In March 2022, College of Charleston’s Professor Scott Persons and colleagues suggested that Tyrannosaurus rex should be reclassified as three species: the standard Tyrannosaurus rex, the ‘robust’ Tyrannosaurus imperator, and the ‘gracile’ Tyrannosaurus regina. Now, a research team led by Carthage College’s Dr. Thomas Carr has published a rebuttal to the ‘multiple Tyrannosaurus species’ study.
Tyrannosaurus rex. Image credit: Mark Witton.
“Tyrannosaurus rex remains the one true king of the dinosaurs,” said co-author Dr. Steve Brusatte, a paleontologist at the University of Edinburgh.
In the new research, Dr. Carr, Dr. Brusatte and their colleagues revisited the original data and also added measurements from 112 species of birds, which are living dinosaurs, and from four non-avian theropod dinosaurs.
They found that the multiple Tyrannosaurus species argument was based on a limited comparative sample, non-comparable measurements, and improper statistical techniques.
“The March 2022 study claimed that the variation in Tyrannosaurus rex specimens was so high that they were probably from multiple closely related species of giant meat-eating dinosaur. But this claim was based on a very small comparative sample,” said co-lead author James Napoli, a doctoral student at the American Museum of Natural History.
“When compared to data from hundreds of living birds, we actually found that Tyrannosaurus rex is less variable than most living theropod dinosaurs. This line of evidence for proposed multiple species doesn’t hold up.”
The March 2022 paper declared that variation in the size of the second tooth in the lower jaw, in addition to robustness of the femur, indicated the presence of multiple species.
But the authors of the current study could not replicate the tooth findings, and they recovered different results from their own measurements of the same specimens.
In addition, they took issue with how the ‘breakpoints’ for each species using these traits were statistically determined.
“The boundaries of even living species are very hard to define: for instance, zoologists disagree over the number of living species of giraffe,” said co-author Dr. Thomas Holtz, a researcher at the University of Maryland and the National Museum of Natural History.
“It becomes much more difficult when the species involved are ancient and only known from a fairly small number of specimens.”
“Other sources of variation — changes with growth, with region, with sex, and with good old-fashioned individual differences — have to be rejected before one accepts the hypothesis that two sets of specimens are in fact separate species.”
“In our view, that hypothesis is not yet the best explanation.”
The new findings appear in the journal Evolutionary Biology.
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T.D. Carr et al. Insufficient Evidence for Multiple Species of Tyrannosaurus in the Latest Cretaceous of North America: A Comment on ‘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 July 25, 2022; doi: 10.1007/s11692-022-09573-1
