Santa Fe New Mexican, By Alaina Mencinger amencinger@sfnewmexican.com
ALBUQUERQUE — We know Tyrannosaurus rex, history’s most infamous carnivore, only ate meat. But did the top predator’s cousin prefer it with red, green or Christmas?
New analysis of a leg bone discovered in San Juan County found what may be a close ancestor of T. rex. The analysis raises new questions about the origin of the species — and indicates there were tyrannosaurs much larger than previously known roaming New Mexico 74 million years ago.
The findings, co-authored by two researchers at the New Mexico Museum of Natural History and Science, were recently published in Scientific Reports.
“It informs us about the ancient biodiversity of New Mexico, and that’s a big thing,” said museum executive director and paper co-author Anthony Fiorillo. “We now know more about the history of life in this state.”
T. rex is typically hypothesized to have evolved in Asia or North America. But the analysis of the New Mexico fossil might indicate the species originated in North America, as it’s not the only example of an early, large tyrannosaur in the American Southwest.
That raises questions about what in the environment at the time might have led to the larger body sizes, Fiorillo said.
“These really big tyrannosaurs may have originated in the southwestern part of the U.S., possibly Mexico,” Fiorillo said. “The follow-up question to that has to be: Why?”
The almost-complete tibia, found in the Bisti De-Na-Zin Wilderness, has an unusual shape and size for its age, estimated at 74 million years old. That’s between 6 million and 8 million years before T. rex roamed the Earth.
While most of the dinosaur’s contemporaries, and T. rex’s predecessors, would have weighed between 2 tons and 3 tons, this one is estimated to have weighed double that. T. rex, the largest of the tyrannosaurs, weighed in at around 10 tons; many animals evolve larger over time.
“Now we know there were tyrannosaurs this big living at 74 million years ago,” said Spencer Lucas, New Mexico Museum of Natural History and Science curator and fellow co-author.
Lucas was part of a paleontological survey in the late 1970s that uncovered the leg bone.
It didn’t seem to be anything special at the time — Lucas, who was a University of New Mexico student when the bone was discovered, said he didn’t even recall finding it. The bone went into the university’s collection, then was moved to the museum’s a few years later.
(PHOTO: 031226_DinoTibia04rgb.jpg Spencer Lucas, New Mexico Museum of Natural History and Science curator and part of a paleontological survey in the late 1970s that uncovered a dinosaur leg bone, walks through the collection hall Thursday toward a tibia that may belong a close ancestor of the T. rex. Gabriela Campos/The New Mexican)
But with a few decades more knowledge on tyrannosaurs and a pair of fresh eyes — in this case, Nick Longrich, lead author and senior lecturer at the University of Bath, England — researchers found this tibia was unusually large for the era and had a shape more like T. Rex than other tyrannosaurs.
Fiorillo pointed to a triangular shape near what would be the animal’s ankle and the straightness of the bone shaft, both of which are more characteristic of T. rex than its ancestors.
“Those are features that these highly derived tyrannosaurs, like tyrannosaurus and tarbosaurus, have,” Fiorillo said. “When we get into some of the more primitive kinds of tyrannosaurs, [the shaft] has a bow to it, and this shape is different.”
There are still questions about the bone. It’s unknown if the tyrannosaur was male or female, full-grown or juvenile. Lucas sees the need for additional fieldwork to uncover more samples. The badlands are constantly eroding, Lucas said, exposing new fossils.
“This bone has been here for about 40 years,” he said. “I want to believe it’ll be here 40 years from now. There may be a totally different understanding by then.”
