I have not seen any fossil mounds so far and neither have I seen any player mention of them. The Tek Rexes appear to not be tied to the event and are a permanent addition to the game according to wiki and forum posts. Hence Tek Rexes showing up but no mounds.

It would be cool to have the event activated. Thread with person who had similar experience on his server and the solution: https://steamcommunity.com/app/34611...3726582447161/