The Tyrant Lizard King, Queen and Emperor: the T-rex might 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.

But what if there were new species hiding right under our noses?

Taking a close look at collections of Tyrannosaurus fossils, researchers from the College of Charleston, US, believe that we’ve been unwittingly lumping three closely related species of this most famous dinosaur into the one basket, and that it’s time to recognise not just the tyrant lizard king, but the queen and emperor too.

Publishing their analysis in Evolutionary Biology, the researchers point to physical differences in the femur and dental structures of 37 Tyrannosaurus specimens, saying that the observed variation in morphologies doesn’t fit with patterns expected through mere sex differences.

The authors used measures of the length and circumference of the femur bones to calculate their robustness, finding a distinct split between specimens with more robust or more gracile bones. The two femur types weren’t evenly split across the collection of specimens, suggesting they weren’t associated with sex. They also didn’t correlate with overall size – juveniles presented with the same discrepancies in femur thickness as adults.

Tyrannosaurus teeth also held clues – some had two slender incisor teeth on each side of the front end of the jaw, while others had only one. Although only 12 of the studied specimens had both femur and teeth present, this limited dataset nonetheless suggested that a single incisor tooth correlates with more gracile bones.

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

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

The first gracile femur makes its appearance in the middle layer, followed by five in the upper layer – a distinct increase in prevalence through 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 forms, or “morphotypes”.

“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,” says Gregory Paul, lead author and palaeoartist. “The differences in femur robustness across layers of sediment may be considered distinct enough that the specimens could potentially be considered separate 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 one species. But even though robust femurs were also found in the more recent sediment layers, they were more likely to be accompanied by a single tooth in these newer specimens, indicating a likely second species. Add in the specimens with the slender forearms, and suddenly we have a collection of three where before there was only one.

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

The researchers were able to clearly recognise the king of lizards in the bones of more recent specimens, matching them to the established phenotype of this thoroughly documented dinosaur – so these specimens will retain the famous moniker.

The researchers propose that T. rex’s gracile cousin, found in the same sediment layers, should be dubbed “Tyrannosaurus regina” – the tyrant lizard queen.

The most ancient specimens, with their robust forearms and double incisors, likely retained their features from an earlier tyrannosaurid ancestor. The team proposes bestowing it with the title of emperor, “Tyrannosaurus imperator”.

But this expansion of the Tyrannosaur royal family may not be without contest. While confident in their proposed new species, the authors acknowledge that assigning fossil vertebrates to new species is fraught with challenges. The observed variation could yet prove to be an example of extreme individual differences, or atypical sexual dimorphism.