Final answer:
The cause of the K–Pg mass extinction is primarily supported by evidence of a catastrophic asteroid impact, indicated by the worldwide iridium layer and the Chicxulub crater, as well as an increase in fern abundance post-extinction.
Step-by-step explanation:
The evidence supporting the cause of the Cretaceous–Paleogene (K–Pg) mass extinction predominantly points towards a catastrophic asteroid impact. This cosmic event caused widespread consequences, such as a thick cloud of debris in the atmosphere that blocked the sun. This led to a dramatic decrease in photosynthesis, affecting the food chain and resulting in the extinction of many species, including dinosaurs. The immediate aftermath included a drop in temperature and light, but the longer-term effects were even more severe because plants and animals died, disrupting ecosystems.
Paleontologists have identified a layer of sediment worldwide that is rich in the element iridium, which is rare on Earth's crust but abundant in asteroids, providing strong evidence for an asteroid impact at the time of the K–Pg mass extinction. Additionally, the discovery of the Chicxulub crater beneath the Yucatán Peninsula in Mexico aligns with this hypothesis. This event's timeline matches that of the mass extinction, further solidifying the asteroid impact theory.
Analyses of pollen grains and fern spores from sedimentary layers demonstrate that ferns were more abundant after the mass extinction, indicating a massive disruption in plant life at the K–Pg boundary. While multiple factors may have contributed to the end-Cretaceous mass extinction, the asteroid strike is the primary cause supported by strong geological and palynological evidence.