I also don't know much about plesiosauri teeth, I'm just going based on what my few minutes' research is saying. So if you want to look into that and see what you come up with, I'd love to know more
And-for sea mammals-I think the Narwhal ought to find a place in the game, as should human/dolphin transformers-or just dolphins.
As an aside: If/when we get the ability to edit the first post in a given thread, I'm planning on presenting a middle-era Aboleths nation based on real-world Mamluk culture. Alongside it, I'll be offering a list of spells for EA Aboleth prehistoric marine creature national summons.