These Tiny Fossil Fish Offer a Glimpse Into a Once-Lush Wyoming

A 50-million-year-old slab preserves hundreds of freshwater herring—and a window into an ancient landscape unrecognizable today

At first, the roughly four-foot-wide slab looks like little more than a broad expanse of stone. Then the fish begin to appear. A tiny backbone here. A cluster of fin bones there. Rows of delicate ribs emerge across the rock until what seemed almost empty becomes crowded with hundreds of small fish, their skeletons still remarkably intact after roughly 50 million years.

That slow reveal stopped paleontologist Stewart Edie in his tracks as he searched the collections of the Smithsonian’s National Museum of Natural History for specimens to include in “From These Lands: Sharing Our Natural and Cultural Heritage”, an exhibition featuring more than 600 specimens and cultural objects representing every U.S. state, the District of Columbia, and the five U.S. territories where people live.

“I stopped, turned back, and looked more closely at it,” recounts Edie, the museum's curator of fossil molluscs. “And once your  eyes refocus, you start to see all these little fish emerge, their tiny little vertebrae and their other little bones. They’re exquisitely preserved.”

The fish are Knightia eocaena, an extinct species of freshwater herring that swam through Wyoming’s lakes between 50 and 48 million years ago. Collected by an amateur fossil hunter in southwestern Wyoming’s Sweetwater County, the slab was donated to the Smithsonian in 1998. It is now part of the museum’s National Fossil Collection and on display in the “From These Lands.”

Knightia was no rarity. Fossils of the little schooling fish occur in staggering numbers in the Green River Formation, a series of Eocene lake deposits famous for both the abundance and exquisite preservation of its fossils. In Sweetwater County, those deposits record ancient Lake Gosiute, which at its maximum extent covered nearly all of the county. But the Smithsonian slab preserves something more than the anatomy of a species. It captures a catastrophe.

Rapid changes in temperature or lake chemistry periodically caused schools of Knightia to die en masse. So-called “mortality blocks” of Knightia are common, and researchers have investigated possible causes, like toxic algal blooms or seasonal water changes. Scientists cannot say exactly what killed the fish on this particular slab. Whatever the cause, hundreds sank to the lake floor. 

Sediment buried the fish before scavengers could pick them apart. Oxygen-poor water slowed decay. Calm conditions kept tiny bones from being scattered. Layer by layer, the lake bottom sealed away the aftermath. “This slab is a snapshot of that mortality event some 50 million years ago,” Edie said. Fossils like these, he added, can preserve “more than just the anatomy of an extinct animal”—they can preserve an event.

Similar mass-mortality beds elsewhere in the Green River Formation, including at Fossil Butte National Monument, contain dense layers of Knightia, evidence that catastrophic die-offs repeatedly swept through these ancient lakes.

These ancient conditions helped turn the Green River Formation into an unusually detailed archive of Eocene life. In some deposits, calm water with low levels of oxygen protected remains long enough for even fragile organisms to survive the journey into stone. The resulting fossil record includes not only fish but delicate plants, insects, and other organisms that might disappear under more ordinary conditions for fossil preservation. “You end up with just an incredible snapshot of what life was actually like back then,” Edie said.

"Our landscapes are really a tapestry of ancient environments. When we start to peel back those layers, we start to appreciate how much life has changed through time."
— Stewart Edie, NMNH Curator of Fossil Mollusca

Follow those little fish backward through time, and modern Wyoming begins to disappear. The dry grasslands and craggy badlands give way to lakes and lush vegetation. Palms rise along the lakeshore. Crocodilians inhabit a landscape far warmer and wetter than the one that stretches across Wyoming today.

The early Eocene was one of the warmest intervals of the past 66 million years, and the Green River fossil record captures just how different that world could be. Among its mammals were tiny early horses, a fraction of the size of their modern descendants. Elsewhere in the formation, Fossil Butte preserves a remarkable diversity of fossils from a sub-tropical ecosystem.

That wealth of fossils allows scientists to reconstruct more than a roster of extinct species. Piece enough of them together and an ecosystem begins to take shape. Plant fossils preserve clues about vegetation and climate. Animals reveal which creatures could inhabit those environments. Newer chemical techniques can extract isotopic evidence that helps scientists investigate ancient temperatures and precipitation. Across the Greater Green River Basin, rocks preserve a record of Eocene environments stretching across portions of present-day Wyoming, Utah, and Colorado.

These ancient ecosystems can also preserve combinations of climate and biodiversity unlike anything living together today. Edie calls these “non-analog” environments, which offer another way to understand the range of conditions life has occupied through deep time.

And it all begins with that slab of fish that made Edie stop and turn back.

Look closely enough at its crowded surface and the rock becomes something else: hundreds of fish sinking through an ancient lake, a quiet lake bottom preserving their bones, and beyond them a humid Wyoming filled with palms and crocodilians. “Our landscapes really are just this tapestry of ancient environments,” Edie said. “When we start to peel back those layers, we start to appreciate how much life has changed through time.”

Lily Rasmussen | Read More

Lily Rasmussen is a science writing intern at the Smithsonian’s National Museum of Natural History, where she covers natural history research and collections news for the museum’s blog, Smithsonian Voices. She is currently completing her masters in science writing at Johns Hopkins University. Born and raised in Appalachia, she writes at the intersection of nature, people, and place, with a particular fondness for weird plants, unusual animals, and current ecological research. She loves translating technical scientific breakthroughs into stories that spark public wonder. In her off-hours, she is an avid artist, singer, and outdoorswoman.