We Used to be Big
Our fellow synapsids, that is. Before the dinosaurs and their fellow diapsids took over the planet, before the great CO2 catastrophe at the end of the Permian, early synapsids, some of them ancestral to mammals, were the boss, ecologically speaking. The great dying at the end of the Permian, likely caused by a CO2 greenhouse catastrophe, wiped out the big guys, and nearly all of the synapsids, as well as nearly all marine life.
It took ten years for life to acquire major diversity again, after which the diapsids (dinosaurs and others had their day.