You can't put indefinite article before "extinction", because it is uncountable noun. I would omit the article. If we want to talk more specifically and to differentiate this extinction from another, we should add a noun, for example "instance of extinction", though in my opinion it looks a bit clumsy.
It is the same as with "evidence", "information" or "advice". We have to use "piece of" if we want to make it plural.