WSGS Paleontology Page: The Mesozoic Era

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WSGS Paleontology Page: The Mesozoic Era
An Ichthyosaur fossil from the Jurassic period.

Wyoming’s Fossil Record

Geologic formations in Wyoming bear a wide range of fossils, representing life forms from nearly every geologic time period. Some of the oldest fossils on earth are found here. From single-celled organisms to wooly mammoths; from bacterial algae to palm fronds, Wyoming hosts a diverse array of terrestrial and marine plant and animal fossils.

This post is the third of four posts about Wyoming’s fossils. The first post may be accessed here and the second post at this link.

An Ammonite fossil from the Triassic period.

Mesozoic Fossil Record

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

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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.

By the end of the Jurassic Period, seas withdrew. 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.

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 the dinosaurs, most marine reptiles, ammonites, and belemnites.