10 Facts About Tyrannosaurus Rex, the King of the Dinosaurs

Tyrannosaurus rex is by far the most popular dinosaur, having spawned a huge number of books, movies, TV shows, and video games. What's truly amazing, though, is how much what was once assumed as fact about this carnivore has later been called into question and how much is still being discovered. Here are 10 facts known to be true:

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Not the Biggest Meat-Eating Dinosaur

T-Rex, artwork
SCIEPRO / Getty Images

Most people assume that the North American Tyrannosaurus rex—at 40 feet from head to tail and seven to nine tons—was the biggest carnivorous dinosaur that ever lived. T. rex, however, was equaled or outclassed by not one but two dinosaurs: the South American Giganotosaurus, which weighed about nine tons, and the northern African Spinosaurus, which tipped the scales at 10 tons. These three theropods never had the chance to square off in combat, since they lived in different times and places, separated by millions of years and thousands of miles.

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Arms Not as Tiny as Once Thought

Tyrannosaurus and comet

One feature of Tyrannosaurus rex that everyone makes fun of is its arms, which seem disproportionately tiny compared to the rest of its massive body. T. rex's arms were over three feet long, however, and may have been capable of bench pressing 400 pounds each. In any event, T. rex didn't have the smallest arm-to-body ratio among carnivorous dinosaurs; that was the Carnotaurus, whose arms looked like tiny nubs. 

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Very Bad Breath

Tyrannosaurs rex skeleton

The dinosaurs of the Mesozoic Era obviously didn't brush their teeth or floss. Some experts think shards of rotten, bacteria-infested meat constantly lodged in its closely packed teeth gave Tyrannosaurus rex a "septic bite," which infected and eventually killed its wounded prey. This process likely would have taken days or weeks, by which time some other meat-eating dinosaur would have reaped the rewards.

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Females Bigger than Males

Tyrannosaurus rex dinosaur

There's a good reason to believe, based on fossils and the shapes of their hips, that female T. Rexes outweighed males by a few thousand pounds. The likely reason for this trait, known as sexual dimorphism, is that females had to lay clutches of T. rex-size eggs and were blessed by evolution with bigger hips. Or maybe females were more accomplished hunters than males, as is the case with modern female lions.

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Lived About 30 Years

Silhouette of dinosaur sculpture at sunset, Moab, Utah, USA
Dave and Les Jacobs / Getty Images

It's difficult to infer a dinosaur's life span from its fossils, but based on analysis of existing specimens, paleontologists speculate that Tyrannosaurus rex may have lived as long as 30 years. Because this dinosaur was atop the food chain, it would most likely have died from old age, disease, or hunger rather than attacks by fellow theropods, except when it was young and vulnerable. Some of the 50-ton titanosaurs that lived alongside T. rex might have had life spans of more than 100 years.

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Both Hunters and Scavengers

Artwork of a Tyrannosaurus rex hunting
Science Photo Library - MARK GARLICK / Getty Images

For years, paleontologists argued about whether T. rex was a savage killer or an opportunistic scavenger—that is, did it hunt its food or tuck into the carcasses of dinosaurs already felled by old age or disease? Current thinking is that there's no reason Tyrannosaurus rex couldn't have done both, as would any carnivore that wanted to avoid starvation.

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Hatchlings Possibly Covered in Feathers

Tyrannosaurus rex dinosaur prowling in marsh
ac productions / Getty Images

It's accepted as fact that dinosaurs evolved into birds and that some carnivorous dinosaurs (especially raptors) were covered in feathers. Some paleontologists believe that all Tyrannosaurs, including T. rex, were covered in feathers at some point during their lives, most likely when they hatched, a conclusion supported by the discovery of feathered Asian tyrannosaurs such as Dilong and the almost T. rex-size Yutyrannus.

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Preyed on Triceratops

Tyrannosaurus rex skull, illustration

Imagine the matchup: a hungry, eight-ton Tyrannosaurus rex taking on a five-ton Triceratops, a not-inconceivable proposition since both dinosaurs lived in late Cretaceous North America. Granted, the average T. rex would have preferred to tackle a sick, juvenile, or newly hatched Triceratops, but if it was hungry enough, all bets were off.

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Incredibly Powerful Bite

Tyrannosaurus rex

In 1996, a team of Stanford University scientists examining a T. rex skull determined that it chomped on its prey with a force of 1,500 to 3,000 pounds per square inch, comparable to that of a modern alligator. More recent studies put that figure in the 5,000-pound range. (The average adult human can bite with a force of about 175 pounds.) T. rex's powerful jaws may have been capable of shearing off a ceratopsian's horns.

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Almost Named Manospondylus

Majungasaurus in a barren environment.

 Stocktrek Images/Getty Images

When famous paleontologist Edward Drinker Cope excavated the first T. rex fossil in 1892, he briefly considered naming his find Manospondylus gigax, Greek for "giant thin vertebrae." After further impressive fossil finds, it was up to the then-president of the American Museum of Natural History, Henry Fairfield Osborn, to select the immortal name Tyrannosaurus rex, the "tyrant lizard king."