For a long time, paleontologists (scientists who study ancient organisms) have debated about the “tempo and mode” of the extinction that wiped out the dinosaurs. Everyone agrees that an asteroid did them in, but the question is: were they on their way out anyway? The study by myself and colleagues suggests that dinosaurs weren’t slowly marching towards extinction, but instead that North American ecosystems as a whole were changing, and that focusing too intensely on dinosaurs may have led people to miss broader patterns.
Diagram representing how the terrestrial animals of North America interacted 65 million years ago, at the end of the age of dinosaurs.
Way back in the day, when giant dinos roamed the land, North America was cut in two by the Western Interior Seaway. Effectively, this meant that there were two separate continents: east North America (Appalachia) and west North America (Laramidia).
By the start of the latest Cretaceous (~72 million years ago) this seaway had begun to retreat, and the two isolated landmasses (and the animals that lived on them) started to come together once again. What my collaborators and I did was use a mathematical model developed by one of us (Peter D. Roopnarine) to try and understand how food webs changed in the lead up to the asteroid impact. What we found was that the later ecosystems were more fragile than the earlier ones. That isn’t to say that the ecosystems were about to fall apart, or that the extinction would have happened without the asteroid impact “pulling the trigger”, but that if the asteroid had struck a little earlier, the extinction wouldn’t have been as potent in the terrestrial realm of North America.
A major part of the increased fragility was a change in how species were distributed ecologically. In the latest Cretaceous, there were more very large herbivores and more very small carnivores and omnivores at each fossil site examined (10 for the earlier time bin, 7 for the later one). Very large herbivores, like Triceratops, were important ecologically because, when they were first born, they were quite small, and so small carnivores could eat them. As they aged and grew, they became prey for medium sized carnivores. When they were fully grown, only the largest carnivores could eat them. In other words, these very large animals provided important links between the smallest and the biggest predators in the food web, so when they started dying off, the entire structure of the food web started to unravel. Likewise, the very small carnivores and omnivores (mostly mammals) provided crucial stability by being eaten by the diverse array of smaller predators, and the young of very large carnivores. Without these very small species, the babies of incredibly large carnivores, such as T. rex would have starved long before they got large enough to terrorize the countryside.
Another interesting aspect of this study concerns predicting future events. Ecologists and conservation biologists are working frantically to try and develop ways of predicting extinctions before they happen, so we know what animals to focus on to save. In the fossil record, we know what animals eventually go extinct (we can “see the future” because we live in it). This means that we can explicitly test the predictions of our models, and (as far as I know) this study is the first time anyone has ever taken a food web model and tested its predictions against the observed extinctions in the ancient past (and it did okay! over 78% correct!). This study may help open the door for more collaborations between people who study ancient mass extinctions and those trying to prevent the next one.
Jonathan S. Mitchell, Peter D. Roopnarine, and Kenneth D. Angielczyk. 2012. Late Cretaceous restructuring of terrestrial communities facilitated the end-Cretaceous mass extinction in North America PNAS; published ahead of print October 29, 2012, doi:10.1073/pnas.1202196109s