Controversial paper suggests there are three Tyrannosaurus species

Palaeontologists have reacted with scepticism over suggestions that one of the world's most famous dinosaurs is actually three separate species.

A study published in the journal Evolutionary Biology has proposed splitting Tyrannosaurus rex into three different species - a redefined T. rex, T. regina and T. imperator - based on differences in their leg bones and teeth.

Its authors suggest that T. rex and T. regina evolved out of T. imperator, with the two former species potentially living side by side in the Late Cretaceous. If the theory were to gain widespread acceptance, it would require specimens around the world to be reinvestigated. 

The study is led by Gregory S. Paul, a paleoartist and independent scientist. He says, 'We found that the changes in Tyrannosaurus femurs are likely not related to the sex or age of the specimen. 

'We propose that the changes in the femur may have evolved over time from a common ancestor who displayed more robust femurs to become more gracile in later species. The differences in femur robustness across layers of sediment may be considered distinct enough that the specimens could potentially be considered separate species.'

However, Museum dinosaur expert Prof Paul Barrett, who was not involved in the study, has poured cold water on the paper's claims.

'I'm very sceptical of these results, and would be very surprised if the experts who work on these dinosaurs would support them,' says Paul. 

'I don't think the authors of this study have enough evidence to say definitively these are three species, and I suspect the different animals they have described would all have been able to mate with each other; the proposed differences between them being subtle and potentially due to other reasons.' 

How hard is it to describe a dinosaur species?

Taxonomy, or the study of relationships between organisms, is an area of science that is always changing. New information, new species and new hypotheses mean that family trees are constantly being updated to reflect the most recent ideas on how organisms are linked.

However, what constitutes a species can sometimes be difficult to determine, especially for fossil remains.

'When assigning a dinosaur to a species, scientists want that species to be well defined so the animals can be clearly distinguished,' Paul explains. 'This definition should be easy for other people to come along and recognise the features you have described. There also needs to be enough material for others to assess those features in future, and you need fossils that are not too deformed or broken as to be unrecognisable. 

'For extinct animals, as with living ones, there are always issues with variation. For instance, male and female dinosaurs may differ from each other in subtle and not so subtle ways, while there is also ontogenetic variation, which is the variation an organism goes through as it grows. A human baby looks very different from a 90-year-old, for example. 

There is also individual variation, which is the variation in any population that sees individuals differ from each other but still allows them to breed. People, for example, are different heights but are all still human.

'In most cases, we can't tell the sex, age or natural variability of a dinosaur species,' says Paul. 'As a result, we're always striving for clear definitions. In the past, the rules were much laxer, and that's part of the problem we face today.'

Tyrannosaurus has previously seen changes to its taxonomy. In the same paper in which T. rex was first described, Henry Fairfield Osborn described another skeleton as Dynamosaurus imperiosus. However, just a year later he realised that the specimen was actually another T. rex.

Since then, T. rex has remained the only species in its genus. The new paper seeks to change that. 

What are the suggested Tyrannosaurus species?

The study looked at 38 specimens of T. rex in collections around the world, including the specimen housed at the Museum. The researchers sought to explain the variation observed within fossils assigned to the species, focusing on features such as the 'stoutness' of the femur bone and the presence of incisiform teeth, which are substantially smaller than others in the jaw.

The measurements taken from different parts of the skeleton were compared to those of close relatives, such as Albertosaurus. Comparisons of the femur dimensions suggested that Tyrannosaurus was distinct from these relatives, but that there was more variation than expected within its own genus.

The researchers claim that the best way to explain this variation is to split the genus into three new species. Using knowledge of the rock layers some of the specimens were found in, they suggest that the thicker femur bones buried deeper gradually give way to a more diverse set of femurs, some of which are significantly thinner. 

In their suggested transition, this means T. imperator gave rise to T. rex and T. regina. During the process of evolution, both daughter species lose an incisiform tooth, but while T. rex maintains robust femurs, T. regina's got more slender.

The authors admit that their results are not an 'ideal proof' of multiple Tyrannosaurus species, but claim that their hypothesis is borne out more by the evidence given by other suggestions, such as natural variation. 

Many other palaeontologists, however, disagree.

'I don't think these differences will be enough to convince T. rex experts that these dinosaurs are anything other than one species,' Paul says. 'The differences in tooth anatomy are relatively minor, and while the differences in proportions are less subtle, they have been known for some time among those who work closely on T. rex.

Moreover, we have no idea how much variation to expect, especially in a species which was evolving over two million years. It may be that this is one species which is evolving through time, or it could be that this is an animal which has slightly different variations when living in different areas, but not enough to make it a new species. This is a phenomenon known as ecotypic variation and is quite common in living animals.'

One of those T. rex experts is Dr David Hone, a researcher at Queen Mary, University of London who specialises in carnivorous dinosaurs. He says, 'When describing a dinosaur species, you work with the characteristics that you have. 

'For T. rex, we have multiple complete skeletons in good condition with about as much anatomical information as you could reasonably ask for as a taxonomist, so to base these new species on just a couple of traits, both of which have already been suggested to vary within populations, is probably not a strong basis for naming new species.'

If the new species were ever accepted, it could require displays, books and exhibitions around the world to be updated. This wouldn't be the case at the Museum, however, as the revised species descriptions found the specimen held in the collections is still a T. rex.

For now, many researchers are likely to continue to call all specimens T. rex until more evidence suggests otherwise.