What vertebrate did not live during the time of the dinosaurs?

The modern-day mammal, specifically the placental mammals, did not live during the time of the dinosaurs. While some early mammals existed during the Mesozoic Era, they were small and not the diverse group we see today. The major diversification of placental mammals occurred after the dinosaurs went extinct around 66 million years ago. Thus, many of the mammals we are familiar with today emerged long after the age of dinosaurs had ended.