PROFESSOR THOMAS CARR - Tyrannosaurus rex and its immediate relatives, the tyrannosauroids, are instantly recognisable with their gigantic bodies, large heads, huge teeth, short arms, and massive legs and tails.

For a long time the question of how those features developed during an individual’s growth was unknown because juvenile specimens were not closely studied or they were misidentified as pygmy adults of unique species.

In fact, the differences between juvenile tyrannosauroids and their adult counterparts are so extreme that the growth changes in these dinosaurs are as remarkable as the evolutionary transformation that turned nonflying theropods into flying birds over millions of years.

It turns out that all of the best-known large tyrannosauroids (Bistahieversor, Albertosaurus, Gorgosaurus*, Daspletosaurus, Tarbosaurus*, and Tyrannosaurus) share the same basic growth pattern – a pattern so similar that it is difficult to tell the juveniles of the different genera and species apart!

It is only later in growth that the differences between species grow in, making them easier to identify. But, as seen in the photo of the juvenile and adult T. rex skulls (below, at the Museum of the Rockies), the differences between juvenile and adult can be seen quite plainly.

Obviously, the juvenile is small – in this case the juvenile skull is only 57 cm long – but it has a suite of features aside from size that make it look very different from the adult.

Notice the eye socket is large and round and the opening in front of the eye socket is long and low, whereas in the adult the eye socket has the form of a tall keyhole and the hole in front of that is round. In the live animal, a large air sac was lodged in the hole.

The snout in the juvenile is shallow, especially the region below the hole that is in front of the eye; this reflects the size of the teeth, which are numerous, short, and thin. The upper jaw of the adult is massive and deep, which is consistent with the fewer (at least in T. rex), but longer and more massive teeth.

Also, the skull of the juvenile is sleek, whereas the facial bones of the adult – especially around the eye socket – are massive and inflated like bony balloons.

As tyrannosauroids matured, their facial bones were inflated internally by air sacs. Also, the top of the snout in the juvenile skull is smooth, whereas that of the adult is coarse.

Finally, the adult has a large horn above and behind its eye socket, whereas that part of the juvenile is smooth and does not have a horn.

These extremes in appearance result from the combined influence of eating larger prey (deep jaws, massive teeth) and sexual maturity (rough snout, puffy face, horns).

The differences in the jaws and teeth indicate changes in eating habits that tracked the size of the animals, as seen in living crocodiles and monitor lizards. Small tyrannosauroids would have eaten small animals that would have been easy to take, whereas adults required greater strength to subdue large prey.

The plain face of juveniles contains less information for a fellow tyrannosauroid to receive than the ornate face of the adult.

At a glance, any tyrannosauroid could instantly tell the difference between juveniles and adults. In addition to that, the various ornaments on the skull are different in shape between species.

This sort of change can be seen in living birds such as chickens and hornbills, where the comb or crest, respectively, is small in juveniles but large and ornate in adults.

Therefore, the same information that tyrannosauroids used to tell each other apart can be used by palaeontologists – it is the closest humans can get to seeing through the dinosaurs’ eyes!

- Dr Thomas Carr is an Associate Professor of Biology at Carthage College and the Director of the Carthage Institute of Palaeontology. He works on the growth and evolution of tyrannosauroids and he does his fieldwork in the Hell Creek Formation of southeastern Montana.

Illustration: Zubin Erik Dutta

*I use these names in protest; given the choice, I would use Albertosaurus libratus and Tyrannosaurus bataar instead of Gorgosaurus and Tarbosaurus, respectively. I think that names should reflect the evolutionary relationships of species; for instance, A. libratus is the closest relative of A. sarcophagus, and T. bataar is the closest relative of T. rex. I see no reason to use different generic names because that removes useful evolutionary information and it implies a greater difference between the species than there really is.