Fossil skull represents closest known relative to famous T. rex
A newly discovered species of tyrannosaur is making headlines this week after a research team revealed fossils unearthed in New Mexico belonging to a previously unknown relative of the infamous Tyrannosaurus rex. Named Tyrannosaurus mcraeensis, this massive carnivore roamed what is now New Mexico over 71 million years ago during the Late Cretaceous period.
The T. mcraeensis fossils were originally discovered in 1993 in the Bisti/De-Na-Zin Wilderness of New Mexico by NMMNHS volunteer Ned McRae. However, it took over 30 years for paleontologists to confirm that the remains represent a new tyrannosaur species. Now described in detail in a paper published this week in Scientific Reports, T. mcraeensis provides exciting new clues into the origins and evolution of T. rex and other tyrannosaurs.
Adult T. mcraeensis rivaled T. rex in size
The T. mcraeensis holotype specimen consists of a nearly complete skull weighing over 1,500 pounds. Based on analysis of the skull and other skeletal material found with it, researchers estimate the individual was over 30 feet long and around 9 tons as an adult, approaching Tyrannosaurus rex in size.
“It would have rivaled T. rex in weights and lengths,” said study coauthor Lindsay Zanno in an interview.
| Measurement | T. mcraeensis | T. rex |
|---|---|---|
| Length | 30-33 ft | 40 ft |
| Weight | 8-9.5 tons | 6.5-9 tons |
However, where T. mcraeensis stands out from other tyrannosaurs is its proportionally large head, noted Zanno. The skull itself measures over 5 feet long. Thomas Carr, another coauthor on the study, said it is “unusually deep and wide,” giving the dinosaur a blunt snout significantly different from T. rex’s sleeker profile.
Teeth point to scavenging behavior
According to Zanno, the defining characteristic setting T. mcraeensis apart is its unusual teeth. While sporting the classic tyrannosaur tooth counts with long laterals and diminutive premaxillary teeth, the crowns are noticeably more bulbous. Zanno says these teeth indicate T. mcraeensis likely had a stronger bite force than other tyrannosaurs.
However, the teeth seem better designed for crushing bone than slicing through flesh, leading Zanno to hypothesize T. mcraeensis was not solely an active predator:
“My suspicion is T. mcraeensis was not competing with apex predators of the day,” she said. “Its incredible robustness suggests it could crush bone and digest it more efficiently than other theropods, leading me to speculate it was something of an apex scavenger dominating carcasses after faster, cuttier carnivores had taken their fill.”
If true, this behavior contrasts sharply with the strictly predatory lifestyle of T. rex later tyrannosaurs that evolved to rapidly take down live prey with their banana-shaped teeth.
Discovery helps trace tyrannosaur family tree
In addition to adding a new giant to Laramidia’s latest Cretaceous fauna, study lead author Albert Prieto-Márquez says T. mcraeensis helps clarify the early evolution of tyrannosauroids on the continent before they rose to apex predator dominance.
“It provides new information about how the body plan of tyrannosaurids evolved before they became colossal apex predators later in the Late Cretaceous,” said Prieto-Márquez in an interview.
The research team’s analysis places T. mcraeensis high up on the tyrannosaur family tree as the sister taxon to T. rex and its closest relatives like Tarbosaurus and Daspletosaurus. Exactly when tyrannosaurs transitioned from more marginal niches to apex predators is still debated, but discoveries like T. mcraeensis are helping illuminate this mysterious epoch of dinosaur history according to Stephen Brusatte, a tyrannosaur expert not involved with the study.
“The origins of tyrannosaurs encompass a murky ~30 million year ghost lineage during which the group evolved from small-bodied hunters into colossal apex predators,” said Brusatte. “Fossils like T. mcraeensis start to fill that gap.”
Future research
There is still much yet unknown about T. mcraeensis, including details about its evolutionary relationship to specific tyrannosaur groups. The research team plans to conduct further analysis on the specimen, including a full body scan to create 3D models of the skull and other bones. These models will be made available on MorphoSource.org for other paleontologists to study.
The team will also return to the T. mcraeensis excavation site this summer to search for more fossils. Prieto-Márquez says additional skeletal material would allow them to further refine their size estimates for the dinosaur as well as reveal more about how it moved. Complete leg or arm bones may also contain medullary bone that would determine T. mcraeensis’ sex.
For Lindsay Zanno, the T. mcraeensis skull continues to yield new secrets about Laramidia’s tyrannosaurs even after 30 years buried in museum storage. “I never know what’s going to come out of the ground here,” she said. “It certainly keeps me coming back to New Mexico.”
References:
Zanno, L.E., Carr, T.D, Williamson, T.E., Mcrae, N. (2024) Tyrannosaurus mcraeensis, a new tyrant from the American south. Scientific Reports. https://www.nature.com/articles/s41598-023-47011-0
Kluger, J. (2024) T. Rex’s Older and Equally Sizable Relative Discovered in New Mexico. Time Magazine. https://time.com/6241792/new-tyrannosaur-species/
Gibbons, A. (2024) This newfound T. rex relative reveals the origin story of the tyrant dinosaurs. Science News. https://www.sciencenews.org/article/tyrannosaurus-rex-relative-origin-story-dinosaurs
Prieto-Márquez, A. et al (2023) Integrative taxonomy supports a new tyrannosauroid from the early Late Cretaceous of New Mexico, USA. PeerJ Life & Environment. DOI: 10.7717/peerj.14214
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