Mesozoic Fossil Record (252–66 Ma)

Mesozoic time scale
Mesozoic time scale showing significant geologic events and paleoenvironments in Wyoming as well as characteristic life throughout geologic history (modified from Hager, 1970).

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.

Distribution of Mesozoic rocks across Wyoming
Distribution of Mesozoic rocks across Wyoming.

The seas continued to rise and fall during the Cretaceous Period. Clams, gastropods, mososaurs, plesiosaurs, ichthyosaurs, turtles, and fish are common fossils from this period. Toward the end of the Cretaceous, the Laramide orogeny (mountain-building event) began deforming and uplifting the crust, causing the seas to retreat. Dinosaurs returned. Among the Cretaceous dinosaurs discovered in Wyoming are Tyrannosaurus rex, Triceratops (the Wyoming State Dinosaur), Anatosaurus, and Ankylosaurs. Deciduous trees (beech, birch, maple, oak, and walnut) became dominant flora. The end of the Cretaceous saw the extinction of many animals, including dinosaurs, most marine reptiles, ammonites, and belemnites.

Ammonite fossil
Ammonite fossil
Ichthyosaur fossil
Ichthyosaur fossil



Colby Schwaderer,