For nearly 180 million years, dinosaurs ruled over life on Earth; although it wasn’t until the very end of their reign - 66 million years ago - that the ‘king of the tyrant lizards' established its short yet dominant dynasty.
Standing nearly 4m tall, measuring 12.5m in length, and weighing in at 8.5 tons (the weight of two African bush elephants), T.rex was one of, if not the largest meat-eating dinosaur known to science. It lived during the Late Cretaceous in what is now western North America, ranging from Canada in the north to New Mexico in the south.
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T.rex shared its environment with lots of other dinosaurs, as evidenced by the fossiliferous rocks of the famous Hell Creek Formation. These rocks, found near Jordan, Montana, have yielded fossils of many massive herbivores but notably few other large-bodied carnivores. This lack of competition is testament to T.rex’s status as the apex predator of its time.
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While it may have been the presiding predator of the period, T.rex wasn’t invincible. Like any animal, it faced dangers and endured hardships, including the single worst day in Earth’s history.
Here are six animals (and one deadly pathogen) that, given some luck and facing down an ailing individual, might have been capable of killing the king of the dinosaurs…
7 animals that could kill a T.rex
Other T.rex
The biggest danger to T.rex were no doubt other T.rex.
There's a lot of evidence of these giants fighting one another - healed bite marks, gouges, and tooth traces on their snouts suspiciously match the unique dentition of other T.rex. These kinds of wounds are so common on T.rex remains that palaeontologists think these violent, intraspecies encounters were part of their everyday lives.
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As well as fighting one another, there’s evidence T.rex ate one another too. Some feet, toes, and lower leg bones display deep grooves that show no signs of healing, suggesting their owners died as a result of their wounds, or were scavenged shortly after dying from other, perhaps unrelated causes. A 2010 study presented four T.rex bones that bore such marks, each consistent with feeding traces made by T.rex on more typical prey.
When these kinds of feeding traces are found on the remains of predators in other parts of the world, it’s not always clear who made them - there could be as many as three, four, or five other predators capable of making such marks. However, in T.rex’s neck of the woods - western North America - there existed only one animal capable of inflicting such damage: T.rex itself.
Ankylosaurus
These slow-moving, heavily-built brutes were the ‘living tanks’ of the Late Cretaceous, encased in a layer of near impenetrable armour composed of bony plates called osteoderms. Ankylosaurus lived alongside T.rex in western North America and likely developed this defence through a long predator-prey arms race between its ancestors and those of T.rex.
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Ankylosaurus also had a large, bony club on the end of its tail. These clubs were made up of two main parts: the club itself (large, fused osteoderms) and a handle (stiff, interlocking vertebrate).
While they were primarily used to fight rivals during disputes over territory and/or mates, some palaeontologists believe Ankylosaurus’ clubs were also weaponised against their predators. Standing 1.7m tall, Ankylosaurus would have likely been capable of delivering bone-breaking blows to the legs of T.rex.
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Despite living alongside one another for several million years, no evidence of Ankylosaurus and T.rex interacting has yet been found. This doesn’t mean they never interacted. In fact, some palaeontologists think a lack of fossil evidence suggests Ankylosaurus’ armour was extremely effective, or that T.rex usually avoided them - probably after seeing a mate get its legs shattered by a particularly feisty Ankylosaurus.
Triceratops
As far as rivalries go, there are few more infamous than the rivalry between T.rex and Triceratops. These prehistoric giants are often depicted locked in mortal combat, but it’s unclear, based on the evidence palaeontologists have found so far, just how often such encounters took place - not to mention which of the two usually emerged victorious.
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In 1996, palaeontologists described the discovery of a damaged Triceratops pelvis bearing at least 58 puncture wounds, most likely inflicted by a T.rex. These marks were deemed feeding traces, rather than signs of active predation, and they added further weight to the controversial theory that T.rex was a strict scavenger.
A decade later, in 2008, palaeontologists described a damaged Triceratops brow horn that provided the first direct evidence of a live encounter between Triceratops and T.rex. The horn had been gnawed and was missing its tip, but importantly it showed signs of healing - indicating that the Triceratops in question lived to fight another day.
While there’s no unequivocal evidence of a Triceratops injuring a T.rex, there is a specimen known as ‘Lee Rex’ that has a nasty-looking puncture wound on its left femur. Some suggest this hole was made by a Triceratops’ horn during a duel that may have ultimately killed ‘Lee Rex’.
