For years, many people believed that the mass extinction of the dinosaurs opened up the way for mammals to become one of the dominant species on Earth.
However, new evidence suggests that mammals may not exactly have been the lowly creatures that they were initially thought during the time of the massive reptiles.
Researchers at the University of Southampton in the United Kingdom and the University of Chicago in the United States set out to challenge the popular notion that mammals were largely suppressed during the dominance of dinosaurs and only became diversified after the reptiles became extinct.
In their study, featured in the journal Proceedings of the Royal Society B, the researchers discovered that the ancestors of present-day mammals known as therian mammals may have already undergone considerable diversification even before the dinosaurs were wiped out.
The extinction of the giant reptiles may have also produced adverse effects on the diversity of mammals.
Mammals Taking A Backseat To Dinosaurs
The basis of the old hypothesis stems from the discovery of fossils from mostly small and insect-eating mammals, which suggested that these creatures weren't able to diversify enough during their time.
Scientists, however, started to rethink this earlier notion as more and more fossils of larger early mammals were found in recent years. Some of these remains were from hoofed animals that were comparable to the size of dogs. The teeth from these animals were shown to be varied as well.
The researchers examined the molars of hundreds of prehistoric mammals included in the fossil collection of museums. They found that the early mammals that had lived right before the extinction of the dinosaurs already had teeth in various shapes, which means they may have had varied diets.
This finding is important as it revealed the possibility that some species of mammals died out as well along with the extinction of the dinosaurs.
While some mammals were able to diversify long before the dinosaurs were wiped out, the mass extinction of the giant reptiles may not have been the ideal opportunity for mammals to evolve as it was initially thought.
The early mammals themselves went through a selective extinction that coincided with the disappearance of the dinosaurs. Species that were able to live off different types of food were able to survive, while those that had more specialized diets eventually died out.
David Grossnickle, a researcher from the University of Chicago and lead author of the study, said they were surprised to discover that early mammals struggled during the mass die-off of the dinosaurs.
He said that he wasn't expecting to see any drop in the diversification of early mammals immediately following the extinction of the dinosaurs. It didn't match the popular notion that the mammals were able to hit the ground running after the event.
It is still unknown what exactly caused the pre-extinction diversification of mammals, but according to Grossnickle, it may have had something to do with the risk of mammals and flowering plants, which occurred right about the same time.
Grossnickle said these plants may have provided the early mammals with much-needed seeds and fruits for their survival. If early insects also evolved at the same time as these flowering plants did, then they may have served as a vital food source for the mammals as well.
The researchers believe the results of their study can help scientists understand more about the mass extinction event the Earth is currently experiencing.
"The types of survivors that made it across the mass extinction 66 million years ago, mostly generalists, might be indicative of what will survive in the next hundred years, the next thousand," Grossnickle said.