A 74-million-year-old leg bone from a fossil bed in New Mexico could be an early relative of Tyrannosaurus rex, suggests a new study published in Scientific Reports.
The find adds weight to the theory that T. rex's lineage originated in what is now the American Southwest, rather than migrating from Asia, and suggests the group was evolving into giants far earlier than previously thought.
The shinbone – discovered in the Kirtland Formation of New Mexico and dating to the late Campanian period – measures 96cm (3.1ft) in length, roughly 84 per cent the size of the tibia belonging to ‘Sue’, the largest known T. rex specimen.
Based on its dimensions, the researchers estimate the animal weighed approximately 4,700kg (10,400lbs), making it the largest tyrannosaur known from this period – around 50 per cent heavier than any contemporary rival.
According to the researchers, there are three possible explanations for the bone's origin. It could belong to an unusually large theropod dinosaur known as a Bistahieversor; it could represent an entirely new and previously unrecognised lineage of giant tyrannosaur; or it could be an early member of Tyrannosaurini – the group that includes T. rex and its closest relatives.
Of the three theories, the authors consider the last most likely. Lead author Dr Nicholas Longrich, of the University of Bath, said the bone bears a striking resemblance to T. rex.
“It's a dead ringer for T. rex,” he told BBC Science Focus. “If this bone was found in the same beds where we get T. rex, no one would doubt it’s a T. rex, it's that similar."
This points to southern North America as the cradle of T. rex's lineage. A giant tyrannosaur, Tyrannosaurus mcraeensis, was previously identified from New Mexico's slightly younger Hall Lake Formation. It was while photographing those specimens that Longrich stumbled across this latest bone on a museum shelf.
The clustering of large, T. rex-like tyrannosaurs in the American Southwest – millions of years before they appeared further north – suggests the lineage evolved there first, before eventually spreading across the continent.
Further excavation of the Kirtland Formation could yet settle the question of this bone’s owner. Longrich said he considers it "extremely likely that new material will turn up", adding that teeth may be a promising avenue, as they preserve better and are therefore easier to find than bone.
A more complete skeleton would allow researchers to formally name the species and determine whether it represents a direct ancestor of T. rex or an early cousin.
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