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 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. rexthat 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”.

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