The Fossil Guild is the shop in Fossil Fighters and comes back in Fossil Fighters: Champions . You can upgrade your tools and your sonar in the guild and you can also buy new Fossil Rocks (which is an easier way to get T-Rex earlier in the game but with a price of 25,000 G). All Sonar Upgrades together cost $194,000. You can also store masks that change your appearance in the Guild. There is also an info clerk in the back that will tell you links to sidequests. You can buy masks here. The lady to the right stores and puts masks on you. She is the mask lady. Medal-Dealer Joe used to work here until he was exiled for stealing medals. In the middle a platform is there that has four leads.
In Fossil Fighters: Champions, the Fossil Guild comes back, located in all 3 of the main Islands of the Caliosteo Islands. It has a Mask Salon, an Info Clerk, and a Shop (both wondrous and miraculous fossil rocks are extremely rare here).