Mammal Varieties Declined When Flowering Plants Evolved, Study Suggests

Mammal varieties declined during the explosion of flowering plant species 100 million years ago, a new study suggests.

The report was published in the journal Proceedings of the Royal Society B, and outlines the unexpected findings derived from an analysis of the jaws and teeth of ancient mammals.

One of the first paleontological examinations of its kind, the study was made possible through a number of fossil discoveries made during the last 30 years, according to the researchers.

"At the middle of the Cretaceous, a time when the early angiosperms are radiating, we find a surprising decrease in the diversity of mammals," David Grossnickle, who conducted the research as part of his master's degree at Indiana University, said in a statement. "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."

The discovery contradicts previous literature suggesting that the spread of flowering plants, or angiosperms, could have triggered an increase in mammalian diversity. However, while there may have been a rise in the number of mammal species, the authors of the new study explain, their variety appeared to diminish during this period, with most of those that survived being small, insect-eating animals.

"From the fossil record, the time of the angiosperm radiation doesn't look like a very good time for mammals," Grossnickle said. "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."

One group that did especially well during the mid-Cretaceous, the scientists noted, were the early therians, the predecessors of many of today's mammals, including humans.

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

The study coincides with a separate report which suggests that flowering plants may have evolved 100 million years earlier than current estimates assume. Led by researchers from the University of Zurich, the report included a look at pollen samples that, the scientists say, date back to the Early Triassic between 252 million to 247 million years ago -- if not earlier.