Explanation:

The major mass extinctions all occurred in the relatively short time frame of several million years (geologically speaking). Different types of phyla in different habitats which are spread across the planet must have become extinct in the catastrophe. The causes of these mass extinctions are still subject to debate and have not left an obvious "smoking gun". Though the effect of mass extinctions can be seen today, even the most recent events do not have a cause that can be called obvious and beyond doubt.