Closest dinosaur relative to T. rex may just have been found

A giant new tyrannosaur species has been discovered in the southern US which may be the closest known relative to Tyrannosaurus rex. The dinosaur could help explain how T. rex got so big.

Jaw bone fossil tyrannosaurus on black background
Jaw of Tyrannosaurus mcraeensis at the New Mexico Museum of Natural History and Science. Note the large scar towards the back of the jaw, which the authors speculate may have resulted from a fight with another Tyrannosaurus. Credit: Nick Longrich.

Tyrannosaurus mcraeensis has been described from a fossilised partial skull found in the Hall Lake Formation in New Mexico. The remains were previously ascribed to T. rex, but new analysis shows they are from a cousin of the dino king.

Subtle differences in the shape of and joins between the skull bones led the palaeontologists to name a new closely related tyrannosaur species.

The research is published in the journal Scientific Reports.

T. rex grew to about 12 m long and weighed more than 7 tonnes. It lived at the end of the reign of the dinosaurs between 68 and 66 million years ago, during the Cretaceous period (101–66 million years ago).

T. mcraeensis lived 73–71 million years ago and rivalled T. rex in size.

“Where and when Tyrannosaurini (T. rex and kin) originated remains unclear,” the researchers write. Competing hypotheses suggest the tyrannosaurin lineage originated in Asia, or an island continent called Laramidia which existed during the Cretaceous and stretched from modern-day Alaska to Mexico.

The new discovery suggests, according to the researchers, that Tyrannosaurini first emerged in southern Laramidia.

“Evolution of giant tyrannosaurs in southern North America, alongside giant ceratopsians [horned dinosaurs], hadrosaurs [“duck-billed” dinosaurs], and titanosaurs [huge, long-necked dinosaurs] suggests large-bodied dinosaurs evolved at low latitudes in North America,” they write.

It’s possible that tyrannosaur gigantism was driven by the growing body sizes of herbivores on which they preyed in southern Laramidia.

Butte in new mexico north american desert landscape
Kettle Top Butte at the Hall Lake Formation in southeastern New Mexico. This fossilised jaw from Tyrannosaurus mcraeensis was uncovered near the base of the butte. Credit: Dr. Spencer Lucas, NM Museum of Natural History & Science. Image courtesy of NM Department of Cultural Affairs.

Recent research has thrown up questions about the tyrannosaur species and, in particular, the most famous and largest – T. rex. This has included suggestions that T. rex might actually be 3 different species – a theory which has since been dumped.

Debate has also raged between palaeontologists about whether smaller tyrannosaur fossils represent juveniles or a separate genus called Nanotyrannus.

Adding a new branch in the tryrannosaur evolutionary tree – the closest to T. rex yet – could shed light on these questions and deepen our understanding of the largest predators to ever walk the Earth.

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