Tyrannosaurus rex: Expert examines brain of dinosaur
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Dinosaur researchers have proposed two potential new species of Tyrannosaurus — Tyrannosaurus imperator, the “tyrant lizard emperor”, and Tyrannosaurus regina, the “tyrant lizard queen” — which lived from around 67.5–66 million years ago. Fossils of T. imperator, which lived earlier in this timespan, are defined by having a robust femur, or thigh bone, and two incisor teeth on each side of the jaw — both more ancestral features — while the younger T. regina has a slender femur and one incisor on each side. T. rex, meanwhile, lived closer to 66 million years ago, alongside T. regina, and sported a more robust thighbone but also only one incisor per side.
In their study, Baltimore-based palaeontologist Gregory Paul and his colleagues analysed the fossilised remains of 38 Tyrannosaurus specimens from North America.
They focussed their analysis on two aspects — the first being the “robustness” of the femur, which is derived from measurements of the bone’s circumference and length.
They also looked at the diameter of the base of each animal’s teeth or the space in their gums to determine if each specimen had one or two slender “incisiform” teeth on each side of the front end of their jaws.
Both of these features have previously been noted by experts to vary significantly across Tyrannosaurus fossil skeletons.
When looking at the femurs, the team noted that there were twice as many robust femurs as more slender — or “gracile” — ones.
This ratio suggests that such disparities in robustness are unlikely to have been caused by sex-related differences, as such would have led to a more even split between the two forms.
Furthermore, the fact that slender femurs were found in adult Tyrannosaurus specimens and robust femurs were found in juveniles only two-thirds the size suggests that the thigh bones were not simply getting more bulky with age.
Instead, it appears that the variations may be indicative of different species.
Mr Paul said: “We found that the changes in Tyrannosaurus femurs are likely not related to the sex or age of the specimen.
“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.”
According to the researchers, while only 12 individual specimens incorporated both a femur and dental remains for analysis, they did find evidence of a correlation between having only one incisor on each side and having a more slender femur.
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Of the Tyrannosaurus specimens included in the study, 29 were recovered from known positions in sediment layers deposited across the end of the Maastrichtian age, around 67.5–66 million years ago.
The team found that the lowest layers of sediment, which are the oldest, contained only Tyrannosaurus specimens with robust femurs.
Comparison with other contemporary theropod dinosaurs suggests that the slight variation in robustness seen among these individuals was unremarkable, indicating that there was likely only one species of Tyrannosaurus alive at this point in time — T. imperator.
However, as the team examined the younger layers, they found that the femurs became more varied, with increasing slender bones alongside their more robust counterparts.
This, they said, suggests that Tyrannosaurus evolved to develop new and distinct forms — T. regina and T. rex — with more variation than might be expected for a single theropod species of the time.
However, the researchers cautioned, it is not yet possible to completely rule out that possibility that the observation variation might be down to extreme individual differences or perhaps an unusual example of distinctions between the sexes.
The full findings of the study were published in the journal Evolutionary Biology.
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