Nanotyrannus, the Tyrannosaur that never was?

Nanotyrannus was described in 1988 by Robbert Bakker as a species of pygmy Tyrannosaur that lived alongside T. rex in what is now Montana, but it's status as a valid species is currently disputed.

Nanotyrannus life reconstruction (image Conty/Wikipedia)

The skull that formed the basis of Bakker's Nanotyrannus has certainly had an interesting second life as a fossil. When first described in 1946 it was named as a new species of Gorgosaurus, given the specific name lancesis after the Lance Formation in Montana where it was found. Subsequently it was referred to the slightly later Tyrannosaur genus Albertosaurus.

But it's moment glory would come in 1988 when it was given it's own genus by Robbert Bakker who named it Nanotyrannus because of it's small size compared to other Cretaceous Tyrannosaurs. The total length of N. lancensis was estimated to by about 5 meters in total length, less than half the size of a mature T. rex.

The status of Nanotyrannus did not go undisputed however, and the discovery of a second specimen in 2001 seemed to bring an end to the status of Nanotyrannus as separate species.

A mature Nanotyrannus

Cast of the holotype (Wikipedia)

The status of Nanotyrannus as first a separate species within Gorgosaurus and Albertosaurus, and it´s ultimate elevation to it´s own genus, all relied on one assumption; that the skull belonged to a mature individual.

If this was indeed a mature individual, it was indeed quite different from T. rex (and Albertosaurus for that matter). Not only was it much smaller, it also had a much more gracile form and narrower snout. The skull overall was much less robust than that of a mature T. rex, with more and smaller teeth. This would mean that it did not have the same bone-crunching bite and probably would have had a different feeding strategy.

Or a juvenile Tyrannosaurus?

The maturity of the specimen was established by the fusion of bones in the skull, which is generally a sign of maturity. In Tyrannosaurs however the nasal bones fuse early in development, so it is of itself not a good indicator of maturity. Analysis of the bone development also showed that the bone was still growing, suggesting that this was a juvenile animal still undergoing significant growth.

Perhaps most damning was a paper by Carr in 1999 in which he established that the differences between the skull of Nanotyrannus and that of a mature Tyrannosaurus were best explained by ontogeny (the changes across an animal's lifespan).

"Jane" a new Nanotyrannus/juvenile Tyrannosaurus (image FunkMonk/Wikipedia)

More specimens, more trouble

In 2001 a second specimen was found, nicknamed Jane it was far more complete than the skull that formed the basis of the genus Nanotyrannus. But rather than further cementing the status of Nanotyrannus as a separate species it led to overwhelming support for the theory that "Nanotyrannus" was really a juvenile T. rex, something most palaeontologists now agree with.

At 6 1/5 metres Jane is actually significantly bigger than the estimated length for Nanotyrannus. Greg Erickson determined her* age to be 11 years, suggesting that the original specimen was younger and thus immature. But other evidence also points to Jane being an immature individual; the arms, legs and feet are disproportionally large compared to the body. Something expected in an immature individual of a larger species, but not in a mature individual of a smaller species. 

The end of Nanotyrannus?

Not all palaeontologists are convinced that "Nanotyrannus" is really a juvenile Tyrannosaurus, most notably Robert Bakker and Peter Larson hold on to the term. And they have recently unveiled a fossil find they named the dueling dinos which shows a Nanotyrannus and an unidentified ceratopsian Dinosaur  locked together in a fight, a fight that possibly killed them both.

It should be noted that since this fossil is on sale there may be an ulterior motive for hanging on to the name for now (their site actually gives "May solve controversy regarding Nanotyrannus vs juvenile T.rex" as a unique selling point).

* The actual gender of "Jane" is unknown, and probably will never be known.