Scientists have long known that flowering plants, as the base of most ecosystems, must have had a profound effect on the evolution of mammals. But it was due to the efforts of a high school biology teacher that this effect was uncovered.
Flowering plants, known as angiosperms, are the most diverse type of plant on Earth. Ranging from mango trees to common lawn grass, today they account for some 12,000 different genera and 250,000 distinct species. They also dominate almost all ecosystems on the planet, on land as well as underwater (exceptions are the tundra, where moss and lichen rule the roost, and the conifer forests where pinecones play the role of flowers.) Everything from insects to fish to mammals to other plants depend upon angiosperms for pollen, seeds, fruit, and leaves for nourishment.
The first angiosperms are estimated to have appeared roughly 200 million years ago, but began to spread and radiate to a large extent about 100 million years ago, during the Middle Cretaceous. This was about the same timeframe in which mammals were getting their start, though mammals didn’t become dominant until much later, after the age of dinosaurs had come to an end.
When high school biology teacher David Grossnickle returned to school to earn a Master’s degree in geology, he decided the coevolution of angiosperms and mammals deserved a second look. With the help of Indiana University professor P. David Polly, Grossnickle collected data on the fossilized teeth and jaws of Cretaceous mammals discovered within the last 30 years. By measuring the age, shape, size, and number of the specimens he was able to estimate what time a species lived, what they ate, and roughly how many there were. His hopes were that this data would paint a picture of what happened to mammals when plants started flowering. Expectations were obvious: the rise of flowering plants would mean more food and shelter and more mammals. Grossnickle and Poly were surprised.
Their paper, Mammal disparity decreases during the Cretaceous angiosperm radiation, published in Proceedings of the Royal Society B, revealed a far more complex story. Contrary to what was expected; species of mammals did NOT become more numerous as angiosperms spread. The number of mammal species actually declined, not to rise again significantly until dinosaurs had begun to die out. Mammals that ate only plants didn’t even evolve until the end of the Cretaceous. The reasons for this are still a mystery.
One set of mammals that did do well, however, were therians. These were small insectivores that later gave rise to modern mammals. If therians had not lived so well during this time, with both flowering plants and dinosaurs, the creatures alive today might look very different.
Grossnickle, D. M., & Polly, P.D. (2013). Mammal disparity decreases during the Cretaceous angiosperm radiation. Proceedings of the Royal Society B: Biological Sciences, 280 (1771): 20132110 DOI: 10.1098/rspb.2013.2110
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