., and Evans, D.C. 2016. Dental disparity and ecological stability in bird-like dinosaurs prior to the end-Cretaceous mass extinction. Current Biology 26, 1–9. http://dx.doi.org/10.1016/j.cub.2016.03.039