The Tyrannosaurus rex may not have been the only representative of its genus. According to a new analysis, skeletal remains that have so far been assigned to the “king of the terrible lizards” show such clear differences in the thigh bones and teeth that they probably belonged to three different species: In addition to the Tyrannosaurus rex, the researchers propose the new species Tyrannosaurus imperator and Tyrannosaurus regina.
The Tyrannosaurus rex is probably the most famous dinosaur of the Cretaceous period. His size, top Predator strength, and oddly small front legs make him an iconic figure that has also found its way into pop culture. Until now, it has been assumed that the T. rex is the only species in its genus. Although there had previously been great variability in the skeletal remains found, precise analyzes of the extent to which all of these fossils actually belong to T. rex have so far been lacking.
Differences not by age or sex
The US paleontologist Gregory Paul and his colleagues from the College of Charleston have now compared the bones and teeth of 37 known Tyrannosaurus specimens. They measured the length and circumference of the preserved femurs and also analyzed the dentition of the respective individuals. “It has long been known that there are considerable differences in the stature of adult Tyrannosaurus specimens,” the researchers explain. “In particular, the femurs of similarly sized specimens vary so greatly in robustness that this is visually obvious.” It had also been shown previously that some specimens had two canines on each side of the jaw, while others only had one.
Without further investigation, paleontologists had previously assumed that the variations were due to differences in age between individuals or sexual dimorphism.
Variation in femur and dentition
But Paul and his colleagues came to a different conclusion based on their systematic measurements: “We found that the changes in the femurs of Tyrannosaurus are probably not related to the sex or age of the specimen,” says Paul. The researchers found twice as many specimens with robust thigh bones as with graceful thigh bones. If the difference were due to sex, the split should be more even. The authors also consider the individual age of the specimens to be an unlikely influencing factor. Sturdy femurs were also found on some juvenile remains, while there were large adult specimens with slender femurs.
There were also differences between individuals in tooth structure. Although both femurs and dentition were preserved for only twelve specimens, the researchers were able to identify patterns of different combinations: The reference specimen of Tyrannosaurus rex has robust femurs and only one canine tooth on each side. However, other specimens with only one canine have delicate femurs, and some robust specimens have two canines.
Robust ancestor
In addition, the researchers evaluated the sediment layer in which the respective fossils were found and how old they were accordingly. “In the lowest, i.e. oldest, layer, all femurs were robust,” the researchers report. In the middle layer, in addition to several robust femurs, there was also a delicate one, while in the top, i.e. youngest layer, there were a similar number of delicate and robust thigh bones.
“We suspect that the femur changes evolved over time from a common ancestor that had more robust femurs to become more graceful in later species,” says Paul. “The differences in the robustness of the femurs in the different sediment layers can be considered so marked that the specimens can possibly be considered as distinct species.”
Two new species of tyrannosaurus
Based on their analysis, the scientists describe two other species in addition to the Tyrannosaurus rex with its robust thigh bones and only one canine per jaw side: The oldest species is therefore the Tyrannosaurus imperator, to which the researchers assign the robust thigh bones found in the lower and middle sediment layer became. This species had two canines, according to Paul and his colleagues. Specimens with delicate femurs and only one canine tooth, which were found in the upper and, to a lesser extent, in the middle sediment layer, are called Tyrannosaurus regina by the researchers.
“As it stands, the only specimen that is a T. rex with any degree of certainty is the holotype,” the researchers write. “Some other specimens are also likely members of the species, but the vast majority of Tyrannosaurus specimens, including the most complete specimens, would remain taxonomically in limbo unless assigned to new species.” While the evidence for more than one Tyrannosaurus paleospecies does not meet the highest standards yet achieved for dinosaur speciation, the available taxonomic evidence is more extensive and statistically more robust than most other species assignments in recent dinosaur research.”
Source: Gregory Paul (Baltimore, USA) et al., Evolutionary Biology, doi: 10.1007/s11692-022-09561-5