Alamosaurus
Standing taller than a telephone pole and weighing in at 35 tons (the average weight of an adult humpback whale), Alamosaurus was the largest dinosaur that ever lived in North America - only Argentinosaurus, another long-necked titanosaur from South America, was larger. Courtesy of its bulk, a fully-grown Alamosaurus was likely untouched by predators, even T.rex.
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Alamosaurus is one of the last North American sauropods and it was long debated whether, despite being around at the same time as T.rex, it ever crossed paths with the continent’s apex predator. However, in 2005 palaeontologists described the discovery of a partial T.rex skeleton from the North Horn Formation of central and eastern Utah. This series of rocks had previously yielded Alamosaurus bones, proving the two at least shared a habitat.
Currently, there’s no evidence of T.rex preying on Alamosaurus, though it’s hard to imagine a predator as formidable as T.rex didn’t target juveniles and/or old, weakened individuals. Still, if a T.rex targeted a weaker Alamosaurus, it would likely have had to contend with other, stronger members of its herd. Just one kick or tail whip from an adult Alamosaurusmay have been enough to immobilise an attacking T.rex, or even kill it.
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Nanotyrannus
For nearly half a century, palaeontologists have debated the validity of Nanotyrannus. Some argue it was a distinct species of tyrannosaur, while others claim it was merely a juvenile T. rex. A 2025 study may have shifted consensus toward the former view and finally established Nanotyrannus as a valid species, though some still believe Nanotyrannus and T.rex represent the same animal.
Regardless of its validity, at half the length, half the height, and roughly one-tenth of the body mass, Nanotyrannus was physically no match for T.rex. However, it had comparatively longer legs, a slender snout, and larger arms. These differences allowed Nanotyrannus to occupy a different ecological niche and survive in the shadow of its larger cousin, hunting swift prey such as young hadrosaurs and ornithomimids.
As part of the aforementioned 2025 study, palaeontologists identified the tyrannosaur in the famous ‘Dueling Dinosaurs’ fossil as a Nanotyrannus. This fossil preserves a nearly complete, articulated Nanotyrannus and Triceratops buried together. While there’s no unequivocal evidence the two were actually ‘dueling’, the existence of such a fossil certainly suggests Nanotyrannus may have preyed on Triceratops, just like T.rex did.
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If Nanotyrannus had the mettle to take on a Triceratops, it might have also been capable of tussling with a T.rex - though likely only a very old, decrepit individual whose senses had been dulled after a lifetime of warring with other T.rex.
Giant Crocodiles
While T.rex may have lorded over the land in prehistoric western North America, the region’s waterways were the domain of giant crocodiles many times larger than today’s biggest beasts.
The largest known from this part of the world is Deinosuchus, which measured 10m in length and weighed more than five tons. These ‘terror crocodiles’ were capable of hunting large dinosaurs, but living from 82 to 73 million years ago they likely never overlapped with T.rex.
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That said, there was a species of large crocodile that lived in Hell Creek alongside T.rex 66 million years ago. This aquatic predator is known as Borealosuchus and it’s estimated to have reached lengths of 5m, making it roughly the same size as today’s largest saltwater crocodiles.
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Borealosuchus was likely no match for T.rex on land, but in the water it may have been a different story. T. rex was probably a capable swimmer, like many other dinosaurs, yet fending off a swarm of hungry Borealosuchus could have been a difficult, if not near-impossible task. If Borealosuchus possessed bite forces comparable to their modern relatives, they might have made short work of a struggling T. rex.
Parasites
The most deadly threat to T.rex may not have been another large predator, but a tiny microscopic organism that still affects birds of prey today. This parasitic protozoan, Trichomonas gallinae, is known to cause ulcers in the mouth and throat of infected animals. Over time, these ulcers can erode jawbones, eventually leading to death by starvation.
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Marks resembling those made by Trichomonas gallinae on modern birds have been observed on the famous T.rex specimen known as Sue, whose jaws are covered in pits and holes. A 2009 study found trichomonosis-style holes in 15% of a sample of 61 tyrannosaurs, indicating that such infections were quite common. The researchers also hypothesised tyrannosaurs may have spread the microscopic parasite through biting others.
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Not everyone agrees these pits and holes on the jaws of T.rex are signs of an infection. A 2022 study on Sue discovered signs that her jawbones were in the process of repairing themselves, suggesting whatever caused the holes didn’t kill her. The authors of this particular study proposed such damage may have been caused by ‘love bites’ inflicted by other T.rex as part of courtship behaviour.
