An Ancient Sea Once Split North America Down the Middle. The Beautiful Multicolored Ammonite Shells From Its Waters Are So Perfectly Preserved That They Still Shimmer Today

Spectacular marine fossils tell the story of the long-gone Western Interior Seaway and the planet’s past aquatic life

Riley Black | Science Correspondent

July 10, 2026 12:30 p.m.

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South Dakota has no oceanfront. The landlocked state is more than a thousand miles from both the Pacific and the Atlantic, and a rural spot within its borders is regarded as the “continental point of inaccessibility” from the ocean. And yet, signs of the salty water are still there, enclosed in rocks laid down by a long-vanished seaway that once split North America in two. Back then, more than 70 million years ago, what’s now prairie was a warm sea full of marine lizards, knife-toothed sharks and gorgeous coil-shelled ammonites jetting through the sunny waters.

Some of those shining cephalopod shells are now part of an exhibition at the Smithsonian’s National Museum of Natural History called “From These Lands,” which showcases hundreds of specimens and artifacts from across the United States.

While fossils such as shark teeth and prehistoric corals can certainly be found along America’s coasts, ocean has touched every part of the country at some time or another in the deep past. “We find marine fossils in every single state,” says Stewart Edie, a paleobiologist and curator at the museum. “It’s fascinating that you can walk through Minnesota and you can find fossils of starfish.”

These fossils are remnants of times very different from our own. The South Dakota ammonite shells come from a period when leathery-winged pterosaurs still flew through the air and immense dinosaurs still trod through forests. In the seas, the now extinct shelled cephalopods proliferated in huge numbers, often serving as food for the large aquatic reptiles of the time. But the ammonites selected for the display did not become a mosasaur’s lunch. The shells are so delicately preserved that they continue to shine.

“They still have this iridescent sheen to them,” Edie says, because of the material ammonites used to make their shells. Many ammonite shells were at least partly composed of nacre, he notes, which is the same material that creates oyster pearls. The material, also known as mother-of-pearl, refracts and bounces light back. The fossilization process allowed the mother-of-pearl to be better exposed than it would have been during the ammonite’s life, Edie explains.

The water that the South Dakota ammonites jetted through, the Western Interior Seaway, existed for more than 30 million years. Around 100 million years ago, during the hothouse world of the Cretaceous, sea levels rose to the point that ocean water spilled down the middle of North America—from the Arctic Ocean down to what’s now the Gulf Coast. The waters were relatively shallow, no more than 3,000 feet deep compared with the average ocean depth of around 12,000 feet now, but they hosted so much life that paleontologists to this day find gigantic seagoing reptile specimens in Kansas and reef-building clams the size of toilet seats in Utah. And all this life isn’t just ancient history: The fossil-filled rocks eroded to the nourishing soils that allowed the Midwest to become so agriculturally important. The seaway eventually vanished as the planet cooled during the end of the Cretaceous and as polar ice formed. Sea levels dropped, and the two split subcontinents finally joined by land in what we now recognize as North America.

Researchers are constantly unearthing new fossils from the layers of the long-lost Western Interior Seaway. Fort Hays State University paleontologist Amanda Peng notes that “countless” discoveries of mosasaurs have been made in Kansas and surrounding states. With such a large sample, she says, experts can better study how the anatomies and diets of the various seagoing lizard species that lived in those waters differed. “I have a student working on understanding bending strength in mosasaur mandibles,” Peng says, and that helps scientists learn more about “the larger picture of mosasaur ecology.”

The continent’s rocks don’t only hold the remains of this one sea. Thanks to continental drift, changing sea levels and the nature of fossil-bearing rock formation, America’s geologic layers contain the vestiges of various bodies of water from before the time of the Cambrian explosion through the Ice Age. Paleontologists and amateur fossil hunters can find traces of some of the earliest animal life in the deserts of Utah, reefs that witnessed the emergence of some of the first fish in Ohio and the teeth of 20-million-year-old sharks in South Carolina.

Some states rest entirely on what’s left of past oceans. “There is evidence of the ancient environment all around us in northeast Ohio,” says Cleveland Museum of Natural History paleontologist Caitlin Colleary. The bedrock beneath Ohio is made up of various rocks that formed during the time when plants were only just beginning to grow on land and our ancestors had yet to live on it. Little brachiopods, clam-like organisms that have been on Earth for more than 500 million years, and the stems of feathery starfish relatives called crinoids can be found there.

“When you start to pay attention to these rocks that you pass by every day, you can sometimes see fossils or evidence of the ancient body of water itself, like ripple marks,” Colleary says.

Classic finds from the area include amazing fossils of the shark-like fish Cladoselache, with some detailed enough to have preserved gut contents. And the construction of highway I-71 during the 1960s led to massive excavations of fossils that are still being prepared and studied by researchers for new insights into why the area’s sharks stayed in such great shape and what was happening in the seas just before a major shake-up. “These rocks preserve an ecosystem right before the mass extinctions at the end of the Devonian period 358 million years ago, so they are a really important part of the story of how things changed on Earth after that,” Colleary says.

Coastal states boast fossils in the National Museum of Natural History display, too. One of the featured objects is Ecphora gardnerae, the state fossil of Maryland. The shells, the mobile homes of prehistoric snails, are more than 20 million years old and come from a time when the immense prehistoric shark Otodus megalodon was swimming through the world’s oceans.

While also carnivorous, Ecphora was not quite so imposing as a 50-foot-long shark, but the lovely shell of the mollusk still has an important connection to American history as well as prehistory. “It’s a beautiful specimen, named after a prominent paleontologist from Maryland, Julia Gardner,” Edie says, “and we’re glad to tell the story here in this exhibit.” Gardner “wrote the book on Southern marine invertebrate paleontology,” he says. What might seem like an unassuming snail speaks volumes of the history of science and the people who’ve studied these creatures.

After all, fossils fascinate many more people than the professional paleontologists who study them. Peng notes that the Western Interior Seaway has had a deep effect on the local culture in Kansas and the broader Midwest, something she regularly encounters at the Sternberg Museum of Natural History. “Every visitor I’ve talked to at the museum about the seaway or about the fact Kansas used to be underwater has been completely knowledgeable about this,” Peng says, adding that she is sometimes called by members of the public with questions about fossils they’ve found. “These people come from all walks of life; they tell me about how they or their family love fossil hunting, and they’ve found all sorts of fossils on their land, or in a road cut, or in a field.”

Marine fossils are invitations to connect our present moment to the planet’s past. Selecting fossils to display in “From These Lands” led Edie to think about the relationships between times and places that at first might seem to have little to do with each other. “In the charge to find stories from places like Rhode Island and Washington State, far from where I grew up, I learned a lot,” Edie says. “I hope our visitors have a similar experience. I hope they see themselves in the show.”

The items on view in the exhibition are like snapshots from an evolutionary epic that has involved mass extinctions, climate changes, the rearranging of continents and animal adaptations like shells, jaws and teeth. Together, they convey a lesson on life’s resilience. “There are all kinds of impending stressors today, and we know from past events that they can be pretty bad, they can knock biodiversity down quite a bit,” Edie says. What researchers like him are taking an interest in now is the fossil record showing how life bounces back from disaster. “It will, it has time and time again,” he says. The proof is in the petrified seashells and bones buried by ocean sand so long ago.

Riley Black | | Read More

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.