What Other Dinosaurs Lived With T Rex?

Tyrannosaurus rex shared its world with a surprisingly rich cast of dinosaurs, from the iconic three-horned Triceratops to herds of duck-billed Edmontosaurus, small burrowing ornithischians, crested oviraptorosaurs, and swift raptor-like predators. Most of what we know about this community comes from the Hell Creek Formation, a stretch of rock spanning parts of Montana, the Dakotas, and Wyoming that preserves the final chapter of the age of dinosaurs. The ecosystem was more varied than many people realize, though T. rex’s sheer dominance shaped it in ways that made it look quite different from earlier dinosaur communities.

The Landscape They All Shared

The Hell Creek Formation dates to the last couple of million years before the asteroid impact that ended the Cretaceous, roughly 68 to 66 million years ago. The environment was a broad, low-lying coastal plain drained by meandering rivers, built up between rising mountains to the west and the retreating Western Interior Seaway to the east. Soils and fossil pollen tell us this was a warm, subtropical place with distinct wet and dry seasons, forested mainly by flowering plants with seasonally dry swamps dotting the floodplains.¹ The rivers and moist lowlands supported a multi-tiered plant canopy and a rich fauna of dinosaurs, crocodilians, turtles, lizards, and small mammals.² Think less “barren volcanic wasteland” and more “humid, forested floodplain with a vaguely Gulf Coast feel.” That lush setting is key to understanding why so many species could coexist here.

Triceratops, the Most Famous Neighbor

If T. rex is the face of Late Cretaceous predators, Triceratops is its herbivorous counterpart. The two are found in the same rocks so reliably that they’ve become inseparable in the popular imagination, and for good reason: Triceratops specimens are among the most common large dinosaur fossils in the Hell Creek Formation. The animal’s massive frill and three horns almost certainly served as both display structures and defensive weapons, and at an adult weight of several tons, a healthy Triceratops would have been a formidable meal even for a tyrannosaur.

Triceratops wasn’t static over the time span recorded in the Hell Creek. Two recognized species, T. horridus and T. prorsus, are separated by rock layer, with T. horridus found lower in the formation and T. prorsus appearing in the upper third. Transitional forms show up in between, suggesting one species gradually evolved into the other rather than both existing side by side.³ So while T. rex always lived alongside some version of Triceratops, the exact version changed over geological time.

Triceratops also had its own habitat preferences. Ceratopsians as a group tended to occur more frequently in mudstone deposits, which represent environments farther from active river channels. That suggests Triceratops spent more time in the wooded interiors of the floodplain rather than right along the riverbanks.⁴ T. rex, by contrast, showed no preference for either environment, turning up in both settings with roughly equal frequency.

Edmontosaurus and the Hadrosaur Herds

Edmontosaurus was the other great herbivore of the Hell Creek, and it was enormous. Adults reached lengths of around 12 meters and likely weighed several tons. These were duck-billed hadrosaurs, flat-headed members of a group that elsewhere in the Late Cretaceous sported elaborate crests. In the Hell Creek, Edmontosaurus appears to have been the dominant large herbivore alongside Triceratops, and fossil census work shows its skeletons are about as common as those of T. rex in certain parts of the formation.⁵

Unlike Triceratops, Edmontosaurus preferred areas close to rivers. Their fossils show a strong association with sandstone deposits, the kind left by active channels and nearby floodplains.⁴ That habitat separation between hadrosaurs and ceratopsians meant the floodplain wasn’t just one undifferentiated buffet; different herbivores carved out different zones.

There’s also strong evidence that Edmontosaurus lived in herds. A mass-death site in South Dakota preserves a group of primarily subadult and adult individuals that died together in a flooding event, with their bones showing signs of brief scavenging by theropods before burial.⁶ Herding behavior would have offered some protection against predators, though clearly not always enough. One particularly vivid Edmontosaurus skull from the Hell Creek was found with a tyrannosaurid tooth embedded in its nasal bone. The tooth broke off during a bite delivered from the front, and the lack of healing around the wound means the Edmontosaurus died at or around the time it was bitten. The bite pattern is most consistent with a predatory attack intended to control or kill the animal.⁷

