[Rockhounds] New research shows “Juvenile T. rex” fossils are a distinct species of small tyrannosaur

[Kreigh Tomaszewski]

A new analysis of fossils believed to be juveniles of *T. rex* now shows
they were adults of a small tyrannosaur, with narrower jaws, longer legs,
and bigger arms than *T. rex*. The species, *Nanotyrannus lancensis*, was
first named decades ago but later reinterpreted as a young *T. rex*.

The first skull of *Nanotyrannus* was found in Montana in 1942, but for
decades, paleontologists have gone back and forth on whether it was a
separate species, or simply a juvenile of the much larger *T. rex*.

Dr Nick Longrich, from the Milner Centre for Evolution
<https://www.bath.ac.uk/research-centres/milner-centre-for-evolution/> in
the Department of Life Sciences at the University of Bath (UK), and Dr Evan
Saitta, from the University of Chicago (USA) re-analysed the fossils,
looking at growth rings, the anatomy of *Nanotyrannus*, and a previously
unrecognized fossil of a young *T. rex*.

Measuring the growth rings in *Nanotyrannus* bones, they showed that they
became more closely packed towards the outside of the bone - its growth was
slowing. It suggests these animals were nearly full size; not fast-growing
juveniles.

Modelling the growth of the fossils showed the animals would have reached a
maximum of around 900-1500 kilograms and five metres - about 15 per cent of
the size of the giant T. rex, which grew to 8,000 kilograms and nine metres
or more.

The researchers have published their findings in *Fossil Studies*
<https://www.mdpi.com/2813-6284/2/1/1>.
https://www.bath.ac.uk/announcements/new-research-shows-juvenile-t-rex-fossils-are-a-distinct-species-of-small-tyrannosaur/