The dinosaur king may yet retain its crown. Earlier this year a study proposed that the iconic Tyrannosaurus rex should be split into three distinct species, but a new analysis says there isn’t enough evidence for such a drastic shake-up.
Since it was first discovered over a century ago, Tyrannosaurus rex has been the sole species in its genus. But in March, a team of paleontologists proposed that the T. rex was actually one of three species of Tyrannosaurus, along with two new ones the scientists named the T. imperator and the T. regina.
Their reasoning for this split was based on differences in physical characteristics between specimens. Some have a single slender incisor tooth on either side of the front of their jaw, while others have two on each side. Examining the ratio of femur length to circumference, some skeletons have “robust” femurs, while others are more “gracile.” The groupings of these characteristics, as well as the layers of sediment they were found in, apparently suggested the presence of three species.
However, other scientists were quick to question the validity of the findings, and now some of those critics have hit back with their own study. This team, made up of scientists from the American Museum of Natural History and Carthage College, concluded that there isn’t enough evidence to make such a bold claim.
In the new study, the researchers revisited the original data, performing their own measurements and analysis of the same specimens. They found no statistical significance in the clustering of the physical features that clearly indicated multiple species.
The team also says that the original study only compared the Tyrannosaurus features to other similar dinosaurs, which are obviously also extinct and as such impossible to verify their own species status. To rectify that, the researchers compared them to measurements from 112 species of living birds, and found that the level of variation in the T. rex bones was consistent with natural differences between individuals of a single species.
“Their study claimed that the variation in T. rex specimens was so high that they were probably from multiple closely related species of giant meat-eating dinosaur,” said James Napoli, co-lead author of the new study. “But this claim was based on a very small comparative sample. When compared to data from hundreds of living birds, we actually found that T. rex is less variable than most living theropod dinosaurs. This line of evidence for proposed multiple species doesn’t hold up.”
The team acknowledges that it is possible that there could be multiple species of Tyrannosaurus, but as the old adage goes, extraordinary claims require extraordinary evidence.
“The boundaries of even living species are very hard to define: for instance, zoologists disagree over the number of living species of giraffe,” said Thomas Holtz, co-author of the study. “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 research was published in the journal Evolutionary Biology.
