“Juvenile T. rex” might be a separate species after all

Fossils believed to be juvenile Tyrannosaurus rex actually belong to a different species, first named more than 80 years ago – Nanotyrannus lancensis.

In new research published today in Fossil Studies, a re-analysis of the fossils and comparison with a previously unrecognised young T. rex fossil appears to have tipped the scale toward N. lancensis being its own species.

The first Nanotyrannus skull was found in Montana in 1942. Since then, palaeontologists have debated whether the animals represented a new species, or a growth stage of the much larger T. rex.

Growth rings inside the fossilised Nanotyrannus bones became more closely packed toward their surface, which shows the animal’s growth was slowing. This suggests it was near full size and not a fast-growing juvenile.

Modelling the growth shows Nanotyrannus would have grown to about 5m long and weighed 900-1,500kg. This is about one-sixth the size of T. rex which has been estimated to grow to 12m and more than 7 tonnes.

“When I saw these results I was pretty blown away,” says Dr Nick Longrich from the University of Bath, UK. “I didn’t expect it to be quite so conclusive. If they were young T. rex they should be growing like crazy, putting on hundreds of kilograms a year, but we’re not seeing that.”

“We tried modelling the data in a lot of different ways and we kept getting low growth rates. This is looking like the end for the hypothesis that these animals are young T. rex.”

Further suggesting that Nanotyrannus is not a juvenile T. rex, the researchers found no anatomical links in the features of the two species. Nanotyrannus was more slender, agile and had longer arms.

“If you look at juveniles of other tyrannosaurs, they show many of the distinctive features of the adults,” says Longrich.

“A very young Tarbosaurus – a close relative of T. rex – shows distinctive features of the adults. In the same way that kittens look like cats and puppies look like dogs, the juveniles of different tyrannosaurs are distinctive.

“Nanotyrannus just doesn’t look anything like a T. rex. It could be growing in a way that’s completely unlike any other tyrannosaur, or any other dinosaur, but it’s more likely it’s just not a T. rex.”

The idea that some dinosaur species might actually be the young of other species gained traction after a study by palaeontologists Jack Horner and Mark Goodwin. This led them to claim in 2009 that up to one-third of dinosaurs might actually be the young of other known species.

Their analysis of bone-headed dinosaurs suggested that Dracorex hogwartsia and Stygimoloch spinifer might be juvenile forms of Pachycephalosaurus wyomingensis. Though it is now believed Stygimoloch lived in a different period, the jury is still out on Dracorex.

Less convincingly, they had also suggested that Triceratops might be the juvenile of the larger three-horned dinosaur Torosaurus.

It is difficult to say from the scattered remains of animals which have been dead for tens of millions of years. But it seems for now that Nanotyrannus has cemented its place as a distinct species.

But if Nanotyrannus is not a juvenile T. rex, then it raises a question – where are all the young Tyrannousaurus rex?

“That’s always been one of the big questions,” says Longrich. “It turns out we actually had found one, but the fossil was collected years ago, stuck in a box of unidentified bones in a museum drawer, and then forgotten.”