You can obtain fossils by doing a variety of activities on Fossil Island. This includes killing monsters, fighting the Deranged Archaeologist, and chopping Sulliusceps. As of this writing, chopping Sulliusceps is the fastest way to obtain fossils.

What Are The Main Uses for Fossils?

Once you’ve obtained a complete set of fossils, you can place them in display cases in the Varrock Museum. To complete this entirely you’d need:

70 small fossils 60 medium fossils 25 large fossils 25 plant fossils

Each display you complete will grant you with 2 Kudos, and an experience lamp, which can be used to get some experience in any skill of your choice! The Fossil collector will also buy your fossils from you, for a rate between 100 and 500 numulite depending on the size & rarity of each fossil. This can be useful too, since Numulite is the currency used on Fossil Island. And obtaining Numulite in other ways can be tiresome.
Fossils can also be taken to the Mycelium Pool to turn them into enriched bones. These aren’t super crazy, but you can use them to gain prayer experience. With these enriched bones, you can gain anywhere from 250 to 2.5k prayer experience per enriched bone, depending on if it’s small or rare. Lastly, you can also use fossils to charge an Ancient Wyvern Shield. This method is far more economical than using Numulite to charge it. So if you’d make use of this shield often, I recommend saving all of your fossils for this purpose!