I'm Just a Teenage T-rex, Baby

Two smallish specimens only twice the size of horses, found in Montana, weren’t teacup tyrannosaurs: Jane and Petey were juveniles of the real thing

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A larger member of the extended Tyrannosaurus family
A larger member of the extended Tyrannosaurus familyCredit: REUTERS

New dinosaur species seem to get announced every other day. Some really are new to science. Others have turned out to be the kids of well-known species, which the paleontologists didn’t recognize because juvenile animals tend to look different from adults. This has now happened with two dinosaur skeletons found almost 20 years ago. They are not teacup tyrannosaurs or Nanotyrannus as once postulated: they were teenage Tyrannosaur rexes.

Their newly identified family affiliation of the terrible twain was announced Wednesday in Science Advances, in the article "Growing up Tyrannosaurus Rex".

Aside from misidentifying juvenile characteristics as unfamiliar adult features, some suggest that the explosion of dinosaur species is because scientists can name new species after themselves, perpetuating their name for posterity. Or they can commemorate someone else, be it a mentor, their pet rat or Donald Trump. Some clever scientists even had the idea of auctioning the right to name new species of snail-sucking snakes they had found.

Don’t be judgy about the bias toward new species. Juvenile forms of animals really can look different from the mature version. Think tadpoles versus frogs. Imagine a paleontologist tens of millions of years in the future unearthing fossil tadpoles and fossil frogs, and having no live ones to compare with. You can see how the future paleontologist might not realize the one was the baby of the other.

Baby T-rex isn’t as different from the gargantuan parent as a tadpole from a toad. But a recent artist’s impression of the infant T-rex shows there is some mental ground to cover.

Baby T-rex, to die forCredit: Natural History Museum, YouTube

We wish to point out at this stage that adult T-rexes are not thought to have been feathered head to talon. Although feathering seems to be the basal dinosaurian condition, like hair is to mammals, the giants were apparently bare-skinned.

Bushy-tailed or bald, the adult T-rex could reach about 40 feet (12 meters) in length. Just its skull was 5 feet long. The specimens found in the early 2000s in Carter County, Montana, were clearly titchy tyrannosaurs of some type, but they were no taller than a horse and only twice as long – about 8 feet.

Their proud possessor of the teen rexes, the Burpee Museum of Natural History in Rockford, Illinois, named them Jane and Petey. As for their identity: As is entirely normal in paleontology, the identity of the titanic twain was not immediately clear. There was thinking that they were adults of a pygmy variant called Nanotyrannus.

See Jane run. See Petey run too. Run

It is now clear that the tyrannosaurian range was vast, reaching all the continents extant during their time and while some were very, very big, some were not. In the mid-Cretaceous, Suskityrannus hazelae was a rex relative that would have reached your hip, though including the tail it stretched a whole 9 feet in length. Moros intrepidus, “harbinger of doom,” loomed over Suskityrannus by a few inches.

Skull of adult T-rexCredit: REUTERS
Skull of a very small tyrannosaur ancestorCredit: AP

Suskityrannus and Moros are thought to have weighed about as much as a fat Labrador or trim Great Dane – say, 80 kilograms (around 175 pounds). T-rex weighed maybe 14,000 kilograms, or almost 31,000 pounds. Jane and Petey were in between, size-wise.

It was microscopic analysis of the bones that led Holly Woodward of the Oklahoma State University Center for Health Sciences and the team to the conclusion that the skeletons were juvenile T. rexes, not a newly discovered nano-species.

Happily, when an animal fossilizes, so does bone tissue at the microscopic level. Measuring rings in their leg bones – which are sort of like tree rings – led Woodward to realize that Jane was only 13 years old when she died and Petey was 15.

The team also learned that in lean years, the teenage T-rexes didn’t grow as much. But when food was plentiful, they grew like weeds.

Suskityrannus hazelaeCredit: Andrey Atuchin / Virginia Tech /

Because it took T. rex up to 20 years to reach adult size, they did change a lot as they grew up – though not as much as tadpole to toad.

Thinking about the behavior of T-rexes changes a lot: Were they heavy-footed lumbering monsters or nimble? Did they hunt in packs? Did they have feather mohawks?

Woodward believes the adults were “lumbering bone crushers” while the teenage tyrannosaurs – sort of like teenage humans – were “fast, fleet-footed and had knife-like teeth for cutting.”

“The spacing between annual growth rings record how much an individual grows from one year to the next,” she explains. “The spacing between the rings within Jane, Petey, and even older individuals is inconsistent: Some years the spacing is close together, and other years it’s spread apart.”

The paper’s co-authors included Jack Horner of Chapman University; Nathan Myhrvold, founder and CEO of Intellectual Ventures; Katie Tremaine of Montana State University; Scott Williams of the Museum of the Rockies; and Lindsay Zanno of the North Carolina Museum of Natural Sciences.