A new specimen of one of the most controversial species of dinosaur has the potential to overturn decades of research on the T rex.
Author
- Abi Crane
Postgraduate Researcher in Palaeontology, University of Southampton
Nanotyrannus, the "miniature T rex", has been the centre of one of the fiercest debates in palaeontology . Scientists have long argued over whether the Nanotyrannus is a separate species or just a young T rex .
The controversy was ignited in 1999 when the only known fossil of a Nanotyrannus was found to belong to a juvenile . More complete fossils have since failed to produce any conclusive answers because they were all also found to be juvenile.
But the debate surrounding the identity of Nanotyrannus may finally be settled. A new fossil specimen , described in the journal Nature, is the smoking gun researchers have been looking for: an adult Nanotyrannus.
Known as the duelling dinosaurs, this fossil preserves an almost-complete Nanotyrannus and Triceratops entombed together. They seem frozen in combat (whether they were actually fighting when they became buried in the Earth's sediment remains to be tested).
Although the fossil was discovered in Montana, US back in 2006, it was under private ownership until the North Carolina Museum of Natural Sciences purchased it in 2020 . Now accessible to scientists, the true nature of this remarkable fossil can be revealed for the first time.
The researchers have confirmed that Nanotyrannus is a separate miniature type of tyrannosaur by demonstrating this specimen belonged to a near fully-grown adult. The age and maturity of dinosaurs can be assessed by looking at the inside of their bones.
Dinosaurs grew in cycles of faster and slower growth which produced distinct layers of bone. When cut open and examined under a microscope, these marks can be counted like rings in a tree.
Using this method, the researchers could determine that the Nanotyrannus in the duelling dinosaurs was at least 14 years old when it died. The researchers also found its rate of growth had slowed significantly in its final years, indicating that this individual was nearly at full body size.
So just how small was this miniature T rex? Nanotyrannus is only around one tenth of the size of a fully grown T rex. Being one of the largest predators to ever walk the Earth, however, T rex would make most animals look small. The duelling dinosaurs Nanotyrannus is over four metres long and estimated to have weighed over 700kg - that's as heavy as some of the very largest polar bears .
Other specimens of Nanotyrannus are even bigger. The almost complete skeleton known as Jane , discovered in 2001 also in Montana, is estimated at over a ton, larger than any land predator alive today.
The researchers have found enough differences in the shape of bones in the skulls of the duelling dinosaurs fossil and the larger Jane to separate them into two different species: Nanotyrannus lancensis and the newly-named Nanotyrannus lethaeus.
Other than small size, another feature that the researchers have used to distinguish Nanotyrannus from T rex is the number of teeth. Despite its much smaller mouth, Nanotyrannus could no doubt pack a powerful bite with its over 60 teeth. T rex had 40-50 teeth in its jaws.
The teeth themselves are also different. Nicknamed " lethal bananas ", the teeth of T rex are curved and serrated like steak knives . These unique teeth are perfect for slicing into flesh and could crush bone . By contrast, some of the teeth of Nanotyrannus are straight, chisel-like and without serrations, more closely resembling those of other types of carnivorous dinosaur.
T rex had famously tiny arms, the source of many jokes and dinosaur impressions. Nanotyrannus does not share this feature. Its arms are almost the same size as those of T rex, despite belonging to an animal ten times smaller.
Nanotyrannus is overall more lanky than the stout T rex, with proportionally longer legs to match its longer arms. The foot bones of the small duelling dinosaurs Nanotyrannus are nearly as long as those of the largest known T rex. Research suggests that, while T rex couldn't manage anything much faster than a walk , Nanotyrannus was well adapted to run and chase after its prey.
Aside from confirming the existence of this long-controversial dinosaur species, this discovery overturns decades of T rex research. Much of what scientists think about the life and growth of T rex is based on the assumption that many gangly "teen" tyrannosaurs fossils would have grown up into the well-known bulky giants. In light of this discovery, scientists must reevaluate what life was like for a young T rex and go back on the hunt for genuine juvenile T rex fossils.
It has been widely assumed that T rex was the only dinosaur of its type living in the US in the last days of the dinosaurs. Some researchers have proposed that T rex was so dominant that other large predators could not live alongside it, with its own juveniles outcompeting any other medium-sized predators. This total dominance is now called into question.
T rex has now been joined by not one but two species of smaller carnivorous dinosaur, hinting at a more complex and diverse ecosystem thriving in the US towards the end of the reign of the dinosaurs.
Despite being arguably the most well-known and best studied dinosaur, T rex and its close relatives continue to surprise us from beyond the grave. There is still much left to learn about the largest and fiercest predators ever to walk the Earth.
Abi Crane does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
