Over time, hummingbird-pollinated flowers evolve 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.
Flowering plant species grew rapidly in number and variety about 100 million years ago, during the mid-Cretaceous period. From what we understand about co-evolution, such rapid diversification should drive diversification in species that interacted with those plants.
But did 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.” But that turned out to be because co-evolution isn’t the only factor affecting species survival.
In terms of diet, mammals can be divided into four groups: insect-eaters (shrews, for example), plant-eaters (koalas), fruit-eaters (squirrels), and omnivores (most primates). The size and shape of mammals’ jaws and teeth are quite different in each of these groups 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 body parts. They found that most mammal groups that were present at the time did not flourish despite the rapid growth of flowering plants.
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This seems to go against what co-evolution would predict. But Grossnickle and Polly suggest that this period of Earth’s past was actually stressful for mammals. The changes took place during what’s termed the “Cretaceous terrestrial revolution,” and the boom in flowering plants happened at a time when enormous ecological changes were occurring.
During this period, the mammals that were most successful were those that had evolved to eat insects or plants. Small size and the ability to move on treetops were added advantages. These mammals generally survived the disruption. For many other species, 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 toward the end of the Cretaceous. Fossil records show that mammalian diversity started recovering at about the same time. 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.
“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.