Tyrannosaurus rex could actually be three species

Everyone loves a new dinosaur. We unearth previously unidentified specimens on a semi-regular basis, and the excitement that greets each discovery speaks to the wonder we find in these ancient animals.

What if new species were hiding under our noses?

Take a close look at the collections of Tyrannosaurus fossils, researchers from the College of Charleston, USA believe we have unwittingly lumped together three closely related species of this most famous dinosaur into one basket, and it’s time to recognize not just the tyrant lizard king, but the queen and emperor too.

Editing their analysis in evolutionary biologyresearchers point to physical differences in the femoral and dental structures of 37 Tyrannosaurus specimens, saying that the observed variation in morphologies does not match the patterns expected by simple sex differences.

The authors used measurements of the length and girth of the femur bones to calculate their sturdiness, finding a distinct separation between specimens with sturdier or more slender bones. The two types of femurs were not evenly distributed in the specimen collection, suggesting that they were not associated with sex. They also did not correlate with overall size – juveniles had the same femur thickness discrepancies as adults.

Tyrannosaurus the teeth also contained clues – some had two thin incisors on either side of the front end of the jaw, while others had only one. Although only 12 of the specimens studied have both a femur and teeth, this limited data set nevertheless suggests that a single incisor tooth correlates with more gracile bones.

As a final corroboration, the authors fit their findings to a geological timeline. Of the 37 specimens studied, 29 were unearthed in the The Upper Maastrichtian formations in North America, in sediments dating from between 67.5 and 66 million years ago. The distinctly banded sediment layers at this fossil site allowed researchers to organize their specimens chronologically, with those found in the lowest layers representing the oldest in the collection.

Importantly, the most slender femurs were entirely absent from the lower layers. Instead, femurs from this layer showed only the normal degree of variation expected in any population. Researchers believe that at the time these lower layers of sediment were deposited, a single Tyrannosaurus species roamed the Earth.

The first gracilis femur appears in the middle layer, followed by five in the upper layer – a marked increase in prevalence over time. In these most recent layers, the level of variation in these bones is no longer within the bounds of normal population differences, instead painting a picture of the emergence of distinct body shapes, or “morphotypes”.

“We propose that changes in the femur may have evolved over time from a common ancestor that exhibited more robust femora to become more slender in later species,” says lead author and paleoartist Gregory Paul. “The differences in femur robustness between sediment layers can be considered distinct enough that the specimens could potentially be considered distinct species.”

Why three new species, and not two?

The authors believe that the oldest of their specimens, equipped with robust femurs and two incisors, represents a species. But even though stout femurs were also found in the more recent sediment layers, they were more likely to be accompanied by a single tooth in these more recent specimens, indicating a likely second species. Add the tapering forearm specimens, and suddenly we have a collection of three where before there was only one.

So which retains the title of tyrannosaurus rex – the tyrant lizard king?

Researchers were able to clearly recognize the king of lizards in the bones of more recent specimens, matching them to the established phenotype of this well-documented dinosaur – so these specimens will retain the famous nickname.

The researchers suggest that T. rex’s gracile cousin, found in the same layers of sediment, should be nicknamed “tyrannosaurus regina– the tyrant lizard queen.

The oldest specimens, with their sturdy forearms and double incisors, likely retained features of an earlier tyrannosaurid ancestor. The team proposes to confer on him the title of emperor, “Imperator Tyrannosaurus”.

But this expansion of Tyrannosaurus the royal family may not be without contests. While confident in their proposed new species, the authors acknowledge that assigning fossil vertebrates to new species is fraught with pitfalls. The variation observed could however turn out to be an example of extreme individual differences, or of atypical sexual dimorphism.

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