About 67 million years ago, in what is now north-west Montana in the United States of America, a dinosaur died.

How it died is unknown, but its death was recorded all the same. Over geological time, bone turned to rock. The skeleton was saved in stone.

In 2003, palaeontologists from the Burpee Museum of Natural History in Illinois retrieved it. Untouched by erosion at the surface and tectonic forces below, the fossilised skeleton was nearly complete, reaching 20ft (6m) long and 7ft (2m) tall.

With a skull full of sharp teeth and long hind limbs, it was clearly a predator. Its sex was unknown, but nevertheless its discoverers called it "Jane".

What was Jane? There were two possibilities.

Some thought she might be a Nanotyrannus, a kind of pygmy relative of the mighty 13m-long Tyrannosaurus rex. That would mean there were two species of tyrannosaurs roaming the forests of North America during the late Cretaceous

Others said Jane was a juvenile T. rex that died too young – and that there was never any such species as Nanotyrannus. If that is true, then Jane can tell us what T. rex was like as a gangling, awkward youngster.

As many palaeontologists are coming to realise, to understand the Age of Dinosaurs, you first have to understand the age of the dinosaurs you are studying.

Jane is not the only fossil that might belong to Nanotyrannus.

In 1942, David Dunkle from the Cleveland Museum of Natural History in Ohio had unearthed a slightly compressed skull, similar in appearance to Jane's. It was labelled as CMNH 7541 and, for a long time, this is the only name that palaeontologists were sure of.

To understand the Age of Dinosaurs, you first have to understand the age of the dinosaurs you are studying

After poring over the skull's features, Charles W. Gilmore – the doyen of tyrannosaur research at the time – classified it as a species of Gorgosaurus. This was one of several smaller relatives of T. rex that lived during an earlier stage of the Cretaceous period.

However, in 1970 CMNH 7541 gained another moniker. A new study suggested it really belonged to a different tyrannosaur genus called Albertosaurus, named after dinosaur-rich deposits in Alberta, Canada.

Then in 1988, Bob Bakker of the University of Colorado and his colleagues changed the name again. They proposed that CMNH 7541 was something completely new among tyrannosaurs, a genus that no one had appreciated before. Based primarily on its thin face and small sharp teeth, they renamed the skull Nanotyrannus, literally translating as the "pygmy tyrant".

But even that was not the end of the story.

In 1999, using the excellent fossil record of Albertosaurus as a guide, Thomas Carr, now at Carthage College in Wisconsin, found that the skulls and teeth of tyrannosaurs became more robust over time.

Large dinosaurs – just like other animals – went through a remarkable sequence of sizes and shapes

Adults were big-boned, thick-toothed, and heavy, while juveniles and adolescents were sleek and lightly-built with thin teeth. In other words, the defining features of Nanotyrannus may have just been a sign of immaturity rather than a distinct identity.

Although backed by unprecedented detail and data, this idea was nothing new.

Working behind the borders of the USSR, the Russian palaeontologist Anatoly Rozhdestvensky first proposed that the Cleveland skull was just a juvenile T. rex in 1965, a time when it was still under its original name, Gorgosaurus.

Rozhdestvensky was one of the first palaeontologists to appreciate that dinosaurs changed dramatically in appearance as they grew – something many of his contemporaries did not realise. After they hatched from their grapefruit-sized eggs, he realised, large dinosaurs – just like other animals – went through a remarkable sequence of sizes and shapes.

For instance, between 1941 and 1951 Chung Chien Young – "the father of Chinese palaeontology" – examined 70 specimens and managed to describe five new species of prosauropods; early relatives of the sauropod group that includes giant herbivores like Diplodocus and Brachiosaurus.

Fossils can be misinterpreted as exciting evidence of a brand-new dinosaur species

However, when Rozhdestvensky looked through the same collection, he found only one: Lufengosaurus huenei. The other four "species" were actually just snapshots of L. huenei at different points in its development from egg to adult.

As Rozhdestvensky wrote in 1965: "Growth changes are therefore of the greatest importance in determining the scope and boundaries of a species."

As Carr laconically puts it: "Growth happens, and it changes everything." Without taking this fact into account, fossils can be misinterpreted as exciting evidence of a brand-new dinosaur species, when really they are just evidence of a known species at different ages.

"Just imagine if you found a skeleton of a toddler," says Stephen Brusatte of the University of Edinburgh in the UK. "If you knew very little about how humans grew, you might conclude that it was some pygmy primate species."

Although often ignored by western science during his time, Rozhdestvensky's thinking has now come to dominate dinosaur palaeontology.

"We just have a lot more knowledge about how dinosaurs grew," says Brusatte. "We recognise that when we find a new fossil and it's different from everything else, it could be a new species… or, it could be just a growth stage of something already known."

Growth happens, and it changes everything

Thankfully, the dinosaur fossils themselves can help researchers decide between the two alternative interpretations.

Fossil bones contain a record of the annual rhythms of their owner's lives. Just like the rings of a tree, each year of growth is delineated against the next as growth halts in winter. This leaves a "line of arrested growth", or LAG. The number of rings provides an accurate proxy for the age at the time of death.

What's more, widely-spaced LAGs reveal that the dinosaur was growing quickly during that year, adding more bone before the harsh seasons arrived. In contrast, older dinosaurs had LAGs very close together in their final years of life. Their growing days were over.

Although researchers had the option of studying LAGs throughout the second half of the 20th Century, it only became commonplace in the last decade or so.

