a 2022 study, Gregory S. Paul and colleagues argued that Tyrannosaurus rex, as traditionally understood, actually represents three species: the type species Tyrannosaurus rex, and two new species: T. imperator (meaning "tyrant lizard emperor") and T. regina (meaning "tyrant lizard queen"). The holotype of the former (T. imperator) is the Sue specimen, and the holotype of the latter (T. regina) is Wankel rex. The division into multiple species was primarily based on the observation of a very high degree of variation in the proportions and robusticity of the femur (and other skeletal elements) across catalogued T. rex specimens, more so than that observed in other theropods recognized as one species. Differences of general body proportions representing robust and gracile morphotypes were also used as a line of evidence, in addition to the number of small, slender incisiform teeth in the dentary, as based on tooth sockets. Specifically, the paper's T. rex was distinguished by robust anatomy, a moderate ratio of femur length vs circumference, and the possession of a singular slender incisiform dentary tooth; T. imperator was considered to be robust with a small femur length to circumference ratio and two of the slender teeth; and T. regina was a gracile form with a high femur ratio and one of the slender teeth. It was observed that variation in proportions and robustness became more extreme higher up in the sample, stratigraphically. This was interpreted as a single earlier population, T. imperator, speciating into more than one taxon, T. rex and T. regina. However, several other leading paleontologists, including Stephen Brusatte, Thomas Carr, Thomas Holtz, David Hone, Jingmai O'Connor, and Lindsay Zanno, criticized the study or expressed skepticism of its conclusions when approached by various media outlets for comment. Their criticism was subsequently published in a technical paper. Holtz and Zanno both remarked that it was plausible that more than one species of Tyrannosaurus existed, but felt the new study was insufficient to support the species it proposed. Holtz remarked that, even if Tyrannosaurus imperator represented a distinct species from Tyrannosaurus rex, it may represent the same species as Nanotyrannus lancensis and would need to be called Tyrannosaurus lancensis. O'Connor, a curator at the Field Museum, where the T. imperator holotype Sue is displayed, regarded the new species as too poorly-supported to justify modifying the exhibit signs. Brusatte, Carr, and O'Connor viewed the distinguishing features proposed between the species as reflecting