[dghughes]

I'm not a math wizard but going by that pattern it seems the periods are getting shorter.

I tried to find a sequence https://www.wolframalpha.com/input/?i=415,+322,+300,+145,+33....

[mrec]

The periods (intervals between events) there would be [93, 22, 155, 112] My. That doesn't look like they're getting shorter.

[swamp40]

But they are getting closer! :)

[marcosdumay]

My bet is that is because of observation biases. We don't know a lot about distant past, and may have lost some events that we'd call great extinctions if they were recent.