Where The Tyrannosaurids Roam

Tyrannosaurus rex is arguably the most famous dinosaur to have walked the Earth. It is important, however, to remember that it is just one member of a much larger group of dinosaurs known as the tyrannosaurids. All species within this collection of reptiles were carnivorous, but many led completely separate lifestyles and had their own unique anatomical and morphological quirks. What is more there is a huge difference between northern North American and southern North American tyrannosaurids.

This resulted in a puzzle which vexed the minds of palaeontologists for decades: where did the tyrannosaurids come from and why is there such a difference between certain groups? Yet earlier this year the bones of a strange creature, which has since been named Lythronax argestes, were uncovered by palaeontologists working at the Grand Staircase-Escalante National Monument in desert Utah.

The cranial elements of the 85 million year old
tyrannosaurid Lythronax argestes

In a rare display, alongside bones from the legs, elements of the skull were present which allowed a research team, led by Dr Mark Loewen from the Natural History Museum of Utah, to identify the remains as belonging to a new species of dinosaur.

At around 8 metres in length it would have been a powerful predator with curved, serrated teeth and front facing eyes; both hallmarks of a hunter. It inhabited the shores of the island of Laramidia roughly where west USA is today and as a result would have been spoiled for choice in terms of prey, living alongside hundreds of gregarious herbivorous dinosaurs - an ideal environment for a such predator.

Its importance, however, lies in its age. Lythronax is around 85 million years old. What is interesting is that this makes it just 10 million years older than its famous counterpart, Tyrannosaurus rex, showing that the so called wide-skulled tyrannosaurids, including T. rex, are older than previously thought. The physiology also demonstrates something profound: a split between the tyrannosaurids in the north and the south of Laramidia. This then begs the question as to what was responsible for this?

A map showing the geographical position of
Laramidia during the Cretaceous

Using geological evidence, evolutionary relationships and the geographical distribution of the Laramidian tyrannosaurids, the research team found that sea levels rose rapidly in just a few million years, separating the north of the island from the south, allowing the occupants of each half to diversify and grow away from the other.

This hypothesis explains why North American dinosaurs are so different from their southern counterparts. It also helps explain both the size and widespread distribution of various tyrannosaurid species across Laramidia.

'Lythronax is a wonderful example of just how much more we have to learn about the world of dinosaurs,' said Dr Phillip Currie, a co-author of the paper published on the new species. 'Many more exciting fossils await discovery in Grand Staircase-Escalante National Monument.'

The study of the geographical impact on the evolution and diversity of a species is biogeography. This subject is often overlooked with respect to both palaeontology and evolutionary theory, but its study is vital in interpreting the past and the origins of life. We must examine the full relationship between geography and the tyrannosaurids if we are to find a comprehensive answer as to where they originally came from.