Evolution of Flowering Plants 100 Million Years Ago Caused Decline of Mammal Species

It turns out that the explosion of flowering plant species that occurred about 100 million years ago wasn't necessarily a good thing for evolving mammals. Although scientists once thought that mammals thrived under these conditions, it turns out that they instead declined as flowering plants appeared in the fossil record.

Previous research indicated that the spread of angiosperms, along with the evolution of pollinating insects, may have spurred an increase in the diversity of mammals. This theory seemed to make sense. After all, the radiation would have likely resulted in more food sources for the mammals.

Surprisingly, though, this seems not to be the case. In order to examine the correlation between mammal diversity and plant diversity, researchers examined mammal jaws and teeth from fossils that dated back to the Cretaceous. The scientists discovered that instead of an increasing diversity of mammals, there was a decrease.

"At the middle of the Cretaceous, a time when the early angiosperms are radiating, we find a surprising decrease in the diversity of mammals," said David Grossnickle, one of the researchers, in a news release. "It's not until the end of the Cretaceous, close to the time of the extinction of the dinosaurs, that we actually see a rebound in mammalian diversity and the first appearance of purely herbivorous mammals."

While the number of mammal species may have increased, their variety decreased. The mammals that did survive were all small, insect-eating animals.

"From the fossil record, the time of the angiosperm radiation doesn't look like a very good time for mammals," said Grossnickle in a news release. "There's not as much variation as there was before and after that time, and there's not as much as you would expect at a time when angiosperms were radiating."

The findings reveal a little more about the evolution of mammals. In particular, it reveals that these early creatures  were all similar. These early therians eventually gave rise to the most modern mammals, including humans.

"Without the ecological changes brought about by the Cretaceous radiation of flowering plants, the world would be a very different place and may not have triggered crucial adaptations of our clever primate ancestors," said Grossnickle.

The findings are published in the journal Proceedings of the Royal Society B: Biological Sciences.