These rexes, nicknamed "Jane" and "Petey," were slightly taller than a horse — about 6 feet — and twice as long.
The bone tissue revealed that Jane and Petey were both growing teenagers, ages 13 and 15, respectively, when they died.
Experts can count the number of rings to determine how old a T. rex was when it died. They can also determine how fast a dinosaur grew at different ages by comparing the spaces between growth rings.
Woodward's team determined that these two T. rexes had been growing at the same speed as modern-day mammals and birds. But Jane and Petey were not done growing when they died — the two juveniles had not yet experienced a major growth spurt, according to the study.
What's more, Jane and Petey's bone rings told the paleontologists that the two rexes grew more in years when food was plentiful.
"Jane and Petey had inconsistently spaced growth rings — some years they did a lot of growing, other years they didn't," Woodward said, adding, "we think the amount of growing has to do with food resource abundance."
According to a 2004 study, the T. rex grew fairly gradually until its teenage years. By the time it was full-grown, the Tyrannosaurus rex weighed about 6 to 9 tons. It stood about 12 to 13 feet tall at the hip and was about 40 to 43 feet long.
A juvenile T. rex would have weighed about five times more than a 4-year-old boy and was as large as any other predatory dinosaur in its habitat.
Some scientists thought they weren't even T. rexes at all. But Woodward said her new study shows the Nannotyrannus species is what she calls "an invalid taxon."
"Our study found that Jane and Petey were immature individuals, and the most likely explanation is that they are juvenile T. rex, not a new species of still-growing tyrannosaur," she said.
The study also suggests that a Nanotyrannus specimen at the Cleveland Museum of Natural History was misidentified and is actually a juvenile T. rex, too.
An adult T. rex, on the other hand, rocked a mullet of feathers on its head, neck, and tail.
That means T. rexes used their massive heads — and jaws — as their main means of attack during a hunt.
A full-grown T. rex had a bite force of 7,800 pounds, equivalent to the weight of about three Mini Cooper cars. T. rexes had the rare ability to bite through solid bone and digest it. Paleontologists know this thanks to studies of the dinosaur's fossilized poop; they've discovered T. rex feces containing tiny chunks of bone eroded by stomach acid.
No other known animal could bite with such force. A study of the Jane T. rex skull published in March showed that not even a juvenile would not have had the bone-crushing power of adult T. rex. But its teeth were more knife-like than those in an adult, Woodward said.
"This in of itself suggests a different feeding behavior for juveniles compared to adults: Juvenile T. rex were more adapted for precision bites and cutting flesh, while adult T. rex could bite into flesh and crush the bone," she added.
An adult T. rex had a long stride, helping it reach speeds of 10 to 25 mph. But the dinosaur never reached a suspended gait, since it had at least one leg on the ground at all times.
"If an adult T. rex was hunting a duck-billed dinosaur almost as big as it was, that duck-billed dinosaur was also likely too large to move very fast either," Woodward said.
When it walked, an adult T. rex kept its legs fairly straight, much like an elephant. Walking with bent legs would have placed immense stress on its bones and joints, quickly exhausting the creature's leg muscles.
But Woodward said juvenile T. rex "could possibly have hunted faster prey," which meant the kids and adults were likely not competing.
"This allowed T. rex to dominate various carnivore niches throughout its life," she added.
T. rex hatchlings were fairly weak and helpless — the young dinosaurs looked somewhat like ducklings and were covered in a downy layer of peach fuzz. A baby T. rex had more than a 60% chance of succumbing to predators, disease, accidents, or starvation during its first year of life.
The T. rex was like "the James Dean of the dinosaurs," Gregory Erickson, a paleontologist from Florida State University, previously told Business Insider. (The actor, often affiliated with the quote "Live fast, die young, and leave a good-looking corpse," died in a car crash at age 24.)
"Historically, many museums would collect the biggest, most impressive fossils of a dinosaur species for display and ignore the others," Woodward said in a press release. "The problem is that those smaller fossils may be from younger animals. So, for a long while we've had large gaps in our understanding of how dinosaurs grew up, and T. rex is no exception."
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