Hummingbird-pollinated flowers evolved perfectly to suit the bird’s bill shape, its colour vision and even its taste buds. This is the beauty of co-evolution, where two species interact so closely that they evolve together.

These sort of flowering plants grew rapidly in numbers and variety about 100m years ago during the mid-Cretaceous period. From what we understand about co-evolution, such rapid diversification in flowering plants should drive diversification in species interacting with those plants.

But does it? That is the question David Grossnickle and David Polly of Indiana University ask in a new study published in the Proceedings of the Royal Society B. Their answer, based on fossil records of mammals, is “no”. That means there is something about co-evolution we do not understand.

Mammals come in four kinds: insect-eaters (anteaters, for example), plant-eaters (koalas), fruit-eaters (squirrels) and omnivores (felines and most primates). The size and shape of their jaws and teeth are quite different and can be used to figure out what food they prefer.

Grossnickle and Polly looked at the Cretaceous fossil record of jaw bones and teeth, which are among the best preserved samples. What they find is that most mammals groups then did not flourish despite the rapid growth of flowering plants.

“It is 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,” Grossnickle told me.

This seems to contradict co-evolutionary theory, but that would be the wrong conclusion. Grossnickle and Polly suggest this period of Earth’s past was actually stressful for mammals. It is termed as the “Cretaceous terrestrial revolution”. A bloom in flowering plants happened at a time when enormous ecological changes were occurring.

During the Cretaceous “blooming” of flowering plants, the mammals that were most successful were those that had evolved to eat insects or plants. Small size and the ability to move on tree tops were added advantages. It was these mammals that survived the disruption. For many other mammals, the changes happened too fast to adapt, which caused a reduction in mammalian diversity.

While flowering plants were spreading quickly, they only started dominating the ecology of our planet towards the end of the Cretaceous. Fossil records show that this start of dominance for flowering plants was also the end of the suppression of mammals. For the few mammals that had survived, this new abundance provided rich-pickings. As flowering plants spread to new ecological spaces, these mammals took advantage and spread out too. Of course, these mammals are our ancestors.

“The study provides an interesting and novel story about early mammal evolution, diet and paleoecology,” Grossnickle said. More importantly, it provides a good example of how simple narratives don’t always explain nature’s messiness.