According to a study published in Evolutionary Biology, a new analysis of Tyrannosaurus skeletal remains reveals physical differences in the femur, other bones, and dental structures across specimens, suggesting that Tyrannosaurus rex specimens should be re-categorized into three distinct groups or species.
To date, Tyrannosaurus rex is the sole known species of the Tyrannosaurus genus of dinosaurs. Previous study has noted differences in Tyrannosaurus femur (thighbone) skeletal remains and specimens having one or two narrow incisor teeth on either side of the front ends of the jaw.
The bones and dental remnants of 37 Tyrannosaurus specimens were studied by Gregory Paul and colleagues. The researchers compared the robustness of the femur in 24 of the specimens, which is a measurement based on the length and circumference of the bone and indicates its strength. They also checked for one or two narrow incisiform teeth by measuring the diameter of the base of the teeth or the space between the gums.
The femurs differed amongst specimens, with some having more robust femurs and others having more gracile femurs, according to the investigators. The investigators discovered that robust femurs were two times more common than gracile femurs across all specimens, indicating that the difference is not due to sex, which would likely result in a more equal split. The authors further claim that femur variation is unrelated to specimen growth, citing the discovery of robust femurs in juvenile specimens two-thirds the size of an adult and gracile femurs in specimens that were full adult size.
The dental anatomy of the specimens varied as well, however the number of specimens possessing both femur measures and dental remnants was minimal (12 specimens). Specimens with one incisor tooth were shown to have greater femur gracility more often.
In the Lancian upper Masstrichtian strata in North America, 28 Tyrannosaurus remains were found in separate levels of silt (stratigraphy) (estimated to be from between 67.5 to 66 million years ago). Tyrannosaurus specimens were compared to other theropod species identified in lower strata of silt by the researchers.
In the lowest layer of silt, only strong Tyrannosaurus femurs were discovered (six femurs). The variance in femur robustness in the bottom stratum was similar to that of other theropod species, indicating that just one Tyrannosaurus species existed at the time. Only one gracile Tyrannosaurus femur was found in the intermediate layer, with five more gracile femurs and additional robust femurs found in the top layer. In the uppermost layer of the sediments, the range in Tyrannosaurus femur robustness was greater than in some earlier theropod fossils. This shows that, in comparison to lower-layer individuals and other dinosaur species, Tyrannosaurus specimens discovered at higher strata of soil grew into more differentiated forms.
The main author, Gregory Paul, stated: “We discovered that alterations in Tyrannosaurus femurs are unlikely to be connected to the specimen’s sex or age. We believe that the alterations in the femur developed through time from a common ancestor with more robust femurs to subsequent species with more gracile femurs. The changes in femur toughness across sediment levels may be substantial enough that the specimens may be classified as different species.”
Based on their findings, the authors propose two new Tyrannosaurus species. Tyrannosaurus imperator (tyrant lizard emperor) refers to specimens discovered in the lower and intermediate levels of sediment, and is distinguished by more robust femurs and frequently two incisor teeth. These characteristics, according to the authors, were passed down from previous relatives (tyrannosaurids). The second, Tyrannosaurus regina (tyrant lizard queen), had slenderer femurs and just one incisor tooth, and is found in higher and maybe intermediate levels of sediment. Tyrannosaurus rex (tyrant lizard king) was discovered in the top and probably middle layers of the sediment, with individuals classified as having more robust femurs but only one incisor tooth. Because some individuals’ remnants could not be recognized, they were not attributed to a species.
The authors admit that they can’t rule out the possibility that the observed variance is attributable to severe individual variations or abnormal sexual dimorphism rather than distinct groups, and they also warn that the position of certain specimens within sediment layers is unknown. The authors talk about how difficult it is to designate prehistoric vertebrates to a new species.