Link of tyrannosaur evolution discovered

92 million-year-old T. rex relative closes crucial gaps in the Dino pedigree

92 million years ago, a tyrannosaur cousin lived in North America, which already possessed crucial features of the T.rex - but in a small format. © Andrey Atuchin
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Missing Link: A 92-million-year-old fossil gives an insight into a key phase of tyrannosaur evolution. The T. rex cousin discovered in the south of the US was only about 2.70 meters long, but already possessed crucial characteristics of his great descendant - including the strong snout, the large brain and optimized for fast running feet. For the first time, the fossil clearly shows that the giants of the Cretaceous started small.

The Tyrannosaurus rex was the king among the giants of the late Cretaceous. The up to 13 meter long carnivore was able to hunt and overwhelm even larger prey animals thanks to its maneuverability, sharp teeth and enormous biting power. Together with large herbivores such as the Triceratops, the duckbill dinosaurs or the heavily armored Ankylosaurians the T. rex dominated the life of the Cretaceous period from around 80 million years ago.

Dark era of tyrannosaur evolution

But how did the Tyrannosaurus become this successful and big predator? Although it is known that early precursors of tyrannosaurids such as Proceratosaurus from the middle Jurassic were still rather small. However, how these rather slender precursors developed into the gigantic megaprädators during the Middle Cretaceous is largely unknown - there is a lack of fossils.

"This is a dark age of tyrannosaur evolution, " says Sterling Nesbitt of Virgina Tech in Blacksburg and his colleagues. Because of strong sea-level rise and other environmental changes at that time remained hardly remains of dinosaurs conserved. "Previously, only a single diagnostically meaningful tyrannosaurid fossil from this mid-Cretaceous gap was known." However, only the brain's skull and some isolated bones are preserved from this specimen.

Tyrannosaurus cousin as missing link

Exactly in this gap, however, fit the fossils discovered in New Mexico: two partial skeletons of a 92 million year old representative of Tyrannosauroidea. They include a skull discovered in 1997, as well as an almost complete skeleton found in 1998 not far from it. "For many years we simply did not know that we had a cousin of the Tyrannosaurus rex in front of us, " says Nesbitt. display

The true nature of these fossils has only now revealed new analyzes of the bones. They prove that the Suskityrannus hazelae baptized tyrannosaur is a true missing link in tyrannosaur evolution. Because Suskityrannus was with 2.70 meters length, although much smaller than his fearsome descendant T.rex. Nevertheless, he had already developed the features that characterized the Tyrannosaurus later.

Covered with fur-like feathers and only horse-groomed: that's how Suskityrannus hazelae might have looked. Andrey Atuchin

Heavy dentition and optimized running foot

Thus, the Suskityrannus already possessed the strong skull and the umbilical snout of its larger cousin and probably also a comparatively large biting power. He could therefore have been an effective hunter and carnivore. His footbones also show a characteristic of T.rex adaptation to the fast and manoeuvrable running. "Suskityrannus represents the earliest evidence of this key signature among the tyrannosaurs, " the researchers said.

"Thus, many of the notable components of the tyrannosaur scheme are already present in a medium-sized species that evolved long before the tyrannosaurs attained ecologic dominance, " say Nesbitt and his colleagues. "With that, Suskityrannus fills an important gap in the evolutionary tree of the tyrannosaurs - he gives us an insight into the evolution of these dinosaurs before they took control of the planet."

Transition to the last high plateau of the Dinos

But the Suskityrannus is not only a long-sought "missing link" of the tyrannosaur evolution, it also provides an insight into an important transitional period of the entire North American dinosaur fauna, as the researchers explain. Because it dates back to the time when the early Cretaceous fauna changed to the classic dinosaur era of the late Cretaceous and from which there are hardly any fossils anywhere in the world.

Both the Suskityrannus and the fossilized plant-herb dinosaur fossils found in the same formation represent smaller representatives of all the dinosaur forms that were to become the main protagonists of the last dinosaur boom. (Nature Ecology & Evolution, 2019; doi: 10.1038 / s41559-019-0888-0)

Source: Virginia Tech, Stony Brook University

- Nadja Podbregar