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New analysis of fossils has revealed the truth about T. rex – those we thought were young T. rex are, in reality, adults of a small tyrannosaur species named Nanotyrannus lancensis. This species has narrower jaws, longer legs, and bigger arms than T. rex. It was initially thought that the fossils were juveniles of T. rex, but that has now been proven false.
The first skull of Nanotyrannus was discovered in Montana in 1942. For years, scientists debated whether it was a unique species or simply a juvenile of the much larger T. rex.
Dr. Nick Longrich, from the Milner Centre for Evolution at the University of Bath, and Dr. Evan Saitta, from the University of Chicago, re-evaluated the fossils. They looked at growth rings, the anatomy of Nanotyrannus, and a previously unrecognized fossil of a young T. rex.
By examining the growth rings in Nanotyrannus bones, the scientists determined that the species was nearly full size, suggesting it was not rapidly growing like a juvenile.
The fossils were modeled to show that the species would have reached a maximum of around 900–1,500 kilograms and five meters, which is about 15% of the size of a full-sized T. rex.
The researchers have published their findings in Fossil Studies.
Dr. Longrich said, “When I saw these results I was pretty blown away. 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 modeling 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.”
Supporting the existence of distinct species, the researchers found no evidence of fossils combining features of both the Nanotyrannus and T. rex. Every fossil they examined could be confidently identified as one species or the other.
Neither did the patterns of growth in other tyrannosaurs fit with the hypothesis that these were young T. rex.
Dr. Longrich said, “If you look at juveniles of other tyrannosaurs, they show many of the distinctive features of the adults. 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. 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.”
But that raises a mystery—if Nanotyrannus isn’t a juvenile Tyrannosaurus, then why hasn’t anyone ever found a young T. Read More
