The Cloverly Formation is a sequence of Lower Cretaceous sedimentary rocks exposed across Wyoming and Montana that has produced some of the most iconic dinosaur fossils in North American paleontology, including the predator Deinonychus and its frequent prey, Tenontosaurus. Deposited by ancient river systems over a span that recent dating work suggests may stretch from roughly 140 million to 98 million years ago, the formation captures a period of ecological transition between the better-known Late Jurassic faunas below it and the Late Cretaceous world above. Its colorful mudstones, channel sandstones, and conglomerates have drawn field crews to the northern Bighorn Basin for the better part of a century, and they continue to reshape how scientists understand Early Cretaceous life on the western interior of North America.
What the Rocks Actually Look Like
The Cloverly Formation sits on top of the Morrison Formation, the famous Jurassic unit known for its Diplodocus and Allosaurus bones. In most exposures the contact between the two is conformable, meaning sediment piled up more or less continuously rather than being interrupted by a dramatic gap in time. The Cloverly itself has been divided into three members, each with a distinctive look in the field.
The lowest member, formally named the Little Sheep Mudstone, is dominated by bentonitic mudstones that weather in shades of gray, purple, olive, and dull red. Mixed in are layers of bentonite clay, chert, thin coaly beds, calcareous nodules, and sandstones studded with chert pebbles. In the northernmost part of the Bighorn Basin, a separate unit called the Pryor Conglomerate sits at roughly the same stratigraphic level, distinguished by its conspicuous black-chert pebbles and an unconformable contact with the Morrison below.1GSA Bulletin. MORRISON, CLOVERLY, AND SYKES MOUNTAIN FORMATIONS, NORTHERN BIGHORN BASIN, WYOMING AND MONTANA – Section: Abstract
The upper member, the Himes Member, is more variable. It often begins with an olive-gray or reddish-brown sandstone bound by a clay matrix, then grades into kaolinitic claystones and mudstones streaked with iron-oxide veins and hardpans. Clean quartz sandstones that once filled river channels cut through these finer sediments, sometimes producing the resistant ridges that make the formation visible on hillsides and roadcuts across the Bighorn country.2GSA Bulletin. MORRISON, CLOVERLY, AND SYKES MOUNTAIN FORMATIONS, NORTHERN BIGHORN BASIN, WYOMING AND MONTANA – Section: Abstract
A Much Longer History Than Originally Thought
For decades, most references placed the Cloverly Formation within the Aptian and Albian stages of the Early Cretaceous, roughly 125 to 100 million years ago. That estimate turns out to have been far too narrow. Radiometric dates obtained from zircon crystals in fluvial sandstones and bentonitic claystones now indicate that deposition spanned from the Valanginian stage through the Cenomanian, approximately 140 to 98 million years ago.3Bulletin of the Peabody Museum of Natural History. Chronostratigraphic Revision of the Cloverly Formation (Lower Cretaceous, Western Interior, USA) – Section: Abstract That more than doubles the formation’s depositional timeframe and carries real consequences for how fossils found within it are interpreted.
The revised timeline also connects different parts of the formation to different tectonic events. The Pryor Conglomerate at the base was laid down between about 140 and 130 million years ago, driven by sediment shed westward off rising mountains during the early phases of the Sevier Orogeny. The Little Sheep Mudstone above it accumulated during a quieter interval of low sediment supply, roughly 124 to 109 million years ago. Then, in the mid to late Albian and into the early Cenomanian (about 109 to 98 million years ago), sediment sourced from the east built up the Himes Member and the Greybull Sandstone that caps the sequence.4Bulletin of the Peabody Museum of Natural History. Chronostratigraphic Revision of the Cloverly Formation (Lower Cretaceous, Western Interior, USA) – Section: Abstract
This means fossils found in the lower part of the Cloverly could be tens of millions of years older than fossils found near the top. Lumping everything into a single “Aptian–Albian” bin, as many older publications did, obscures genuine evolutionary change happening within the formation itself.
The Blurry Boundary With the Morrison Formation
One question that has occupied stratigraphers is whether the contact between the Morrison Formation and the Cloverly represents a significant gap in the rock record. At some localities the Pryor Conglomerate cuts into Morrison beds along a clear erosional surface, which does look like a real unconformity. But across much of the basin, the picture is muddier than that. Sandstone composition, clay mineralogy, and the architecture of ancient river channels all change gradually through the section rather than shifting abruptly at the formation boundary. Scour surfaces, the kind of erosional contacts that might signal a time gap, are common within each formation and do not necessarily line up with the contact between the two.5Sedimentary Geology. Evaluation of unconformities in upper jurassic and lower Cretaceous nonmarine deposits, Bighorn Basin, Wyoming and Montana, U.S.A. – Section: Abstract
The implication is that the upper Morrison and lower Cloverly may record continuous, though episodic, deposition by a river system evolving through time rather than a dramatic break followed by a fresh start. For paleontologists, this matters because it means faunal changes across the boundary could be gradual transitions rather than sudden replacements triggered by some environmental upheaval. It also complicates any attempt to draw a clean line between “Jurassic” and “Cretaceous” ecosystems in this part of the world.
