Ok so Idk why Mosasaur and Plesiosaur have the chance to drop this skin.But I do notice the SLIGHT reference here.You see remember jurrasic park and the Spinosaur was the star?Well if you do you probably know that this skin is a reference to the gun in that movie.The gun was the weapon that driven off the Spino!It lit a fire and chased off the Spino this is why Spino can drop this skin accasionally.