Biodiversity increased significantly during the Mesozoic Era. The first birds and flowering plants originated during this time, as did dinosaurs and reptiles.

The Triassic Period was accompanied by a series of sea level transgressions and regressions. Common fossils are ammonites, belemnites, snails, and clams. Phytosaurs (crocodile-like reptiles) are a vertebrate fossil from the Triassic. Dinosaur tracks have been documented in Triassic Chugwater Formation exposures in parts of Wyoming.

Seas withdrew by the end of the Jurassic Period. Common fossils from this time are marine belemnites, clams, crinoids, and sea urchins. Ichthyosaurs and plesiosaurs are common marine reptiles. Snails, reptiles, and terrestrial flora are also common Jurassic fossils. Dinosaurs were abundant during this period. The Jurassic Morrison Formation is a world-renowned dinosaur-bearing formation in which many significant dinosaur discoveries have been made in Wyoming, including Allosaurus, Apatosaurus, Diplodocus, and Stegosaurus, to name a few.