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WASHINGTON — Scientists reevaluating a partial skull first discovered in 1983 in southeastern New Mexico have concluded that the fossil represents a new species of Tyrannosaurus — a fearsome predator that lived in western North America at the end of the Age of Dinosaurs — that preceded the famous Tyrannosaurus. T. rex.
Slight differences observed from Tyrannosaurus rex in the skull merit the dinosaur's recognition as a separate species called Tyrannosaurus macroensis that lived several million years before Tyrannosaurus and was similar in size, researchers said Thursday. The skull was previously identified as a T. rex.
Other researchers expressed doubts that it represents a new type of dinosaur, saying that the differences between it and other T. rex skulls are not noticeable, and that the study's conclusion that the fossil dates back to between 71 and 73 million years ago represents a problem.
Tyrannosaurus was the only species of the genus Tyrannosaurus to be recognized since the dinosaur was first described in 1905. The genus is a broader group of related organisms than species. Tyrannosaurus rex fossils date back to 2 million years before an asteroid struck Earth 66 million years ago, decimating the dinosaurs.
The first parts of a New Mexico skull were found near the base of Kettle Top Butte in 1983, with more discovered later.
About 25% of the skull has been collected, said paleontologist Anthony Fiorello, executive director of the New Mexico Museum of Natural History and Science and one of the authors of the study published in the journal Scientific Reports. Most of the brain and upper jaws are missing.
“Compared to T. rex, the lower jaw is shallower and more curved backwards. The sharp wasps above the eyes are fewer than in T. rex,” said paleontologist Nick Longrich of the University of Bath in England, one of the other researchers. .
“It's the nature of the species that differences tend to be subtle. The main thing is that they're consistent. We looked at a lot of different tyrannosaurs, and our animal was consistently different from every known T. rex, in every bone,” Longrich added.
The differences involve important features, the researchers said. Fiorello noted that the feature above the tyrannosaur's eyes has been interpreted as being important in sexual selection.
“So these differences imply that our animal ate differently and perhaps chose different mates from the more famous Tyrannosaurus rex,” Fiorello said.
The tyrannosaur had a massive head, tremendous bite force, and walked on strong legs and weak arms. Perhaps the largest known Tyrannosaurus rex is a specimen named Sue located at the Field Museum in Chicago, which measured 40 feet long.
The presence of T. mcraeensis changes understanding of the dinosaur's origins, the researchers said. T. mcraeensis was larger than other dinosaur relatives that inhabited northern North America before T. rex, suggesting that giant species from this group evolved millions of years earlier than previously thought and came from southern North America, the researchers said.
Paleontologist Thomas Carr of Carthage College in Wisconsin, who has studied diversity within T. rex, called the new study “unconvincing.”
“The features in the New Mexico specimen that supposedly make it unique can be seen among other T. rex fossils,” Carr said. “The differences are not discrete and obvious; they are like shades of gray or shapes in the clouds. I have seen the specimen myself several times and nothing comes to mind that is different from other adult dinosaurs.” Which can be seen to be explained by relative maturity and individual diversity.”
Carr said the fossil was not dated based on the skull itself, but rather on the rocks underneath. Carr added that existing evidence indicates that the dinosaur appeared 67 to 68 million years ago.
The question of possible additional species of Tyrannosaurus has been controversial. Some researchers in 2022 proposed that there were three species of Tyrannosaurus, based on femur and tooth differences between the fossils. Other researchers disagreed, seeing insufficient evidence.
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