Mammals 'thrived despite dinosaurs'
Toothy mammals Mammals did not need the dinosaurs to die off in order to thrive, new research on fossil teeth suggests.
Until now, most scientists believed it took the extinction of the dinosaurs, around 66 million years ago, before mammals were able to develop, says Evans. But his latest research suggests this is not the case.
Before the death of the dinosaurs, the most common mammals were multituberculates - so called because of their teeth.
"Many of the multituberculates have very bumpy teeth and each of the bumps are called a tuberculate," says Evans. "It's a very long name for 'bumpy teeth'".
Evans and colleagues carried out a comprehensive survey of teeth from about 48 multituberculate species held in fossil collections around the world.
They used software normally used to analyse land topography to create high-resolution 3D images of the teeth.
By analysing the complexity of the teeth bumps, Evans and colleagues were then able to map the evolution of multituberculates.
170 million years ago, their teeth were very simple but about 90 million years ago - long before the dinosaurs died off - their teeth started getting more complex.
Previous research has shown that increasing complexity in teeth enables animals to shift from eating insects and other meat to plant material.
Plants are easier to come by, but they also take longer to digest so a shift to herbivory is also generally associated with an increase in body size. Larger animals have a lower relative metabolic rate and have the time to digest slowly.
Significantly, Evans and colleagues found the increase in multituberculate teeth complexity coincided with the evolution of flowering plants.
"There's a nice correlation there between the ecological dominance of flowering plants and the rise of herbivory in multituberculates," says Evans.
At the same time multituberculates increased in size from that of a mouse to a beaver, also supporting the move to herbivory.
Evans and colleagues think these changes gave multituberculates a way of thriving even during the time of dinosaurs.
"We think it might be because they were exploiting this new food source that perhaps dinosaurs weren't using as much as they could have," says Evans.
These changes also gave multituberculates an evolutionary head start compared to other mammals, meaning they were able to do better than others once the dinosaurs did go extinct.
Multituberculates maintained an edge over other mammals for another 30 million years after the dinosaur extinction, and only died out 35 million years ago, due to competition from primates and rodents.
Curator of Vertebrate Palaeontology at Museum Victoria, Dr Tom Rich describes the research as "a new and imaginative approach" to quantifying the adaptation of the multituberculates.
"The authors show quite persuasively that the widely held conventional view that mammalian evolution was 'held back' until the non-avian dinosaurs became extinct is a generalisation that does not hold in detail," he says.
Rich says although there are literally thousands and thousands of multibuerculate fossils in museum collections from sites in the Northern Hemisphere, there is just one specimen known from Australia.
"But we've got one so we know they were here, too! That's all it takes," he says.