"It has taken so long to become really popular because of the idea that you actually have to cut a dinosaur bone to access this information," says Holly Ballard, assistant professor of anatomy at Oklahoma State University. "And a lot of people are very hesitant to do that, especially if you only have one specimen of that particular species."

But it is really not as bad as it sounds, Ballard says.

He estimated Jane's age at death as 12 years, give or take a year

First, only a tiny sample is needed – a few millimetres will do the trick. And second, a replica piece, matching the original in shape and texture, can be made and reinserted back into the skeleton, making it appear whole once more.

The damage is minimal, but the rewards are great. Taking a slice of a non-weight-bearing bone is the best option, since any forces placed on bone can remodel its internal structure and blur any annual lines in the process.

That makes a fossilised skull – a complex mosaic of different bones that are often subjected to extraordinary bite forces – a poor choice. The Cleveland skull, therefore, remains ageless to science.

This is where Jane comes in. In 2003, Gregory Erickson of Florida State University in Tallahassee was given permission to take a small slice out of Jane's fibula; the accessory shinbone to the weight-bearing tibia. After polishing the sample and placing it underneath a microscope, he estimated Jane's age at death as 12 years, give or take a year.

She was going through a growth spurt

That makes Jane a juvenile. Adult tyrannosaurs attain maturity at around 20 years of age, and lived until their early 30s.

In theory, Jane could have belonged to an unusual species of tyrannosaur with an exceptionally short lifespan. But it seems unlikely. In the years just before her death, Jane's annual growth rings were very far apart and the microstructure of the bone was highly porous, indicative of blood vessels nourishing the cells that secreted the bone.

In other words, she was nowhere near a fully-grown adult when she died. Instead, she was going through a growth spurt.

In 2005, after slicing through the bones of adult T. rex specimens, Erickson demonstrated exactly how this species grew to be a giant among giants. After comparing their LAGs to the smaller – but still 33ft (10m) long – Albertosaurus and Gorgosaurus, he demonstrated that tyrannosaurs all seemed to go through a growth spurt that lasted for about 10 years.

I have my whole life to decide whether I think Nanotyrannus is a T. rex

The difference was the rate. While the smaller species were struggling to put on 1lb (500g) of weight per day, T. rex was adding over 4lb (2kg). That is like adding a classic Mini Cooper, complete with driver and a passenger, in mass every year.

"There's this huge boost in growth through the teenage years," says Tom Williamson of the New Mexico Museum of Natural History and Science. "It's much greater than what you see in other tyrannosaurs. It's just more. With T. rex, everything is more."

From deep within her bones, Jane's true identity had started to emerge. If she had not died when she did, she would probably have become a giant herself. "[Jane] would fit right in with being a T. rex," says Erickson.

But he is still reluctant to conclude that Jane really was just a juvenile T. rex – and that Nanotyrannus never existed as a separate species. "I have my whole life to decide whether I think Nanotyrannus is a T. rex," says Erickson. "And on my deathbed I may not even make a decision."

Jane, no doubt, is a juvenile

However, other palaeontologists are more confident in the evidence.

Carr is currently working on a monograph of the Jane specimen, detailing its every nook and cranny and profiling her features. Taken as a whole, he finds little evidence that Jane could be anything but a young T. rex.

"You have to work really hard at ignoring all of the evidence that shows that it's a juvenile and that, in terms of identity, it's T. rex," he says. "You have to close your eyes and plug your ears."

Brusatte agrees. "Jane, no doubt, is a juvenile," he says. "Absolutely no doubt. Some people have argued that it's not, which is insanity. When that monograph comes out it will mostly shut the book on Nanotyrannus."

That means Jane provides a rare glimpse into an iconic giant's early years as a smaller predator.

She was in an "awkward gangly teenage phase"

"The fossil record was really poor for young tyrannosaurs," says Carr. "And Jane is virtually complete. [She] will help set out the growth changes of how we get from a juvenile to a full adult."

Even without a monograph, many changes are obvious with little more than a cursory glance at the skeleton. Unlike the thick bones and deep skulls of adulthood, Jane's skeleton is lightly-built, sleek, and long-limbed.

Within her skull, her teeth are thin and serrated like steak knives. Jane would have been an agile hunter, more akin to a large raptor than her older kin. She was in an "awkward gangly teenage phase," says Brusatte.

She may also have been clad in a coat of feathers. The fossils of T. rex's ancestors show the trademark imprints of simple hair-like outgrowths covering large portions of the body, more akin to the coat of New Zealand's kiwi than a New Caledonian crow. "The only logical conclusion is that T. rex itself had feathers," says Brusatte.

The adult T. rex was such a freak of evolution, or a feat of evolution

Jane's early death is a metaphor for her entire species.  "T. rex was the James Dean of dinosaurs," says Erickson. "These things lived fast and died young."

During a decade of adulthood, T. rex was the size of a double-decker bus. Its jaws and teeth could crunch through bone, inflicting deep gouges into the skulls of adolescent Triceratops – as well as the faces of other T. rexes.

"The adult T. rex was such a freak of evolution, or a feat of evolution," says Brusatte. "Whatever you want to call it, it's really both. But understanding how that adult grew, how it developed, what it had to go through to get to that stage of being seven tonnes, 13 metres long… That's a fascinating biological, anatomical question. And we're really getting at it. That's really amazing. That's what we should be revelling in."

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