I'll try Bill, but I know I am not as glib as most in explaining words.

A cusp as I know it is the very most tip of a thing. So behind you, you've got everything you know about dinosaures and it seems that you have reached the top of your knowledge when you look down and there is a whole new road, stretching down as far as the eye can see.

Oh and they'd call it a cusp because the knowledge is not in the same old direction but a completely new direction.