With its colossal size, some 12 meters long, weighing between 6 and 9 tons , dagger-shaped teeth and razor-sharp claws, Tyrannosaurus rex was the best-known prehistoric predator on Earth. Now, a team of scientists studying its fossils have suggested that this creature may not have been the only species of tyrannosaurus, but could instead be three different species.
One of the most popular dinosaurs
American researchers reanalyzed nearly 40 fossilized Tyrannosaurus rex skeletons found by paleontologists over the course of more than a century. Specimens studied included ‘Sue’, a complete Tyrannosaurus skeleton now housed at the Field Museum of Natural History in Chicago, and ‘AMNH 5027’, found in Big Dry Creek, Montana, in 1908.
They discovered, in the studied specimens, physical differences in the femur, as well as in the dental structures and other bones, clues that suggest three different species of Tyrannosaurus and not one, as previously believed. Examination of the fossilized femurs revealed that some were slimmer while others were more robust, with a ratio of 1:2 respectively among the 24 femora analyzed. And it was not a difference attributable to sex or mixing of juveniles and adults (as they found robust femurs in some two-thirds adult-size juveniles and lighter femurs in some full-size adult specimens), which meant that there was something else at play to explain this variation. They also measured the diameter of the base of the teeth or the space in the gums to assess whether the samples had one or two thin incisor teeth. Tooth structure also varied between specimens , although there were a fairly low number of specimens with femur measurements and dental remains available for study (12 specimens).
If neither sex nor age explained this discrepancy… what made the difference? three species.
Three species!
Until now, differences in the skeletal morphology of Tyrannosaurus specimens have been attributed primarily to sexual dimorphism, when male and female members of the same species appear different. But only one last possible explanation is the most surprising: the differences represent distinct species within the genus Tyrannosaurus.
According to the experts, who publish their study in the journal Evolutionary Biology, the largest specimens found should be attributed to a new species called Tyrannosaurus imperator ; while the smaller and more slender specimens should be assigned to a species called Tyrannosaurus regina.
“We found that the changes in the Tyrannosaurus femurs are probably 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 that displayed more robust femurs to become more gracile in later species. Differences in the robustness of the femur across layers of sediment can be considered distinct enough that the specimens can be considered separate species,” says paleontologist Gregory S. Paul (and dinosaur specialist from the movie Jurassic Park).
The team says that it is to be expected that more than one species of tyrannosaurus evolved during the million years that it walked the Earth, as has been discovered for other dinosaurs that lived at the same time, such as the well-known triceratops.
While paleontologists say they cannot rule out other explanations for these findings, not the entire scientific community seems to be convinced by this new theory and they question the results . In fact, the authors acknowledge that they cannot rule out that the differences in the bones are due to extreme individual differences or atypical sexual dimorphism, rather than separate species, considering that this finding would have multiple implications.
Referencia: Paul, G.S., Persons, W.S. & Van Raalte, J. 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 (2022). https://doi.org/10.1007/s11692-022-09561-5
