The partial skull of a dinosaur found 40 years ago has now been identified as a new species of Tyrannosaurus, and perhaps the closest relative of Tyrannosaurus rex. The research adds a new twist to a long-running debate about how many species of tyrannosaurs there were, and could help to clarify how the iconic predator evolved.
T. rex appeared in North America around 68 million years ago, 2 million years before the mass extinction that wiped out most dinosaurs. Palaeontologists have been stumped as to the carnivore’s origins. Some have suggested that the ancestors of T. rex walked over a land bridge from prehistoric Asia, while others placed its origins in southern North America.
Anthony Fiorillo at the New Mexico Museum of Natural History and Science and his colleagues re-examined a fossil in the museum’s collection, which was found in rock layers known as the McRae Group in western New Mexico.
While the skull had originally been categorised as T. rex, Fiorillo and his colleagues noticed differences with the bones. They propose that the specimen represents an older species, which they named Tyrannosaurus mcraeensis in reference to the rocks in which it was found.
They estimate that the dinosaur was about 12 metres long, comparable in size to T. rex, but it lived about 4 million years earlier.
The differences between T. rex and T. mcraeensis would have been relatively subtle. Whereas T. rex had prominent ridges on its brows and broad, bone-crushing jaws, the same ridges on T. mcraeensis were less well developed and its skull is more slender, says Fiorillo.
Other recent studies have proposed that there were several species of Tyrannosaurus and that some so-called T. rex fossils should be reassigned. Such proposals have been controversial, however, and largely dismissed by dinosaur palaeontologists. The new study will fuel further debate over how many Tyrannosaurus species there were in North America.
“I hesitate to regard Tyrannosaurus mcraeensis as being distinct from Tyrannosaurus rex,” says Jared Voris at the University of Calgary in Canada. He notes that many of the anatomical features that supposedly make the new species unique can also be found in specimens of T. rex.
Regardless of the species assignment, the existence of such a large tyrannosaur several million years before T. rex hints that south-western North America was an important centre for the dinosaur’s evolution. “The proposed age dates for the specimen are peculiar and very much warrant more study,” says Voris, as they might outline a clearer picture of dinosaur evolution in the last few million years of the Cretaceous Period.
The New Mexico tyrannosaur was found in the same rocks as giant horned dinosaurs, duckbilled dinosaurs and long-necked herbivores that could reach 30 metres in length. Fiorillo and his colleagues suggest the lineage leading to T. rex might have evolved their giant size to prey on these large herbivores, later spreading northward as the last of the “tyrant lizards” to stalk the planet.