Sometimes Alpha mosasaur spawns in the western underwater cave. However it does not aggro and only runs when hit. So can be an easy trophy source.(along with other normal mosasaur).

Just kill cave creatures for it to spawn.

Bring the Alpha Mosasaur to the surface by grabbing it with a Tusotuethis. It won’t even fight back when it’s too close to the surface. Use the tooth for the Alpha Tek Cave.