How did the mass extinction that ended the reptile reign help lead to today's mammal diversity?

The mass extinction event around 66 million years ago, which famously eliminated the dinosaurs, opened ecological niches that had previously been dominated by reptiles. This allowed mammals, which had existed in the shadows of dinosaurs, to diversify and adapt to new environments. With the absence of large reptiles, mammals evolved into a wide variety of forms and sizes, leading to the rich diversity of species we see today. This transition marked a pivotal shift in the Earth's biological landscape, paving the way for mammals to become the dominant terrestrial vertebrates.