Other Extinction Hypotheses
Although the impact hypothesis is the most widely accepted explanation of the K/T extinction, other theories still remain. Evidence of widespread volcanism, particularly at the Deccan traps in India, correlates with this moment in time as well. Prolonged volcanism could have led to atmospheric and climatic changes similar to those proposed for an asteroid impact. However, volcanism does not provide an alternate explanation for the high levels of iridium in the clay layer, because high concentrations of iridium occur deep in Earth's core rather than in the mantle, which is the source of the magma that was erupted.
One debate centers on whether the extinction was truly as sudden as it appears, or whether this is an artifact of the geological record. Some scientists believe that dinosaurs went extinct gradually, and were doing so for millions of years prior to the K/T boundary. Studies in the Western Interior of North America have suggested that the latest layers of Cretaceous sediments contain fewer dinosaur species than those below. These results have been challenged by other researchers, who claim that no such decrease is apparent in the Late Cretaceous record.