Tyrannosaurid dinosaurs are iconic, giant predators from around 85-65 million years ago. They lived in the Late Cretaceous of North America and Asia and were wiped out be the mass extinction that marked the end of the ‘Age of Dinosaurs’. After the discovery of Tyrannosaurus rex and close relatives in the early 20th century, for a long time, this is all we knew.
In the past 5–10 year, palaeontologists have been filling the gaps in the long early history of tyrannosaurs. They are now known as early as 170 million years ago (Middle Jurassic). Early tyrannosauroids were somewhat different to the Late Cretaceous giants. They are almost all the same size, or smaller than a man, and they generally have light skulls and long arms compared to T. rex. We know from Dilong (Early Cretaceous, China) that some at least had filamentous integument called ‘protofeathers’. Guanlong and Proceratosaurus from the Jurassic of England and China tell us that some had striking bony head crests. Raptorex, a Chinese tyrannosauroid reported last year in Science, shows us that the robust skulls and short arms of T. rex appeared in the Lower Cretaceous among fairly small (~80 kg) animals.
But a piece of the puzzle has been missing. All tyrannosauroids thus far have been discovered in the Northern Hemisphere. This is odd, as most long-lived dinosaur clades (tyrannosaurs spanned at least 100 million years) made it across the globe. Early in dinosaur history the continents were assembled into a single supercontinent, so this was easily possible. But perhaps tyrannosauroids never made it.
Today, myself and colleagues reported a new specimen from 110 million year old rocks at Dinosaur Cove Australia, representing the first evidence that tyrannosauroids made it into the Southern hemisphere. The animal is estimated at 3 metres long and a weight of 80 kg, similar to a man based on comparison with the similarly-sized tyrannosaur Raptorex. The fossil is a single pubic bone (from the hips), recovered in 1989 during an expedition funded by National Geographic and the Australian Research Council, and led by Tom Rich at the Museum Victoria, Melbourne. Tom and his team blasted into the rocks at Dinosaur Cove using heavy equipment provided gratis by Atlas Copco, and explosives. They effectively created a ‘fossil mine’, from which hundreds of isolated bones were recovered. Most of these were difficult to identify definitively, but they have provided evidence of a range of dinosaur groups, including megaraptoran allosauroids, oviraptorosaurs, hypsilophodontids, and now tyrannosauroids.
With the tyrannosauroid pubis we got lucky. Tyrannosauroids have distinctive pubes showing coelurosaurian features such as a transversely narrow distal expansion (called the pubic boot). In tyrannosaurs the boot is especially large and, in tyrannosaurids there is a flange-like pubic tubercle. The identification isn’t just based on these features though. In fact, in almost every respect, our specimen is identical to the pubis of a derived tyrannosaurid like Albertosaurus. To show this as clearly as possible included a picture the two next to each other in our paper (thanks to Lara Shychoski for providing a photo of a tyrannosaurid pubis).
We believe that our specimen show that, 110 million years ago, in the middle of its history, the tyrannosaur lineage was globally-distributed. This raises a question. Why then did tyrannosaurs grow to giant size late in dinosaur history in the north, but fade into obscurity in the south, where the apex predator mantle was taken up by other dinosaur groups? Only future discoveries can reveal the details of the radiation of southern tyrannosaurs.
Benson, R. B. J., Barrett, P. M., Rich, T. H. & Vickers-Rich, P. 2010. A southern tyrant reptile. Science 327, 1613.