Study reveals ‘juvenile T. rex’ fossils are a distinct species
New research has the potential to propel a decades-old debate forward by indicating that 'juvenile Tyrannosaurus rex (T. rex)' fossils actually represent a distinct species of small Tyrannosaur. This discovery marks a significant step in our understanding of these iconic creatures.
A study, published in the open access journal Fossil Studies, by researchers from the University of Bath and the University of Chicago unveils a new analysis of fossils challenging the long-standing belief that certain specimens, once considered juvenile T. rex are, in fact, adults of a distinct species of small Tyrannosaur. Identified as Nanotyrannus lancensis, the species, distinguished by narrower jaws, longer legs, and larger arms compared to T. rex, was initially named decades ago. However, the fossil was later reinterpreted as a young T. rex.
Discovered in Montana in 1942, the initial finding of a Nanotyrannus skull marked a significant milestone. However, over the following decades, palaeontologists have engaged in a continual debate, wavering between classifying it as a distinct species and considering it merely a juvenile counterpart to the much larger T. rex.
In this study, researchers re-analysed the fossils, focusing on growth rings, Nanotyrannus anatomy, and a previously unrecognised young T. rex fossil. Measurement of Nanotyrannus bone growth rings revealed a pattern of increased density toward the bone's outer layers, indicating slowed growth. This suggests the species were nearly full size, challenging the notion of fast-growing juveniles.
“When I saw these results I was pretty blown away,” says Dr Nick Longrich, author on the paper. “I didn’t expect it to be quite so conclusive.”
Modelling the growth of the fossils showed the animals would have reached a maximum of around 900-1500 kilograms and five metres - about 15 percent of the size of the giant T. rex, which grew to 8,000 kilograms and nine metres or more.
Longrich adds, “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,” says Dr Longrich.
Further supporting the distinct species theory, the researchers discovered no fossils exhibiting combined features of Nanotyrannus and T. rex, crucial for any transition. Each examined fossil was clearly identified as either one species or the other.
Distinguished by longer arms and claws than a T. rex, Nanotyrannus are revealed as possessing formidable weapons. The researchers suggest the species likely relied on its small, fast, and agile build, shedding light on its unique characteristics.
“T. rex relied on size and strength, but this animal relied on speed,” Dr Longrich explains.
Dr. Longrich notes that the long arms and distinctive features of the species indicate it was only a distant relation to T. rex. The findings lead the researchers to propose the possibility that Nanotyrannus occupied a distinct family of predatory dinosaurs, separate from Tyrannosauridae, to which T. rex belongs.
The researchers suggest that, given how difficult it is to tell dinosaurs apart based on their often-incomplete skeletons, we may be underestimating the diversity of dinosaurs, and other fossil species.
“It’s amazing to think how much we still don’t know about the most famous of all the dinosaurs.”
If you're interested in learning more about this research, you can access the paper published in Fossil Studies here: https://shorturl.at/hkmA9