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Palaeontology: Was the juvenile T. rex a separate species after all?
Size plays an important role in palaeontology. When researchers discover fossil bones that resemble others but are smaller, it raises the question of whether it is a separate species or possibly a juvenile. There is even the possibility that it is an unusual variation such as short stature, similar to the consideration once considered for the extinct human species Homo floresiensis in Indonesia. With regard to dinosaurs, this leads to heated debates, especially around the best-known and best-researched representative, Tyrannosaurus rex. Two years ago, a hypothesis was published stating that the specimens found so far could be categorised into more than one species. Palaeontologist Gregory Paul and his team therefore proposed Tyrannosaurus imperator and Tyrannosaurus regina as two further species designations, a royal trio so to speak. However, criticism of this idea was not long in coming.
Have the researchers been wrong all these years?
Another controversial point, which is strongly related to size, concerns the so-called Nanotyrannus. Some smaller fossils that have been attributed to T. rex are the subject of debate as to whether they are not yet fully grown specimens or whether they are a separate species or genus. Four years ago there was a published study that argued that Nanotyrannus did not exist and instead represented young T. rex.
Now there is a new twist. Researchers Nicholas Longrich from the University of Bath in the UK and Evan Saitta from the University of Chicago in the US wrote about Nanotyrannus lancensis, a distant relative of T. rex, in the journal Fossil Studies. It was smaller and – in contrast to the iconic tiny arms of the famous dinosaur – had slightly longer arms.
Their re-analysis of the controversial fossils, one of which has been known since 1942, should finally settle the debate in favour of Nanotyrannus: “This seems to be the end of the hypothesis that these animals are young T. rex,” Longrich said in a statement from the University of Bath. He himself was once a sceptic of Nanotyrannus, “until I took a closer look at the fossils about six years ago and was surprised to find that we had been wrong all these years.”
A convincing argument in their analysis are the growth rings in the bones, similar to the growth rings of trees. These rings develop due to life phases and nutritional conditions. One interesting study even reconstructed the lifelong travelling route of a mammoth based on its tusks. The dinosaur bones showed denser growth rings towards the outside, indicating that their growth slowed down.
According to palaeontologists, this indicates that they were at the end of their growth phase. Young specimens, on the other hand, should be in phases of rapid growth and gain hundreds of kilograms every year. The results of this analysis surprised Longrich: “When I saw these results, I was quite overwhelmed.”
The model calculations showed that the Nanotyrannus weighed a maximum of 900 to 1,500 kilograms and grew no larger than five metres. This is in contrast to the giant T. rex representatives, which can weigh up to eight tonnes and grow to over nine metres. The authors estimate that the Nanotyrannus was therefore not even a fifth of the size of a T. rex.
Look like “kittens and cats”
Longrich also argues that juveniles of related species resemble adult animals in many typical characteristics. This applies to the East Asian Tarbosaurus, which some experts also classify as a tyrannosaur. Longrich uses an illustrative comparison: the young animals of different tyrannosaurs have a characteristic appearance, “just as kittens resemble cats and puppies resemble dogs.”
However, Nanotyrannus looks nothing like a T. rex. The animal was not only much smaller, but also more agile. It also had slightly longer arms. These could also function as weapons. “T. rex was big and strong, but this animal focussed on speed.”
During their research, the team came across a specimen of a young Tyrannosaurus that had ended up in a museum box together with other unidentified bones and had been forgotten. Nevertheless, according to the authors, there are no known fossils of young T. rex specimens. This particular specimen had a skull bone that resembled that of an adult animal, distinguishing it from Nanotyrannus. “Young T. rex do exist, but they are very rarely found, just like the young of many dinosaurs.”
Not all experts share this view. Thomas Carr from Carthage College in Wisconsin criticised the study and had already criticised the model of T. rex, T. regina and T. imperator. He himself discovered more than ten times as many differences between juvenile and adult tyrannosaurs as the new research. However, this is to be expected: “The authors do not seem to be sufficiently familiar with the growth variations in tyrannosaurs.”
David Hone from Queen Mary University in London notes that although the new study revisits some old ideas, they have not gained any persuasive power – and that some of the new ideas are mostly unconvincing. Nevertheless, he can imagine that there are other species within the Tyrannosaurus genus, although currently only the T. rex is considered a valid species. It is remarkable that the Tyrannosaurus was apparently a uniquely large predator in its ecosystem.
Ultimately, the debate will probably only be settled when evidence for young and adult specimens of T. rex and Nanotyrannus is found – and this is recognised by the majority of experts. Although this evidence has yet to be found, the statements by Longrich and Saitta are food for thought. They emphasise that the mostly incomplete skeletons of dinosaurs make it difficult to distinguish between them.
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