Deinonychus, Tenontosaurus, and the Dinosaurs That Made the Formation Famous
The Cloverly Formation owes much of its celebrity to two dinosaurs collected by field crews led by Barnum Brown in the 1930s and, more influentially, by John Ostrom beginning in the 1960s. Over nearly four decades of combined prospecting, Brown and Ostrom recovered what turned out to be a low-diversity but deeply influential dinosaur fauna, headlined by the dromaeosaurid theropod Deinonychus antirrhopus and the ornithopod Tenontosaurus tilletti.6Taylor & Francis Online (Journal of Vertebrate Paleontology). Vertebrate Paleontology of the Cloverly Formation (Lower Cretaceous), I: Faunal Composition, Biogeographic Relationships, and Sampling
Deinonychus, described by Ostrom in 1969, is the animal that sparked a revolution in how scientists thought about dinosaur biology. Its stiffened tail, large sickle-shaped foot claw, and generally athletic build led Ostrom to argue that at least some dinosaurs were active, warm-blooded predators rather than the sluggish reptiles of older textbooks. That argument rippled outward for decades, influencing everything from museum displays to the raptors of popular film. The holotype and referred specimens all come from the Cloverly.
Tenontosaurus, a medium-sized herbivore roughly five to seven meters long, is by far the most commonly found dinosaur in the formation. It shows up so often, and so frequently alongside shed Deinonychus teeth, that the association between predator and prey has become one of the most discussed taphonomic relationships in the fossil record. Multiple Cloverly sites preserve Tenontosaurus skeletons surrounded by Deinonychus teeth that were apparently lost during feeding, suggesting repeated interactions between the two species.
Tenontosaurus and the Gastrolith Puzzle
A Tenontosaurus skeleton from the Cloverly Formation in Montana added an unexpected detail to what we know about this common herbivore. The specimen was found with at least twelve polished stones concentrated in its gastric region, consistent with gastroliths, or “stomach stones,” that some animals swallow to help grind food internally. If confirmed, these would represent one of only a handful of unambiguous gastrolith records in ornithopod dinosaurs, alongside finds in the more basal ornithopods Gasparinisaura, Haya, and Changmiania.7Cretaceous Research. Gastroliths and Deinonychus teeth associated with a skeleton of Tenontosaurus from the Cloverly Formation (Lower Cretaceous), Montana, USA – Section: Results
The discovery is interesting for a couple of reasons. First, it suggests that Tenontosaurus may not have been as effective a chewer as more advanced ornithopods that appeared later in the Cretaceous. Many of those later species evolved complex dental batteries capable of grinding tough plant material. Tenontosaurus, by contrast, may have needed mechanical help from swallowed stones to process its food. Second, the specimen also carried Deinonychus teeth, reinforcing the familiar predator-prey association and suggesting the animal was scavenged or killed by dromaeosaurids before or after burial.8Cretaceous Research. Gastroliths and Deinonychus teeth associated with a skeleton of Tenontosaurus from the Cloverly Formation (Lower Cretaceous), Montana, USA – Section: Results
The researchers considered whether the stones might instead be clasts deposited by a debris flow, but dismissed this because the surrounding sediment is fine-grained mudstone, not the kind of deposit you would expect to carry large clasts, and because the stones were clustered specifically in the stomach area rather than scattered around the skeleton.
Sauropelta and the Armored Dinosaurs
Not all Cloverly dinosaurs were swift predators or common prey. Sauropelta edwardsi, a nodosaurid ankylosaur, is one of the most completely known armored dinosaurs from the Early Cretaceous anywhere in North America. Several partial skeletons from the Cloverly Formation of Wyoming and Montana have preserved the animal’s bony armor in its original position on the body, allowing detailed skeletal reconstructions and life restorations that give a clear picture of what this tank-like herbivore looked like in life.9Canadian Journal of Earth Sciences. Skeletal reconstruction and life restoration of Sauropelta (Ankylosauria: Nodosauridae) from the Cretaceous of North America – Section: Abstract
Sauropelta was a large animal, estimated at around five meters long, covered in rows of bony scutes and prominent spines along its neck and shoulders. Unlike the club-tailed ankylosaurids that became common in the Late Cretaceous, nodosaurids like Sauropelta relied on their armor and shoulder spines for defense rather than a tail weapon. Its presence in the Cloverly adds a heavily armored herbivore to an ecosystem otherwise dominated by the more lightly built Tenontosaurus, suggesting different feeding strategies and habitat preferences among the plant-eaters sharing this landscape.
