It sounds strange to me. I would expect to read something like:
What we do/did not find is/was the enormous range of...

Phew. :-) So I am not the only one.

Thanks for your reply. I just wondered, because usually the person who wrote this (an American professor) uses a rather sophisticated English. I thought it's my poor English that plays a trick on me, so that I just cannot understand the construction.

Manrico

Your perplexity is entirely justified. Your professor had an off moment.

My guess is that your prof. accidentally missed out a/some word(s) - perhaps she/he intended something like : What has not happened is the appearance of an enormous range of roles ...