It was while working deep in the archives of a dinosaur museum that Hallie Street realized she had a mystery on her hands. Street, a curatorial assistant at the Royal Saskatchewan Museum’s T.rex Discovery Centre, was only meant to be cataloging the thousands of ancient fossils in the museum’s collection. But every time she picked up another tiny plastic bag with a shark tooth or reptile bone in it, the weird markings and sheer number of specimens gave her the same hunch—something dramatic had happened to these animals.

“I kept finding things that I thought were curious,” she says.

Street’s work had taken her into the heart of a special collection of fossilized marine animals, a trove of 75-million-year-old treasures that tells a story of burgeoning life and sudden death.

The collection of about 3,100 fossils had been found tightly packed into an area of about five or six square meters in Herschel, Saskatchewan.

Herschel is about as far from the ocean as you can get in Canada, but 75 million years ago, during the Late Cretaceous, it was underwater. The Western Interior Seaway cut a huge channel through North America, splitting the continent into separate land masses.

Discovering such a dense concentration of marine fossils is rare—and it takes time to understand what the specimens show. Street has now coauthored a paper about the fossils, along with her colleagues Emily Bamforth, also at the T.rex Discovery Centre, and Meagan Gilbert, a geologist at the Saskatchewan Geological Survey.

Though staff from the Royal Saskatchewan Museum have been studiously gathering material from the Herschel site for 30 years, most of the finds had never been formally examined before. In fact, there are still many more fossils at the site waiting to be excavated. “We haven’t found the end of it, it just keeps going back into the hill,” says Bamforth.

Most of the remains are of majestic marine reptiles called plesiosaurs.

“Usually, the Loch Ness monster is modeled after a plesiosaur,” explains Street. Plesiosaurs had paddle-like limbs and long necks, and stretched eight to 10 meters long. The Western Interior Seaway was teeming with them. The Herschel site also holds remains of mosasaurs, which were a similar size but more fearsome-looking. A bit like a giant Komodo dragon with paddle-like limbs, they were top predators in the area.

The sheer number of plesiosaur fossils at the excavation site is striking. Street’s work in the archive revealed the site was awash with juveniles as well as adults. “I think the plesiosaurs were using it as a nursery,” says Street.

Why such a dense clump of bones on the seafloor? The researchers think they were deposited during a time of low sediment input from nearby rivers, perhaps because a nearby marsh soaked up that sediment before it got to the shallows. This happened in a basin-like area just off the coast, says Street—one probably sheltered by barrier islands.

There was another conundrum, however. Some of the bones showed unusual patterns of decay, almost like they had been rapidly eroded. The researchers think that when the plesiosaurs died, their bodies fell to the seafloor and were scavenged by other creatures. The decayed bones, the researchers surmise, were actually plesiosaur remains that had been partially eaten.

The scene was akin to a modern-day whale fall, when the carcass of a whale ends up on the ocean floor only to be scavenged by crustaceans, worms, or snails. Similar creatures probably had their fill of sunken plesiosaurs 75 million years ago.

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