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Closest Known Relative Of T. Rex Discovered From 72 Million-Year-Old Skull

Meet Tyrannosaurus mcraeensis, older and more primitive but still the same size as a double decker bus.

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Eleanor Higgs

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Eleanor Higgs

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Eleanor is a content creator and social media assistant with an undergraduate degree in zoology and a master’s degree in wildlife documentary production.

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Edited by Katy Evans
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Katy Evans

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Katy is Managing Editor at IFLScience where she oversees editorial content from News articles to Features, and even occasionally writes some.

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The teeth of Tyrannosaurus mcraeensis, the closest relative to T. rex ever found, and paleoart depicting the dinosaur.

One of the teeth from the skull is worn at the top, possibly as a consequence of biting bone.

Image Credit: Credit: Nick Longrich (left), Sergei Krasinski (right), edited by IFLScience

Tyrannosaurus rex is arguably the most famous of all the dinosaur species, with starring roles in pretty much every dino movie ever made. However, scientists have identified a new subspecies of Tyrannosaur, Tyrannosaurus mcraeensis, older and more primitive but still the same size as a double-decker bus. This new species also helps researchers understand where T. rex and other Tyrannosaurs might have originated from, a question that has puzzled palaeontologists. 

The new species was discovered when the team took a closer look at a museum specimen of a 71-73 million-year-old partial skull. The skull was discovered in the Hall Lake Formation, New Mexico, USA in the 1980s and is currently on display at the New Mexico Museum of Natural History & Science (NMMNHS). 

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Paleoart reconstruction of Tyrannosaurus mcraeensis similar to a T.rex with small arms but a huge body size. Triceratops are in the background with trees and hills.
Reconstruction of what Tyrannosaurus mcraeensis might have looked like.
Image credit: Sergei Krasinski


“[The] skull/jaw was originally described and assigned to T. rex in 1986," Dr Spencer Lucas, one of the authors of the new paper and Curator of Paleontology at NMMNHS, told IFLScience. "Decades later, much has been learned about T. rex through newly discovered fossils. The anatomical differences between the McRae fossil and other T. rex now stand out as likely taxonomic differences, hence the need for reinvestigation”

Close analysis of the skull revealed that not only was it not a Tyrannosaurus rex skull, but it represented a whole new species, now named Tyrannosaurus mcraeensis, thought to have lived 71-73 million years ago – around 6-7 million years before T. rex was roaming about. It also sheds light on the idea that tyrannosaurs were living in North America millions of years earlier than palaeolontologists previously thought. 

“New Mexicans have always known our state is special, now we know that New Mexico has been a special place for tens of millions of years,” Dr Anthony Fiorillo, Executive Director of NMMNHS, said in a statement seen by IFLScience. 

Given that T. rex is so famous, this helped the team identify key subtitle differences between the bones within the skull of a T. rex and the skull of the new species. 

Jaw of Tyrannosaurus mcraeensis. One of top and one on the bottom of the image on a black background with a scale bar.
While it might not look like it, these jaws are shallower and longer than a T.Rex, suggesting a mouth with less bite force.
Image credit: Nick Longrich


“The differences are subtle, but that’s typically the case in closely related species. Evolution slowly causes mutations to build up over millions of years, causing species to look subtly different over time,” said Dr Nick Longrich, a co-author from the Milner Centre for Evolution at the University of Bath, UK.

Dr Lucas told IFLScience that the longer and shallower jaws in the new species suggest that it “likely had less bite force than in a T. rex.” Regardless it was still an impressively large dinosaur, roughly the same size as a T. rex, which measured up to 12 meters (40 feet) from head to tail and 3.6 meters (12 feet) tall. Given the dates of the fossil, it shows that tyrannosaurs grew to a giant size near the end of the Campanian (toward the end of the Cretaceous period). Like its more well-known cousin, both species would have consumed a largely meat-based diet. 

Phylogenetic analysis also suggests that the species could be a sister species to the T. rex making it the closest known relative. According to Dr Lucas, it could even be a “plausible ancestor”, though the paper notes not a direct one. The discovery suggests that Tyrannosaurs may have evolved south of an island continent called Laramidia that existed between 100 and 66 million years ago. This island was extremely large and would have stretched from what is now Alaska to Mexico. Towards the end of the Cretaceous period, the team posits, Tyrannosaurus started spreading north, alongside other giant species like Triceratops

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The paper is published in Scientific Reports.


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natureNaturenatureanimals
  • tag
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  • dinosaurs,

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  • fossil,

  • t.rex

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