Tyrannosaurus rex’s immense size (but leisurely pace) and popularity in cinema have made it one of the most popular dinosaurs, but new research has highlighted that the "species" may still have a few tricks up its sleeve – including two extra species.

A new analysis looked at skeletal remains and found a lot of variation in the leg and dental structures among “T. rex” specimens, possibly representing a trio of species instead of just one.

Two new proposed species – Tyrannosaurus imperator (tyrant lizard emperor) and Tyrannosaurus regina (tyrant lizard queen) – are put forward to sit alongside T. rex in a new study published in Evolutionary Biology.

Skeletal remains from 37 Tyrannosaurus specimens were enrolled into the new study to see if lead author Gregory Paul and colleagues could spot any significant differences. They were looking mostly for differences in the femurs and dental structures of the Tyrannosaurs, two areas highlighted in previous research as a source of variation among T. rexes.

Combing through fossilized femurs revealed some were slenderer while others were more robust, with a ratio of 1:2 respectively among 24 femurs analyzed. If the difference was down to sex alone, the researchers would’ve expected more of a 50/50 split between femur types, which means the result indicates something else is at play in explaining the variation.

There were also adult-sized slender femurs, indicating that a mix of juvenile and adult specimens likely didn’t explain the differences either.

Tyrannosaurus teeth also showed some differences, but with just 12 specimens having both dental and femur data available, it was hard to match tooth type to thigh bone robustness.

Clues were also discovered in the age of the remains, with specimens from the lowest and oldest layer of sediment only exhibiting thicker thigh bones. The lack of variation here could demonstrate that only one Tyrannosaurus species existed at this time.

As they looked at specimens from the middle layer, they found one slender femur, and by the top and latest layer there were five gracile femurs among robust ones. Comparing the variation seen in the top layer against variation among other theropods revealed it was higher than would be expected.

While extreme individual differences could account for the variation, diversifying from thick-thighed Tyrannosaurs to a range of femur thicknesses over time could support the emergence of more than one species. To this end, the study authors suggest two further species – T. imperator and T. regina – could join T. rex’s genus.

“We found that the changes in Tyrannosaurus femurs are likely not related to the sex or age of the specimen,” said Paul in a statement.

“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.”