On valguero, on the beach next to the chalk hills, there are a bunch of dead ichthy's. You get hide, spoiled meat and organic polymer from harvesting them. It's very useful for narcotics, ghillie armour and fabricated weaponry and structures.
Go to herbivore island (The island) at the right bottom. Go to map if unsure. Look around and you should find some itchyosaurus. But be careful, in the water they look like sharks, don’t mistaken them as sharks they have a pointed snout. Like if you found it helpful! :)