For nearly 200 million years, reptiles were the dominant animals on land, in the air (e.g. pterodactyls), and in the sea (e.g. mosasaurs). They were efficient herbivores, munching on lush vegetation, and also were efficient carnivores (think: T. rex). They were protected by scaly skin and often horns or armor plates. Mammals at this point were typically small, rat-like creatures, hiding in their burrows from the reptiles, and creeping out at night to feed.
However, the Age of Reptiles came to a sudden end 66 million years ago. Dinosaurs and many other large reptiles disappeared, which gave opportunity for mammals to rapidly evolve and proliferate to fill many key ecological niches. What happened to all those reptiles? The leading hypothesis is that a huge meteorite impacted the earth near what is now the Yucatan peninsula of Mexico. The dust and aerosol cloud that was thrown into the atmosphere darkened the skies around the world enough to shut down photosynthesis long enough to starve the reptilian herbivores, which in turn starved the reptilian carnivores. Somehow enough mammals survived the event to repopulate the earth (my guess is they ate insects which ate dead dinosaurs).
The impact blasted tons of molten rock droplets high in the air, which then fell as little glassy spheres or dust particles all over the world, and especially in North America. Where these “tektites” fell in undisturbed places like bogs, they accumulated as a distinct layer. Over time, these spheres decomposed into a clay layer which is distinguished by a high iridium content. Here is a cut-out section of rock which shows this meteorite-derived boundary layer between lower (older) rocks that contain dinosaurs and an overlying layer where dinosaurs are absent:
Rock section showing layers from the Cretaceous Period (when dinosaurs lived), overlaid by boundary layer material from the asteroid strike 66 million years ago, and then younger Paleogene rocks (no dinosaurs). Source: Phil Manning/Uni of Manchester, UK.
Exactly When and How Did the Dinosaurs Perish?
The picture is complicated by the fact that very few dinosaur fossils have been found in roughly three meters (ten feet) of sedimentary rocks immediately below the Ir-rich meteorite layer. This is known as the “three-meter problem”, and suggests that the dinosaurs had already largely died out from other causes; maybe the meteorite impact just finished them off. Shortly before the impact event, there was a massive series of volcanic eruptions in the Deccan Traps area of India which released enormous amounts of sulfur dioxide and other gasses in the atmosphere, which probably altered the climate. It has been proposed that this fatally stressed the dinosaur populations.
Recent finds from the “Tanis” fossil site in North Dakota have brought clarity to this question. Apparently when the meteorite hit in what is now Mexico, it created a forceful earthquake. When this tremor rolled up to North Dakota, it caused several large waves of water to surge upstream in a creek near the sea, which deposited layers of muddy clay on preexisting sandbars. This occurred several hours after the impact. Providentially, that was just when some of the small glassy spheres which were blasted into the atmosphere were raining down on North Dakota. Some of these spheres, and even their little impact depressions from smacking into the mud at terminal velocity, have been found in the layers of sediment deposited on the sandbars. So we know that whatever fossil remains we find in these sediments were entombed there on the very day the meteorite hit.
It turns out that numerous fossils of dinosaurs have been found in these Tanis mud layers, indicating that there was a thriving community of huge reptiles right up until the impact. These finds include a dinosaur hip/leg with exquisite details of skin preserved, and an egg with a partly-developed pterosaur embryo visible in it:
Ornithischian dinosaur hip/leg/skin from Tanis site. Source: BBC
Fossilized egg with bones of pterosaur embryo in it. Source: Yahoo
Also, immediately below the mud deposit layer have been found numerous dinosaur footprints, indicating the juvenile and adult dinosaurs from a variety of species were tramping around shortly before the impact event:
Source: Riley Wehr et al. paper at 2021 GSA Conference
Bottom line: it looks like we humans do owe our existence in large part to this one, seemingly random meteorite impact which cleaned out the dominant reptiles and made room for mammals.