Yellowstone’s Fossil Forests: The Story Behind the 2022 National Fossil Day Art

Most visitors come to Yellowstone for Old Faithful, Grand Prismatic Spring, the Grand Canyon of the Yellowstone, and bison, elk, grizzlies and wolves. But beyond the geysers lies a fossil record reaching back about 500 million years. For Yellowstone's 150th anniversary in 2022, the artwork for National Fossil Day (October 12, 2022) recreated one chapter of it: a forest from the Early Eocene, some 50 million years ago.

The 2022 National Fossil Day artwork. Image from the National Park Service's article on the fossils of the 2022 artwork.

A park built on a supervolcano

President Ulysses S. Grant signed the law establishing Yellowstone as the first national park on March 1, 1872. It lies mostly in northwestern Wyoming, reaching into Montana and Idaho, and its name likely comes from the yellow rhyolite exposed in the Grand Canyon of the Yellowstone. People have been in the region for about 11,000 years, and today about 4 million visitors come each year.

The Yellowstone Caldera, North America's largest "supervolcano," has dominated the center of the park for more than 2 million years — and its heat and eruptions erased much of the older record there. To find fossils, you go to the edges of the caldera, where older sedimentary rocks survive.

Half a billion years of life

A Spirifer brachiopod from Mississippian marine limestone in Yellowstone. Credit: National Park Service.

The petrified forests of Specimen Ridge

The caldera wasn't Yellowstone's first volcanism: in the Eocene, chains of smaller volcanoes spanned the area. On Specimen Ridge, in the park's northeast, volcanic sediments of the Sepulcher and Lamar River Formations hold at least 27 separate Eocene forests, one above another, each buried by ash and debris — the famous Yellowstone Fossil Forests. Mountain men told of them in the 1830s, and paleobotanists have studied them intensively since. Some trees were buried standing upright about 50 million years ago; a large petrified conifer stump is a stop on the park's driving tour. Fossil leaves, cones and nuts have also been found.

A standing petrified tree on Specimen Ridge. Image from the National Park Service's article on the fossils of the 2022 artwork.

Road construction in the 1990s exposed fine volcanic sediments with fossil leaves and pollen of ferns, horsetails, conifers and flowering plants. Plants, with their precise environmental needs, are often better witnesses to ancient climates than animals, and these show a Yellowstone far warmer and wetter than today, but with cooler seasons that made trees drop their leaves.

Reading the artwork

• The forest: deciduous oak, poplar and walnut in fall color, with evergreen conifers rising above.
• The volcano: active in the distance — eventually it will bury this valley in ash.
• The predator: half-hidden in the foliage is Didymictis, a civet-like early carnivore about the size of a house cat, from a group called the viverravids. They lived across North America and Eurasia in the late Paleocene and early Eocene, probably hunting on the ground and sometimes in trees like modern civets and mongooses. None has been found inside Yellowstone, but they turn up at nearby Early Eocene sites in northwestern Wyoming. Their kind was part of the ancient group that eventually gave rise to the ancestors of today's grizzlies, wolves and pine martens.
• The giants in the riverbed: titanotheres — one also appears on the official National Fossil Day logo.

The skull of Miacis, an Eocene early carnivore from Wyoming similar to Didymictis. Image from the National Park Service's article on the fossils of the 2022 artwork.

Sources

Based on "Fossils of the 2022 National Fossil Day Artwork," National Park Service; a work of the United States government in the public domain.