Tyrannosaurus rex could be three distinct species, study claims

Tyrannosaurus rex could be thr...
An artist's impression of the proposed new species, Tyrannosaurus imperator, attacking a triceratops
An artist's impression of the proposed new species, Tyrannosaurus imperator, attacking a triceratops
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An artist's impression of the proposed new species, Tyrannosaurus imperator, attacking a triceratops
An artist's impression of the proposed new species, Tyrannosaurus imperator, attacking a triceratops

Tyrannosaurus rex is one of the few dinosaurs that has its full scientific name in wide use, largely because that name is so cool. But now we might have a few others to learn, as analysis of known fossils reveals that T-rex might actually be three separate species, including the T-imperator and the T-regina.

The T-rex is currently the only recognized species in the Tyrannosaurus genus, which probably helps bolster its rep. But previous studies have identified a few recurring variations in the bones of the beasts that could indicate that known T-rex specimens might actually belong to several different, but closely related, species.

Some skeletons, for instance, have been found to have just one slender incisor tooth on either side of the front of the jaw, while others have two. And some have quite robust femurs, while others are more gracile. So for the new study, researchers analyzed 37 Tyrannosaurus specimens to figure out if there was any particular pattern to those variations.

Robust femurs were found to outnumber gracile femurs two-to-one. The team says this split indicates the variation isn’t a sex difference, because that would be closer to 50/50. It also doesn’t appear to be related to the animals’ ages either – robust femurs were found in some juvenile specimens, and gracile femurs turned up in some adults.

Only 12 specimens had both femurs and teeth intact, but of those, the team noted that specimens that had only one incisor were more likely to have gracile femurs.

The team also examined these variations in the context of which layers of sediment the specimens were found in, with such records existing for 28 of the specimens. Intriguingly, robust femurs were found throughout but gracile femurs only began to appear in the middle layers, with most concentrated in the upper layers. That suggests a lineage from just one Tyrannosaurus species differentiating into several over time.

“We found that the changes in Tyrannosaurus femurs are likely not related to the sex or age of the specimen,” said Gregory Paul, lead author of the study. “We propose that the changes in the femur may have evolved over time from a common ancestor who displayed more robust femurs to become more gracile in later species. The differences in femur robustness across layers of sediment may be considered distinct enough that the specimens could potentially be considered separate species.”

The researchers have even proposed names and defining characteristics for these potential species. They call the oldest species Tyrannosaurus imperator, which are found in the lower and middle sediment layers and tend to have robust femurs and two incisor teeth – features that seem to be retained from earlier ancestors.

The second species, named Tyrannosaurus regina, was found in the upper and possible middle layers, with gracile femurs and only one incisor. And the third species, the existing Tyrannosaurus rex, was found in the upper and middle layers with robust femurs and one incisor.

In keeping with the naming convention of the iconic Tyrannosaurus rex (which means “tyrant lizard king”), the team proposed similarly royal names. Tyrannosaurus regina means “tyrant lizard queen,” while Tyrannosaurus imperator means “tyrant lizard emperor.”

Of course, the team acknowledges that there’s still plenty we don’t know. Not all specimens studied could be neatly assigned a species, and the sediment layer of origin couldn’t be determined for all either. There’s also a chance that these differences are due to other factors. And other paleontologists disagree that the variations constitute new species at all.

“Ultimately, to me, this variation is very minor and not indicative of meaningful biological separation of distinct species that can be defined based on clear, explicit, consistent differences,” Professor Steve Brusatte, a paleontologist not involved with the study, told The Guardian.

Still, it is a testable hypothesis, and further investigations into other specimens will be needed before anything official comes of it.

The research was published in the journal Evolutionary Biology.

Source: Springer Nature

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