Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Serio, graduate student, and Madison Myers, Assistant Professor of Igneous Processes, with the Department of Earth Sciences at Montana State University.

The thermal features and “charismatic megafauna” of Yellowstone are more than enough to draw crowds from all over the world, with the park seeing over 4 million visitors in 2021. But Yellowstone has a secret that very few visitors are even aware of: Yellowstone has fossils! Enough to rival other national parks that are renowned for their fossil riches, like Petrified Forest and Dinosaur National Monument! In Yellowstone, there are fossils that tell the story of volcanic eruptions that buried entire forests, that detail a slow change in Yellowstone’s climate, and that are the basis for entire evolutionary theories!

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Indigenous people undoubtedly knew about Yellowstone’s fossils, and their knowledge was used by early explorers and fossil hunters to locate sites. As a result, from the beginning of its exploration by Euro-Americans, Yellowstone has been a noted spot for fossils. Mountain man Jim Bridger told tales of the petrified forests of the region, and the 1871 Hayden Survey described fossilized wood, petrified forests, and animal fossils, with trilobite fossils (ancient bugs) being some of the oldest recorded in the park. These trilobites lived on the bottom of the sea hundreds of millions of years ago during a time period known as the Cambrian and are what scientist Charles Walcott used as the basis for his theory of a massive radiation of life, called the Cambrian Explosion.

The most notable and well-documented fossil deposits in Yellowstone are petrified trees and forests. About 53 to 43 million years ago, during the Eocene period, volcanic episodes from the Absaroka mountains engulfed ancient trees, burying them in massive debris flows and fossilizing them by slowly replacing organic material with silica. These are akin to the petrified logs in Arizona’s Petrified Forest National Park, but with some very important distinctions: in Yellowstone there are forests preserved almost in their entirety. Studies performed at these stands have shown that a total of 27 layers of petrified forests are preserved, one right on top of the other, highlighting the long and tumultuous history of being a tree during this time! From these deposits we can tell that Yellowstone’s ancient forests looked very different from today. Sequoias, much like the giants that now live in California, were abundant in the Eocene in Yellowstone!

Another notable fossil locality in the park is found in Lamar Cave. These more recent deposits are from after the last ice age and include layers that contain 36 different mammal species, including the gray wolf! One of the most interesting finds from Lamar Cave is the disappearance of the prairie vole roughly 1550 years ago. Extinction you ask? Nay. The prairie vole is still alive today and is known as an endemic resident in dense, tall grassland environments, especially within the Great Plains. Its presence and then disappearance in Yellowstone indicates that a shift in the environment occurred over time—a result that has also been revealed by sediments collected from Yellowstone Lake and from the Lower Geyser Basin.

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As exciting as the known fossil deposits and locations in Yellowstone are the fossils that have yet to be discovered. There has been little modern paleontological exploration done within the park, especially of rocks from Mesozoic time, between 250 to 65 million years old. One notable exception was a one-day expedition led by Dr. Jack Horner with the Museum of the Rockies in the 1990s. Despite this survey only being one day, the group was able to find a piece of turtle shell, dinosaur egg fragments, and what is thought to be a skeleton of an aquatic reptile known as a plesiosaur. A follow-up fossil expedition from the Museum of the Rockies is planned for the coming summer. With all that’s left unexplored there are sure to be some very interesting secrets still hidden away in Yellowstone, so stay tuned for these exciting discoveries.

There are a few parts of the park where guests can see fossils, including a petrified tree stump displayed in front of Albright Visitor Center, another visible in the foundation of Roosevelt Lodge, and a standing stump located roughly 2 miles West of Tower Junction. Perhaps given time, more discoveries from future fossil expeditions will better showcase this lesser-known aspect of the park. Until then, the secret fossils of Yellowstone will continue to wait just beneath the surface.