Answer:
They found evidence of an impact crater in Mexico
Step-by-step explanation:
The K-Pg boundary is the boundary that separates two periods, but also two eras. The two periods that are separated by this boundary are the Cretaceous and the Tertiary, while the two eras are the Mesozoic and the Cenozoic. The boundary is marked by a mass extinction, and the scientists started to hypothesize as to why it happened. There were numerous different theories, but a clue in Mexico, or rather right next to the Yucatan Peninsula, shed new light on it. The scientists found an impact crater, formed either by a large asteroid or a comet. Such an impact would have easily caused sudden and drastic changes on the Earth, killing of the majority of the species. This impact released such a big amount of energy that it killed everything in the surrounding area in seconds. On top of that, it caused an extremely hot wave of air across the whole planet, killing many more organisms. It managed to trigger most of the volcanoes to erupt, as well as the earthquakes, causing also very large tsunamis. The sunlight got blocked for quite some time from all the aerosols in the atmosphere as well. This resulted in a mass extinction of the dominant species, like the dinosaurs, but it opened up the terrain for the evolution of the others, like the mammals.