The first mammals appeared at the end of the Triassic, a little over 200 million years ago, but they remained in the shadow of the dinosaurs until they disappeared during the Cretaceous-Tertiary mass extinction, about 66 million years ago. Thus, at least, the most widespread hypothesis among paleontologists about the evolution of primitive mammals has been up to now. However, a new study published in the journal Proceedings of the Royal Society B points out that these animals were actually in top form 20 million years before the dinosaurs disappeared. So much so that despite their presence they began to diversify massively and colonize all kinds of ecological niches.
The fact is that, for a long time, the fossils that have been found suggested the opposite: it seemed that those mammals were not very different from each other; they were small, somewhat resembling squirrels or some rodents and insectivores. But in recent years, very varied specimens have been found, some even one meter long, such as Repenomamus giganticus , which could feed on small dinosaurs.
The feed put David Grossnickle, an evolutionary biologist at the Field Museum of Natural History in Chicago, who has coordinated the trial, and his collaborators on the trail. After studying the molars of hundreds of primitive mammals, they found that their shape was very different from each other, an indication that their diet was also different.
For Grossnickle it was not only a surprise to discover the great diversity of mammals that lived with the dinosaurs, but, contrary to what was believed, the disappearance of these did not allow the former to dominate the environment. No, at least, immediately. “We expected that mammals would diversify even more just after the mass extinction that occurred at that time, but in reality, it seems that they also suffered deeply from its effects,” says Grossnickle. In general, those who followed a more specialized diet perished, but those who could choose different foods survived .
However, it is still unknown what drove the diversification of mammals in the middle of the dinosaur era. “We cannot be sure, but at about the same time a similar phenomenon occurred with flowering plants, so there could have been more seeds and fruits available and, also, more insects that served as food for primitive mammals,” he points out. Grossnickle.
Image: Nobu Tamura / GNU
