The current database indicates that the terminal decline and extinction, or near extinction, of many groups commonly attributed to an asteroid or comet impact at the Cretaceous-Tertiary (K-T) boundary (e.g., ammonites, bivalves, planktic foraminifera) began during the last 500 k.y. of the Maastrichtian. By the time of the K-T boundary, extinction-prone tropical and subtropical marine faunas and floras were almost gone, or had severely reduced species populations struggling to survive. The K-T boundary kill-effect was largely restricted to these struggling tropical and subtropical populations that accounted for 2/3 of the species among planktic foraminifera, but less than 10% of the total foraminiferal population. No significant extinctions occurred among ecological generalists that dominated across latitudes. No single kill mechanism can account for this mass extinction pattern. The last 500 k.y. of the Maastrichtian were characterized by a series of rapid and extreme climate changes characterized by 3-4°C warming between 65.4 and 65.2 Ma, major volcanic activity between 65.4 and 65.2 Ma, a spherule-producing event between 65.3 and 65.2 Ma, and an impact at the K-T boundary ( 65.0 Ma). All of these events caused major environmental perturbations and biotic stresses that resulted in severe reductions in species populations and extinctions that culminated at the K-T boundary. The mass extinction pattern, and the parallel environmental changes during the last 500 k.y. of the Maastrichtian, suggest that both long-term (climate, sea-level) and short-term (impact, volcanism) events contributed to the K-T boundary mass extinction.