An analysis of over three dozen specimens of Tyrannosaurus rex finds that they exhibit such a remarkable degree of proportional variations that the pattern favors multiple species at least partly separated by time.
Tyrannosaurus rex, or T. rex for short, is the only recognized species of the dinosaur genus Tyrannosaurus to date.
“The name Tyrannosaurus rex, has two parts,” said Professor Scott Persons, a paleontologist in the Department of Geology and Environmental Geosciences at the College of Charleston.
“Tyrannosaurus is the name of the genus. The rex identifies a species within that genus.”
“Normally, it’s a dinosaur’s genus name that everybody knows: Triceratops, Stegosaurus, Velociraptor. T. rex is an exception.”
“We all know the full name, genus and species. Maybe that’s because the species name is short and sweet; maybe it’s because the full name is so evocative and just plain fun to say.”
In the new research, Professor Persons and his two colleagues, U.S. paleontologist Dr. Gregory Paul and Dr. Jay Van Raalte from the Department of Geology and Environmental Geosciences at the College of Charleston, analyzed the bones and dental remains of 37 Tyrannosaurus specimens.
They compared the variability of the data to that of another large carnivorous dinosaur, Allosaurus fragilis.
Unlike the Tyrannosaurus data, which came from fossil sites scattered across the continent, the 14 Allosaurus fragilis skeletons all came from a single spot: the Cleveland-Lloyd Dinosaur Quarry in Utah.
Being from one spot at one point in time, the Allosaurus skeletons are assumed to be one species.
Sure enough, the Allosaurus data were far less variable, indicating the differences in heft observed in Tyrannosaurus were beyond what should be expected in just one species.
When the paleontologists went on to compare the skeletal proportions of Tyrannosaurus with those of its closest relatives (other two-fingered tyrannosaurids like Albertosaurus, Daspletosaurus and Tarbosaurus), the Tyrannosaurus data still came out as unusually variable. But there was another big surprise.
“For about half the specimens, the proportions are far more gracile than what you would expect for a tyrannosaurid or other carnivorous dinosaur of that size,” Professor Persons said.
“That confused the heck out of me. Tyrannosaurus is the biggest of the tyrannosaurids, so you’d think it would be the most robust.”
As animals get bigger, their bones have to support more weight and endure the forces imposed by their heavy bodies while moving.
But, big heavy animals tend to be slower, making it harder to chase and capture prey.
“Instead of adapting their bodies to deal with the greater physical constraints, it’s as though the animals were adapting to deal with greater ecological constraints,” Professor Persons said.
“Rather than compensate for the greater risks and strains of growing big, the gracile Tyrannosaurus has a leaner frame that may have helped it maintain athletic performance even at large size.”
Finally, the researchers divided all the Tyrannosaurus specimens up based on time.
A few patterns emerged: no gracile Tyrannosaurus skeletons were known from older, lower rocks layers, and neither were specimens with just one chisel-like tooth set; all the gracile specimens were from a younger point in time and also had a single set of chisel-like teeth.
This suggests that the burlier and double chisel-toothed form found in the older layers was one species that gave rise to another.
The young gracile descendants have been dubbed Tyrannosaurus regina (meaning ‘tyrant lizard queen’), and, continuing the royal theme, the ancestral species has been christened Tyrannosaurus imperator (meaning ‘tyrant lizard emperor’).
But what about Tyrannosaurus rex? Well, the gracile Tyrannosaurus regina was not the only Tyrannosaurus of its time.
Also found in the younger rocks were burly specimens, but unlike the older Tyrannosaurus imperator, they all also bear only one chisel-like tooth set. It is to this third group that the first-discovered skeleton named Tyrannosaurus rex belongs.
“We found that the changes in Tyrannosaurus femurs are likely not related to the sex or age of the specimen,” Dr. Paul said.
“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.”
“We cannot rule out that the observed variation is due to extreme individual differences, or atypical sexual dimorphism, rather than separate groups,” the authors said.
“And we also caution that the location within sediment layers is not known for some specimens.”
“The physical variation found in Tyrannosaurus specimens combined with their stratigraphy are indicative of three potential groups that could be nominated as two new species, Tyrannosaurus imperator and Tyrannosaurus regina, alongside the only recognized species to date, Tyrannosaurus rex.”
The study appears in the journal Evolutionary Biology.
G.S. Paul et al. The Tyrant Lizard King, Queen and Emperor: Multiple Lines of Morphological and Stratigraphic Evidence Support Subtle Evolution and Probable Speciation Within the North American Genus Tyrannosaurus. Evol Biol, published online March 1, 2022; doi: 10.1007/s11692-022-09561-5
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