Wyoming’s Fossil Record
WYOMING — Geologic formations in Wyoming bear a broad 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. See the previous post about the Precambrian era here.
Paleozoic Fossil Record
In the Paleozoic Era, plant and animal life began to develop greater range and complexity. Insects, reptiles, fish, and invertebrates advanced during this time, as well as vascular plant life.
Cambrian exposures in Wyoming represent a transgressive depositional sequence (sandstones overlain by shale and limestone) deposited as the sea flooded the land. Common fossils in Cambrian units include marine flora and fauna: trilobites, brachiopods, algae, corals, and sponges.
During the Ordovician Period, Wyoming was submerged in a shallow sea. Marine fossils, such as trilobites, brachiopods, cephalopods, and primitive fish can be found in Ordovician formations.
The Silurian Period was a time of uplift and erosion. There are no rocks of this age in Wyoming. As such, there are no Silurian fossils. In neighboring western states, however, a restricted distribution of poorly preserved coral fossils has been documented.
Seas transgressed and flooded the western part of Wyoming in the Devonian Period. Marine plants (crinoids) and animals (amphibians, cephalopods, and primitive fish) are common fossils of Devonian age, as are fossil land plants.
During the Mississippian Period, Wyoming was submerged in a shallow sea, and marine fossils such as trilobites, brachiopods, fish, and crinoids are common in rocks of this age. In the Pennsylvanian, the sea began to regress, but marine fossils are still common for this period.
In the Permian, the shallow sea continued to regress. Fossils include marine flora and fauna such as pelecypods, corals, belemnites, and conodonts, as well as gastropods. Reptiles and coniferous trees originated during the Permian. A mass extinction marked the end of the Permian. Most of the earth’s marine species and over half of the terrestrial reptiles, amphibians, insects, and plant life were lost.