A team of paleontologists has identified a new species of tyrannosaur based on a fossil skull discovered in New Mexico over 30 years ago. The new dinosaur, named Tyrannosaurus mcraeensis, provides key insights into the origins and evolution of the infamous Tyrannosaurus rex.
Chance Encounter Leads to Groundbreaking Discovery
The fossil was originally uncovered in 1993 near Elephant Butte Reservoir by a vacationing family. Paleontologist Sterling Nesbitt, now at Virginia Tech, learned of the specimen in 2020 and realized its potential significance.
“I saw a picture of the fossil online and instantly got goosebumps,” said Nesbitt. “Even though the fossil was encrusted with rock, I could tell from the elongated skull and large, blade-like teeth that this was something special.”
Further examination showed the fossil to be a new tyrannosaur species and the closest known relative of T. rex. The research team named the dinosaur T. mcraeensis after Buster McRae, the New Mexico rancher on whose land the fossil was found.
Key Insights into Tyrannosaur Origins and Evolution
The T. mcraeensis fossil dates back approximately 72-73 million years to the Late Cretaceous period. This makes it slightly older than the known T. rex fossils.
“This discovery fills an important gap in the tyrannosaur family tree between earlier Cretaceous tyrannosaurs and T. rex,” said Nesbitt. “It helps clarify how tyrannosaurs transitioned from long-snouted hunters to the short-faced bone crushers we know from the end of the Cretaceous.”
With its elongated skull and smaller size, T. mcraeensis represents an intermediate stage between earlier, smaller tyrannosaurs like Dilong and later giants like T. rex. This supports the hypothesis that tyrannosaurs steadily grew in size over tens of millions of years before culminating in behemoths like T. rex.
Intriguingly, Nesbitt notes that T. rex likely evolved its massive size and bone-crushing bite independently from South American giant carnivores like Giganotosaurus. This convergence between North and South American apex predators hints at shared evolutionary pressures.
Controversy Over Tyrannosaur Family Tree
While most paleontologists agree T. mcraeensis was an early relative of T. rex, its exact placement in the tyrannosaur family tree remains contentious.
Some argue it is the closest known sister taxon to T. rex, while others contend it diverged earlier and is more basal in the tyrannosaur lineage. Resolving this debate has implications for when key tyrannosaur adaptations like short faces and bone-crushing dentition evolved.
“We originally classified T. mcraeensis in the genus Tyrannosaurus since it’s clearly a close relative,” said Nesbitt. “But further comparisons to other tyrannosaur skulls could reveal enough differences to warrant its own genus.”
Either way, T. mcraeensis demonstrates remarkable new diversity among Late Cretaceous North American tyrannosaurs. More fossils are needed to clarify relationships between T. rex and its closest cousins.
The Hunt for More Tyrannosaur Fossils
The T. mcraeensis specimen started an ongoing hunt for more tyrannosaur fossils in the Southwestern US. Researchers are now scouring parts of New Mexico and Texas that were underwater during the Late Cretaceous when T. rex and kin stalked North America.
“This fossil radically changes our perceptions of tyrannosaur diversity on Laramidia,” said Lindsay Zanno, paleontologist at the North Carolina Museum of Sciences. Laramidia is the name given to the landmass that existed during the Late Cretaceous that is now the western part of North America.
Zanno explains finding T. mcraeensis in New Mexico makes it very likely that distinct tyrannosaur species occupied discrete geographic areas across Laramidia. This is akin to how modern lion subspecies are separated across parts of Africa.
“There could be a whole host of tyrannosaurs still waiting to be found,” said Zanno.
Teams will focus exploration on New Mexico and Texas to uncover more tyrannosaur remains and piece together the evolutionary origins of T. rex. Any new specimens will provide invaluable data to clarify relationships between these apex predators.
“I think soon we will be adding several new species to the tyrannosaur family tree,” said Nesbitt.
Implications and Next Steps
The identification of T. mcraeensis has broad implications for resolving longstanding mysteries about tyrannosaur evolution. It provides a new anchor point connecting T. rex to more primitive tyrannosaur ancestors.
However, as typical with such discoveries, the fossil raises as many questions as it answers. Key next steps include:
- Comparing T. mcraeensis to contempory Late Cretaceous tyrannosaurs species across North America to determine its exact placement on the family tree
- Initiating new expeditions in Southwestern states to search for more tyrannosaur fossils that could clarify evolutionary relationships
- Using geologic data and evolutionary modeling to study what environmental pressures may have driven the steady size increases seen in tyrannosaurs over time
- Exploring what made tyrannosaurs in North America massive and bone-crushing while tyrannosaurs in Asia remains longer-snouted and more gracile
Ultimately, T. mcraeensis gives new momentum to finally unraveling the remarkable evolutionary history of perhaps the most iconic dinosaur species ever – the tyrant lizard king Tyrannosaurus rex.
This fossil and future remains promise to shed more light on how T. rex came to rule the North American landscape for millions of years.
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