To make Mosasaurus Saddle, combine Hide, Fiber, Cementing Paste, Silica Pearls, and Metal Ingot. The Mosasaurus Saddle unlocks at level 78 and requires 60 Engram Points.
tame mosa with an ichthy they won't attack you.
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)