In the badlands of Montana, time doesn't so much flow as it calms down. Under layers of sandstone and ancient river mud, two dinosaurs – a horned Triceratops and a smaller, sharp-toothed predator – have been buried together since the last days of the Cretaceous period. For paleontologists, the fossilized pair, known as the “Dueling Dinosaurs,” were both a spectacle and a mystery: Was the smaller skeleton young? Tyrannosaurus rexor something completely different?
Now this mystery has been solved. New research published in Nature shows that the smaller predator was not young T. rex not at all, but an adult Nanotyranus lanceolata – the kind that rewrites the tyrant's family history.
“This fossil doesn't just settle disputes. T. rex research on its head,” said Lindsey Zanno, co-author of the study. in a press release.
Read more: The origin story of Tyrannosaurus rex may not have started in North America
minor T. rexor Nanotyranus lanceolata?
For more than a century, T. rex was a celebrity in paleontology—a model organism for studying dinosaur biology, from feeding and locomotion to growth and life history. But that fame, experts say, may have blinded science to a crucial truth hiding in plain sight.
“For Nanotyranus to be a teenager T. rex“It's not just unlikely—it's impossible,” said James Napoli, co-author of the study.
This discovery forces a reexamination of decades of research. T. rex research. Many studies of growth, nutrition, and biomechanics may have unknowingly conflated two separate animals—species that actually lived side by side for the last million years before the asteroid hit.
Paleontology's fiercest feud
To ensure that their results would stand up to scrutiny, Zanno and Napoli conducted an exceptionally detailed examination of the fossil's bone tissue—one of the most comprehensive histological studies ever conducted on a tyrannosaurus. Analysis of thin sections revealed growth rings marking about 20 years of life, indicating that the animal had already reached maturity at the time of death.
Lindsay Zanno, assistant research professor at North Carolina State University and director of paleontology at the North Carolina Museum of Natural Sciences, with fossils of fighting dinosaurs.
(Image credit: North Carolina State University/CC BY-NC-ND)
Anatomy told an even clearer story. dinosaur mouth contained noticeably more teeth than typical T. rexand its forelimbs – especially its arms – were proportionately and absolutely longer than those of even the largest tyrant lizards, according to Nature. For such features to be transformed into properties T. rexits limbs would have shortened and its teeth would have disappeared as it grew—a pattern virtually unheard of in vertebrate evolution. Taken together, these details, as well as other subtle differences in the skeleton, made the argument decisive.
Comparing more than 200 T. rex specimens, the researchers also revisited the well-known “Jane” skeleton, determining that it was still growing but destined to remain much smaller than an adult. T. rex. Based on these results, they named the new species Nanotyrannus lethaeus – a tribute to the mythical River Lethe, for the predator's identity had been forgotten for generations.
Their phylogenetic analysis places Nanotyrannus just behind the main lineage of tyrannosaurids, hinting at a more complex evolutionary tree of apex hunters that flourished in the final days before the extinction of the dinosaurs.
Rival predators in the shadow of extinction
The results show that even in the last million years of the planet's existence before the asteroid impact, tyrannosaurs emitted into new forms rather than disappearing.
“This discovery paints a richer, more competitive picture of the last days of the dinosaurs,” Zanno said in a press release. “Possessing enormous size, powerful bite force and stereoscopic vision, T. rex was a formidable predator, but did not reign unconditionally. Flying next to him was Nanotyrannus, a slimmer, faster and more agile hunter.”
Read more: Sleeping on a Tyrannosaurus Rex's Arm Is a Common Habit That Helps Us Feel Safe and Relaxed
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