(CN) — When researchers first uncovered giant dinosaur remains in the Hall Lake Formation in New Mexico, they initially believed that the remains belonged to a Tyrannosaurus rex specimen. The fossilized partial skull at the New Mexico site bore similarities to T. rex, but a team of researchers used phylogenetic analysis to define their newly dubbed Tyrannosaurus mcraeensis as the closet known relative of T. rex, not another member of the famous species.
According to the researchers' study published Thursday in Scientific Reports, the team noticed several subtle differences between T. rex and T. mcraeensis, perhaps the most significant of which was the jaw. Study co-author Anthony Fiorillo said that T. mcraeensis had a slender jaw whereas its more well-known counterpart T. rex possessed a robust jaw, which suggested something beyond a mere difference in shape.
“The slender jaw is not a minor matter because it implies that T. mcraeensis ate differently than T. rex,” said Fiorillo via email, on behalf of himself and in collaboration with the rest of the team. “The most prominent other difference in skull bones is in the postorbital, a bone around the eye orbit. On T. rex, there is a prominent ridge that may have had a role in sexual selection. That prominent ridge is not present in T. mcraeensis. So, based on the anatomy of T. mcraeensis, thinking of the biological implications of these anatomical differences, it seems that T. mcraeensis ate differently and chose its mate differently than T. rex.”
Another discovery that caught the researchers' attention was evidence of how old T. mcraeensis was. Using a combination of dinosaur fauna, the morphology of T. mcraeensis and the age constraints from underlying radiometric dates and overlying dinosaur bones, the researchers reported that T. mcraeensis is likely between five to seven million years older than T. rex.
And it is an older cousin at that, as the researchers say that T. mcraeensis showed several characteristics that T. rex did not, such as a weakly curved prearticular bone. Per the study, this implies that T. mcraeensis is more likely a side branch of the tyrannosaurin evolution rather than a direct ancestor of T. rex.
Regarding tyrannosaurin evolution, researchers posit in the study that T. mcraeensis shows that tyrannosaurini likely evolved into their giant size near the end of the Campanian age, and that tyrannosaurini did so in the southern part of Laramidia, an island continent stretching from modern-day Alaska to Mexico that existed between 100 to 66 million years ago.
Tyrannosaurini roamed a limited portion of the relatively small Laramidian landmass, and the researchers hypothesized that giant tyrannosaurs evolved to prey on giant south Laramidian herbivores, though why herbivorous dinosaurs became that large is one mystery to contend with.
Other unresolved matters include figuring out precise constraints on the age of T. mcraeensis, which the researchers said they cannot do without more radiometric dates, palynostratigraphy or vertebrae fossils. Also, Fiorillo and the team wonder about how New Mexico's environment played a role in T. mcraeensis' existence.
“Now that we know we have something new, we are starting to ask questions regarding the environment at the time this dinosaur lived here in New Mexico,” said Fiorillo. “What was so special about New Mexico at that time that allowed this dinosaur to grow so large? So, some of the future work will be ecologically oriented.”
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