In general, the Mosasaur and Tusoteunthis spawns occur on the edge of the world border on Lost Island map of Ark Survival Evolved. Below are a few of the locations where I have seen them. Hopefully, it helps narrow down where you might find them. It isn't meant to be a comprehensive list, but should help you find some of Ark's creatures. I tend to find more Moses and squid on the western border of the map and by the desert biome.