GMAT Verbal Section | Re: Verb used in the sentence of "ONCE".

Dear Mikemikemcgarry

I have doubt about word "once". I read somewhere that "once" refers to the item coming immediately after "once".

following example was provided in book to support above rule.

Incorrect- Dinosaurs existed once
Correct- Dinosaurs once existed.

Unfortunately, i am not able to understand difference between above two examples. Could you please help me.

Regards,
Amm