They used to be land mammals. Maybe they had a civilization, but they ruined the climate with it, then evolved to be sea mammals but kept their intelligence?
https://en.wikipedia.org/wiki/Silurian_hypothesis
Yes! another fan of the Silurian hypothesis. I know most dismiss it but there really are oddities in the geological record that are somewhat consistent with it.
Agree. No proof, I know, but such an interesting idea to think about!
Do you have any references to these oddities? Always fun to think about things like this, but can't find much about it.