The Smaller Herbivores Most People Forget

Triceratops and Edmontosaurus get the attention, but the Hell Creek supported a range of smaller plant-eaters too. Thescelosaurus was a roughly human-sized ornithischian that, like the hadrosaurs, preferred habitats near rivers.⁴ For a long time it was treated as a fairly generic small dinosaur, but recent work on its brain anatomy has revealed something more interesting: neurological features consistent with burrowing behavior. This makes Thescelosaurus the first ornithischian dinosaur with evidence of neurological specializations for life underground, broadening the range of ecological roles we know non-avian dinosaurs could fill.⁸

Pachycephalosaurs, the dome-headed dinosaurs, also lived in the Hell Creek. Pachycephalosaurus itself is the most familiar, but recent finds suggest there were more species than we realized. Two newly described small-bodied pachycephalosaurines, including one from the Hell Creek of Montana, indicate that small-bodied dome-heads continued to diversify right up to the end of the Cretaceous, even as the large-bodied species were already well catalogued.⁹ This points to a broader theme: we probably undercount small dinosaurs in general.

Mid-Sized Predators and the Raptor Question

T. rex wasn’t the only meat-eater around, but its smaller theropod neighbors are much harder to find. Dromaeosaurids, the family popularly known as “raptors,” were present but are mostly known from teeth found at microvertebrate sites rather than complete skeletons. Two named species from the Hell Creek, Acheroraptor and the larger Dakotaraptor, give us some idea of what was there. Dakotaraptor may have reached roughly 5 to 6 meters in length, making it a serious predator of mid-sized prey. The discovery of a related dromaeosaurid called Dineobellator from the same time period in New Mexico confirms that multiple lineages of raptors were diversifying right up to the end of the Cretaceous, filling similar predatory niches across different parts of Laramidia, the western landmass of what is now North America.¹⁰

Troodontids, another group of small, brainy theropods, also lived in the Hell Creek. Troodon-like teeth turn up at the same quarry sites that produce T. rex material, and their distinctive large, apically angled denticles make them identifiable even as isolated finds.¹¹ Despite being confirmed residents, skeletons of these smaller predators are conspicuously rare. A census of Hell Creek dinosaur material found that while T. rex skeletons are surprisingly abundant, smaller predators like Troodon and dromaeosaurids are represented almost exclusively by shed teeth.⁵ Whether that reflects genuinely low population numbers or just a bias in how smaller bones survive in the fossil record is an open question, and probably a bit of both.

Anzu, the “Chicken From Hell”

One of the more unusual Hell Creek residents was Anzu wyliei, a large oviraptorosaur nicknamed the “chicken from hell.” At roughly 3.5 meters long with a hip height of about 1.5 meters and a body mass of around 200 to 300 kilograms, Anzu was among the largest oviraptorosaurs known. It had a deep, narrow skull topped by a tall, cassowary-like crest, and its overall build suggests it was an ecological generalist, capable of feeding on vegetation, small animals, and possibly eggs on the coastal plains of western North America.¹² Anzu is a reminder that “theropod” doesn’t always mean “apex predator.” The Hell Creek’s theropod diversity included everything from massive tyrannosaurs to beaked omnivores that probably looked more like oversized ground birds than anything from Jurassic Park.

The Nanotyrannus Debate

For decades, one of the most contentious questions about the Hell Creek community has been whether T. rex shared its turf with a smaller tyrannosaur called Nanotyrannus. Certain small tyrannosaur specimens were argued to be juveniles of T. rex rather than a separate species, and bone tissue analysis initially supported that interpretation. Osteohistology of the key specimens showed they were immature individuals around 13 to 15 years old, growing at rates similar to birds and mammals, which pointed to them being young T. rex undergoing a dramatic growth spurt rather than adults of a different species.¹³

That conclusion held as the mainstream view for years, but a 2025 study in Nature reignited the debate with force. Based on an exceptionally well-preserved, near-adult tyrannosaur skeleton, the researchers argued that Nanotyrannus is indeed a distinct genus, not a young T. rex. They went further, identifying two species within Nanotyrannus and placing the genus outside Tyrannosauridae entirely, making it a separate lineage of tyrannosaur relative. If correct, the late Maastrichtian of North America hosted at least two ecologically distinct tyrannosaur-line predators living alongside each other.¹⁴ This is far from settled. The debate has real consequences for how we understand the Hell Creek food web: a world where juvenile T. rex filled the mid-sized predator role looks very different from one where a separate, more lightly built tyrannosaur occupied that niche.

