Did mammals and dinosaurs exist at the same time?

Mammals have been around since the Triassic Period, alongside early dinosaurs like Riojasaurus and Saltopus, according to fossil records. True mammals, such as the shrewlike Juramaia sinensis, first appeared 160 million years ago, during the Jurassic Period, where they would have coexisted with dinosaurs like the brachiosaurus and Stegosaurus.

A global cataclysmic event 66 million years ago heralded the end of the Mesozoic Era and altered the whole trajectory of life on Earth (Fig. 1). This was the end-Cretaceous mass extinction, and whether it was caused by a bolide impact or massive volcanic eruptions is still discussed. Both are supported by geological data. The consequence, on the other hand, is undeniable.

The dinosaurs, which had dominated the terrestrial vertebrate fauna for the previous 125 million years, vanished, as did the related flying pterosaurs and the huge marine reptiles, including ichthyosaurs, plesiosaurs, and others. Birds (which are essentially small, feathered, flying dinosaurs) and crocodiles, lizards, snakes, and turtles were among the vertebrate survivors. All of these groupings quickly diversified, however none of them saw major evolutionary changes in body structure. A few pre-existing animals escaped the extinction, but their stories were extremely different.