The fossil of a dinosaur thought to be a ‘toddler T rex’ is actually an entirely different species, according to a new study which could cost one fossil owner millions of pounds.
Debate has raged for decades over the mystery fossil, found in Montana in 1942.
At first paleontologists classified it as a new dinosaur, Nanotyrannus lancensis, but it was later reinterpreted as a young Tyrannosaurus rex.
However, its long arms and small size helped prove it could not have grown into the most feared creature ever to walk the Earth.
Dr Nick Longrich, from the Milner Centre for Evolution at the University of Bath, and Dr Evan Saitta, from the University of Chicago, analysed the fossils again, and also found a giveaway in the bones’ growth rings – similar to those of a tree.
They found the rings became more closely packed towards the outside of the bone, showing its growth as slowing. That suggests the animal was almost full sized, and probably only grew to be about five metres tall and between 900 and 1,500kg.
In comparison, T rex towered nine metres or more above the ground, and weighed up to 8,000kg – eight tonnes.
‘When I saw these results I was pretty blown away,’ said Dr Longrich. ‘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.’
In addition, the team has unearthed no evidence of any fossils combining features found on both the Nanotyrannus and T rex fossils, which would be found if one turned into the other.
‘If you look at juveniles of other tyrannosaurs, they show many of the distinctive features of the adults,’ said Dr 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. And Nanotyrannus just doesn’t look anything like a T rex.’
Nanotyrannus also had different arms to the T rex, famed for its tiny upper limbs.
‘The arms are actually longer than those of T rex,’ said Dr Lonrich. ‘Even the biggest T rex has shorter arms and smaller claws than in these little Nanotyrannus. This was an animal where the arms were actually pretty formidable weapons. It’s really just a completely different animal – small, fast, agile.
‘T rex relied on size and strength, but this animal relied on speed.’
Its features suggest Nanotyrannus may only be distantly related to T rex, and could even form part of a distinct family of predatory dinosaurs.
If the study finally prompts academic agreement on whether Nanotyrannus is a separate species, it will be awkward timing for a London auction house currently offering a ‘juvenile T rex skeleton’ for sale.
Nicknamed ‘Chomper’, the skeleton has previously found itself at the centre of the identity debate. Whether it will still fetch the same estimated £16 million when it goes under the hammer as a Nanotyrannus remains to be seen.
And in determining that the fossil is in fact a separate species, the authors have prompted another conundrum – where are all the baby T Rexes?
‘That’s always been one of the big questions,’ said Dr Longrich.
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‘Well, 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.
‘Yes, it’s just one specimen, and just one bone, but it only takes one.
‘T rex skull bones are very distinctive, nothing else looks like it. Young T rex exist, they’re just incredibly rare, like juveniles of most dinosaurs.’
The study is published in the journal Fossil Studies.
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