How T. rex Shaped the Community Around It

One of the stranger features of the Hell Creek ecosystem is the apparent absence of mid-sized carnivores. In most modern ecosystems, you find large predators, medium predators, and small predators in a roughly predictable distribution. In the Hell Creek and other dinosaur communities with megatheropods like T. rex, the medium-sized carnivore niche seems to be empty. Research modeling dinosaur community structure found that while herbivores spanned the full body-size range, communities with megatheropods consistently lacked carnivores weighing 100 to 1000 kilograms. The most likely explanation is that juvenile T. rex filled that gap themselves, occupying the mesocarnivore niche as they grew and reducing overall species diversity in the process.¹⁵

This idea fits neatly with the census data from the Hell Creek. T. rex skeletons are strikingly abundant, as common as Edmontosaurus in the upper formation and nearly twice as common in the lower third. That ratio of predator to prey is much higher than what you’d expect from modern mammalian ecosystems.⁵ Modeling work across multiple formations confirms that large theropods appear to have been more abundant relative to large herbivores than modern big carnivores are relative to their prey. One proposed explanation is that herbivore productivity (the rate at which new biomass becomes available through births and deaths) was higher than in modern mammal systems, or that large theropods had lower metabolic needs than comparable mammals, or both. But even with generous assumptions, the numbers are hard to square unless big theropods also ate a wider range of prey than we typically imagine, including smaller dinosaurs and other vertebrates.¹⁶

What the Fossil Record Is Still Missing

Every list of Hell Creek dinosaurs comes with a major caveat: the fossil record is systematically biased toward large animals. Work on the closely related Dinosaur Park Formation shows that while our knowledge of large-bodied dinosaurs is reasonably complete, small taxa are dramatically underrepresented in both diversity and abundance, with many discoveries still to be made.¹⁷ That pattern almost certainly extends to the Hell Creek. The small-bodied pachycephalosaur discoveries mentioned earlier are a case in point: they turned up in a formation that has been intensively collected for over a century, suggesting that our census of small dinosaurs is still far from complete.

This size bias matters when you try to reconstruct the full community T. rex lived in. The list of well-known neighbors (Triceratops, Edmontosaurus, Anzu, a handful of small theropods) is probably just the visible fraction of a much more diverse assemblage. Small ornithischians, juvenile specimens of known species, and entirely unknown small-bodied forms are the most likely gaps. Every few years, a new taxon turns up in rocks that paleontologists have been working for generations, which is a good sign that the Hell Creek still has secrets to give up.

Regional Differences Across Laramidia

The Hell Creek Formation in Montana and the Dakotas gets the most attention, but T. rex wasn’t the only giant tyrannosaur in latest Cretaceous North America, and the communities in other regions looked different. In what is now southern New Mexico, a recently described giant tyrannosaur called Tyrannosaurus mcraeensis lived alongside a fauna that shared no overlap with the penecontemporaneous faunas of Canada. Its neighbors included the giant chasmosaur Sierraceratops, a titanosaurian sauropod related to Alamosaurus, and a giant hadrosaur.¹⁸ The presence of a sauropod is particularly noteworthy. Sauropods had largely disappeared from northern Laramidian ecosystems by the late Cretaceous but persisted in the south, giving those communities a different flavor entirely. A person asking “what lived with T. rex” would get a genuinely different answer depending on whether they meant the northern Hell Creek fauna or the southern Big Bend and New Mexico assemblages.

Pterosaurs and Other Non-Dinosaur Neighbors

Dinosaurs were not the only large vertebrates in the Hell Creek. Pterosaurs, the flying reptiles that were not dinosaurs but are often grouped with them in popular imagination, were present as well. Azhdarchid pterosaurs, the group that includes some of the largest flying animals ever, are known from the formation, and recently described material has expanded the known diversity of pterosaurs in Maastrichtian North America. These were tall, stork-like animals with enormous wingspans that probably stalked the floodplains for small prey on the ground.

Crocodilians were abundant, including the massive Deinosuchus relatives that had populated earlier Cretaceous waterways and smaller forms that would not look out of place in a modern bayou. Turtles were extremely common, and the rivers and ponds supported a range of fish, amphibians, and freshwater sharks. Among the mammals, small multituberculates and early placentals scurried through the undergrowth. The Hell Creek was a complete, functioning ecosystem with every trophic level occupied, and the dinosaurs, spectacular as they were, shared it with a much broader community of animals adapted to every available niche from subterranean burrows to river bottoms to open sky.