The Empty Barrel Theory

Before it was known that Ediacaran organisms survived through the Neo-proterozoic/Cambrian boundary, some theorized that a mass extinction put an end to the Ediacarans. After an extinction, survivors diversify and fill ecological niches left emptied by the extinction.

The "empy barrel theory" had it that the Cambrian explosion of diversity was a response to extinction of the Ediacarans.