Sauropods Were Still Around
One longstanding assumption about Early Cretaceous North America was that sauropod dinosaurs had largely disappeared from the continent, squeezed out after their Late Jurassic heyday in the Morrison Formation. The Cloverly Formation has helped overturn that idea. A titanosauriform sauropod described from the formation demonstrated that large, long-necked herbivores were still present in what is now Wyoming and Montana during the Aptian and Albian stages, and that North American sauropod diversity during the Early Cretaceous was greater than researchers had previously assumed.10Cretaceous Research. A new titanosauriform from the Early Cretaceous Cloverly Formation of Montana – Section: Conclusions
This find is part of a broader pattern emerging from Early Cretaceous sites across the continent. Rather than a clean faunal replacement in which sauropods vanished and ornithopods took over as dominant herbivores, the picture looks more like a gradual ecological reshuffling. Sauropods persisted but became less common, while ornithopods like Tenontosaurus proliferated and diversified. The Cloverly captures a transitional snapshot of that process, a world where both groups coexisted alongside armored nodosaurids and predatory dromaeosaurids.
Beyond the Dinosaurs
Early collecting efforts in the Cloverly focused almost exclusively on dinosaur bones, and the formation earned a reputation as yielding a distinctive but low-diversity fauna. Brown and Ostrom’s decades of fieldwork produced relatively few non-dinosaurian vertebrates.11Taylor & Francis Online (Journal of Vertebrate Paleontology). Vertebrate Paleontology of the Cloverly Formation (Lower Cretaceous), I: Faunal Composition, Biogeographic Relationships, and Sampling That partly reflects genuine low diversity in the preserved ecosystem, but it also reflects how those early expeditions were conducted. Crews were looking for large, impressive skeletons. Microvertebrate fossils, the tiny teeth and bones of fish, lizards, mammals, and amphibians that get recovered by screen-washing bulk sediment, were mostly ignored.
More recent work has expanded the known fauna considerably. Screen-washing and targeted microsite collecting have turned up small mammals, crocodilians, turtles, fish, and various small reptiles that fill out the ecological picture. These smaller animals are important for understanding what the Cloverly ecosystem actually looked like day to day. A landscape dominated by Tenontosaurus and patrolled by Deinonychus also supported a complex understory community of creatures that rarely make it into museum displays but formed the backbone of the food web.
Tectonic Setting and Why It Matters for the Fossils
The Cloverly Formation accumulated in a continental interior setting, deposited by rivers flowing across a broad floodplain between rising mountains to the west and a slowly encroaching seaway to the east. The Sevier Orogeny, the mountain-building event that was pushing up the ancestral Rocky Mountains during the Cretaceous, is the dominant tectonic driver. Sediment eroded from those growing highlands was carried eastward by river systems and spread across the lowlands as the sand, mud, and gravel that eventually became the Cloverly’s three members.
The shift in sediment sources recorded by the formation’s members tracks real changes in the regional tectonic landscape. Early sediment came from the west, driven by Sevier uplift. The middle interval, when the Little Sheep Mudstone’s fine-grained beds accumulated, represents a period of relative quiet when sediment supply dropped. Then the upper Himes Member records a reversal, with sediment arriving from the east.12Bulletin of the Peabody Museum of Natural History. Chronostratigraphic Revision of the Cloverly Formation (Lower Cretaceous, Western Interior, USA) – Section: Abstract These changes in river behavior and sediment supply directly affected where and how fossils were preserved. Channel sandstones tend to concentrate bones and teeth at their bases, while floodplain mudstones are more likely to preserve articulated skeletons buried by overbank floods.
Understanding this depositional framework also helps explain the spotty nature of the Cloverly’s fossil record. Fluvial environments are inherently patchy preservers of life. An animal that died on a well-drained floodplain miles from a river channel had almost no chance of being fossilized. The specimens we have are a biased sample, skewed toward animals that lived near water, died near active channels, or were lucky enough to be buried quickly by flooding events.
How Field Methods Have Changed the Story
The history of collecting in the Cloverly is itself a case study in how field techniques shape what scientists think they know about ancient ecosystems. When Brown first visited the formation in the 1930s, collecting was focused on large, visible bones that could be jacketed in plaster and hauled out by horse or truck. Ostrom’s campaigns in the 1960s and 1970s were more systematic but still oriented toward macrofossils. The result was a fauna that looked deceptively simple: Deinonychus, Tenontosaurus, Sauropelta, a few odds and ends.
Modern collecting has rewritten that story without needing to discover a single new formation. By applying screen-washing techniques to Cloverly sediments, researchers have recovered a microvertebrate record that dramatically expands the faunal list. The same outcrops that Brown and Ostrom walked past still contain information, just at a smaller scale than those earlier workers were equipped to detect. Combined with the revised chronostratigraphy that extends the formation’s age range, the Cloverly has gone from a seemingly simple window into a narrow slice of time to a complex, long-duration record of changing Early Cretaceous ecosystems. The formation is the same rock it always was; it is the questions being asked of it that have gotten more sophisticated.