Prior to the time when Luis and Walter Alvarez proposed that an impact led to the extinction of the dinosaurs, it was generally assumed that the dinosaurs had died out over a period of at least thousands to millions of years. Consider each piece of evidence listed below and sort it into the correct bin based on whether it supports a sudden cause for the demise of the dinosaurs or whether it could also be consistent with a gradual cause leading to their demise over many thousands or more years.