Dinosaurs

 are trapped alive in tree sap in the abandoned mines in the Gravity Falls forest. The summer heat is causing the tree sap to melt, slowly releasing the dinosaurs from the sap. In "Land Before Swine," a pterodactyl escaped the tree sap and began terrorizing the town and ended up stealing Waddles.

Appearance
A number of dinosaur species are seen trapped in tree sap in the abandoned mines.