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The Hell Creek Formation Is North America’s Legendary Boneyard. See the Top Five Discoveries Found in the Iconic Fossil Bed
From preserved plants to T. rex, the material found in these Late Cretaceous rocks has resulted in countless breakthroughs for paleontologists
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In 1902, professional fossil hunter Barnum Brown excavated a stunning set of bones from eastern Montana’s Cretaceous period rocks. The remains were clearly from a large carnivore—far more imposing than the dinosaurs of the earlier Jurassic—but no one had seen a dinosaur quite like this one before. And it was far from the only prehistoric reptile peeking out of the rocks.
The mysterious dinosaur kept clawing Brown back to the area. For years, he and his colleagues returned to find more fossils. He finally named the creature in 1905: Tyrannosaurus rex. The now-famous “tyrant lizard” lived alongside the shovel-beaked herbivore Edmontosaurus, the three-horned giant Triceratops, the living tank known as Ankylosaurus and other dinosaurs.
In part, paleontologists know this because the animals’ bones are encased together in layers of rock dating to between 66 million and 68 million years old. This geologic formation is exposed among the hills of eastern Montana, the Dakotas and Wyoming. Brown called the area the Hell Creek beds, after nearby Hell Creek, and it’s now known to geologists and paleontologists the world over as the Hell Creek Formation.
But dinosaur fossils are only one facet of what makes the formation so special. Ever since that first discovery, paleontologists have found preserved leaves, the bones and teeth of tiny mammals and other clues that have opened a rich window into what Cretaceous life was like just before the devastating asteroid strike. If you want to glimpse the last days of the dinosaurs, Hell Creek has to top the list of places to visit.
Quick fact: The Hell Creek Formation and T. rex
More than 95 percent of T. rex museum specimens come from the Hell Creek Formation.
Institutions like the Museum of the Rockies and the American Museum of Natural History are full of Hell Creek fossils, from microscopic pollen to skeletons of giant dinosaurs. A list of all the remarkable fossils found in the formation so far might run longer than the 160-foot thickness of much of its rock beds. Here, instead, is a sampling of the Hell Creek Formation discoveries that have changed the way we understand the prehistoric past.
1. Tyrannosaurus rex
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The last and largest of the tyrannosaurs, T. rex is practically synonymous with the Hell Creek Formation. Even though fossils of the giant reptile have been found in other geologic formations of the same age—such as Utah’s North Horn Formation and the Frenchman Formation of Saskatchewan, Canada—the bones of the Hell Creek T. rex were the first, revealing that carnivorous dinosaurs grew far larger than paleontologists previously knew.
In life, adult T. rex could be more than 40 feet long and weigh more than nine tons. Although they were not especially speedy hunters, studies of their legs show that they were agile—and they specifically evolved to deliver powerful bites capable of crushing bone. The fact that T. rex appears to be so abundant in the Hell Creek Formation is significant because such a large carnivore could exist in great numbers only in a rich ecosystem with prey to support it. The Hell Creek fossils helped T. rex become the most famous dinosaur of all time: The Cretaceous carnivore is a paleontology ambassador, welcoming visitors to museums and scaring summer movie audiences alike.
2. The K-Pg boundary
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The fossil organisms of Hell Creek are wonderful in their own right. But what makes them even more spectacular is their context in the history of life. Roughly 66 million years ago, an asteroid struck what’s now the Yucatán Peninsula in Mexico. The impact sparked Earth’s fifth mass extinction—called the Cretaceous-Paleogene (K-Pg) extinction—marking the end of the dinosaurs’ heyday and the beginning of our current “Age of Mammals.” Evidence of that catastrophe is recorded across the planet as a thin layer of rock called the K-Pg boundary, part of which is visible atop the Hell Creek Formation.
What makes this section so special is that paleontologists have extensive knowledge of both the fossils below the boundary and those above it. Such Hell Creek wonders as T. rex preserve life as it was before the extinction, and fossils of the survivors of the catastrophe—including small mammals, reptiles, leaves and other early Paleocene organisms—are found in the overlying Fort Union Formation rock. Divided by the stark rock of the K-Pg boundary, the Hell Creek Formation sets the context for what life was like before the asteroid impact, and the Fort Union Formation demonstrates what went extinct, what survived and how the world as we know it came to be.
3. Acheroraptor temertyorum
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The biggest dinosaurs of the Hell Creek Formation get the most attention, but paleontologists are still searching for the elusive, small species that filled the Late Cretaceous world. Not only are fossils of small animals relatively rare, as the bodies of these pipsqueaks were often eaten or rotted away before they could fossilize, but paleontologists historically focused on finding the big, impressive specimens and largely overlooked the little ones. The announcement of Acheroraptor in 2013 indicated that many small Hell Creek animals are still left to uncover.
Named by the Royal Ontario Museum paleontologist David Evans and colleagues, Acheroraptor would have been roughly the size of a turkey—though it likely had an extended, bony tail that would have made it longer. If that sounds like another famous dinosaur, Velociraptor, it’s for good reason. Acheroraptor was a close, later relative of Velociraptor and likely hunted mammals, lizards, baby dinosaurs and other small prey, probably using a hyper-extendable second toe to pin the little morsels. Paleontologists can only say “probably,” because essentially all that’s been found of Acheroraptor is an upper and lower jaw. More is almost certainly out there, if experts think small.
4. Cretaceous plants
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Dinosaur bones don’t mean much without information about their environment. Fossils of organisms that lived in the same places and at the same time as our favorite dinosaurs go a long way toward telling us about climate, rainfall, forest structure and food webs—the essential details of how prehistoric ecosystems worked. Fortunately for us, the Hell Creek Formation is rife with plant fossils, including pollen and beautifully preserved leaves.
Some of the plants of the Hell Creek Formation would have looked familiar: Palms, magnolias and dogwoods are still with us today. Others, like extinct relatives of today’s pineapple-shaped cycads and conifers related to modern monkey puzzle trees with crowns of needle-like leaves, might stand out as more unusual. In all, the plant fossil record indicates that the Hell Creek Formation forests were eaten and trampled by dinosaurs to create lots of open spaces and hosted a combination of very ancient plants and relatively new flowering plants called angiosperms. Pollen records in the rocks, too, show that ferns were some of the first plants to bounce back after the asteroid strike—an overwhelming number of fern spores in rocks above the K-Pg boundary indicates a “fern spike” when life was beginning to recover from the end-Cretaceous catastrophe.
5. Infernodrakon hastacollis
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In 2002, paleontologists uncovered the skeleton of a tyrannosaur nicknamed “Jane”—a specimen that would later become well known for debate over its identity. But that team also found an unexpected bonus in the same quarry, one that for decades was only poorly understood.
Preserved among the tyrannosaur bones was a long, flanged bone later recognized as the fifth neck bone of a pterosaur—one of the leathery-winged, flying reptiles that flew above dinosaurs’ heads. Paleontologists originally thought the neck bone belonged to Quetzalcoatlus, a pterosaur that stood as tall as a giraffe and had a wingspan comparable to that of a single-propeller plane. But a new analysis released last year found that the pterosaur is something new.
Based on the size of its neck bone, the living pterosaur would have stood about as tall as a person, if it had proportions like those of related species. It was what experts identify as an azhdarchid pterosaur, a toothless flier that shuffled around on the ground to snatch small prey, as some storks do today. Paleontologist Emi Thomas and colleagues gave the creature a name that matches the storied site where it was found: It is now known as Infernodrakon—the hell dragon.
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