What would have happened if an asteroid hadn’t killed the dinosaurs? Perhaps mammals wouldn’t have evolved to become the planet’s dominant group of animals and humans would never have existed. We’re certainly used to the idea that it was only after the dinosaurs (apart from birds) became extinct around 66m years ago, that mammals were able to develop and spread around the globe. But new research suggests this picture of Earth’s evolution may not be completely accurate.
Work by a team that includes me and other palaeontologists from the universities of Chicago and Southampton has unveiled a new level of detail of early mammals. We discovered that the therian mammals (the kind that gave rise to most modern mammals) were actually beginning to diversify between 10m years and 20m years before the Cretaceous mass extinction event that wiped out the dinosaurs.
This finding reflects the considerable changes in our understanding of the early evolution of mammals, thanks to exceptional fossils found over the last few years.
Previously, most palaeontologists thought that mammal diversity was suppressed by the dominance of dinosaurs. This was largely based on the fact that many of the early mammal fossils that had been found were from small, insect-eating animals with very similar feeding habits and life histories. But with the help of some astounding new fossils, including several from China, we have shown that mammals at this time were much more diverse than this suggests.
We focused on studying several aspects of early mammal diversity, including the number of species through time and how these animals differed in both their physical structure and the way they lived. We also looked at how these things changed through time, particularly during the period either side of the extinction event. To do this, we studied the shape of teeth of hundreds of early mammal specimens in museum fossil collections.
Mammal teeth are highly developed, with uniquely complicated series of cusps, troughs and pits that perform precise functions for feeding. This means the specific shape of teeth can offer a wealth of information on the lives of long-extinct animals.
Tracking changes in tooth shape through time showed us that the mammals that lived during the years leading up to the dinosaurs’ demise had widely varied diets. This also helped us uncover another surprising finding. Mammals with the most extreme tooth shapes disappeared immediately following the extinction, which suggests that those that had the most specialised diets suffered along with the dinosaurs.
Our research follows other recent findings that dinosaur diversity was already decreasing before the extinction event, exactly as mammals were on the rise. So what caused this growing change in the prehistoric animal kingdom? One explanation is that the rapid spread of flowering plants (angiosperms) during the late Cretaceous period had a significant effect on the ecology of animal groups.
It is possible that mammals may have been able to adapt to this change in plant life and flourish while the dinosaurs could not. Small-bodied mammals may have been early adaptors to the range of new food sources (fruit and seeds), ecosystems and insect prey provided by the flowering plants. Meanwhile, non-avian dinosaurs felt more of the negative effects as these flowering plants took over their habitats, causing their traditional food sources to disappear.
Mammals' shaky start
Another traditional theory that our research questions is that mammals diversified rapidly immediately after the dinosaurs went extinct. Traditionally, the extinction has been seen as an ideal opportunity for mammal evolution to take off, but our findings suggest that early mammals were also hit by a selective extinction event. Some that could live off of a wide variety of foods were able to survive, but many other mammals with specialised diets went extinct.
By offering a more realistic image of the dynamics of mass extinction on mammals, our research may also provide important evidence of how mammals could be affected by man-made climate change. It raises the question of whether we should prepare for a similar selective extinction event in modern mammals as occurred 66m years ago.
This highlights how studying the fossils of long-dead organisms can actually help us to model our own effects on global ecosystems and habitats. In a rapidly changing world, the fossil record is a unique and unadulterated constant that may provide solutions far beyond what most people expect.