There the stronger and slower dino in water

You can tame mosasaurs in a underwater cave at the right side at the top

Do the cave of the strong and immune at level 71.70 and 69

To unlock the mosa saddle with an arthro (strong cave first)

tame mosa with an ichthy they won't attack you.

Use the basilosaurus and target and ride in ichthyosaurus and tame