Woodward, Holly N; Tremaine, Katie; Williams, Scott A; Zanno, Lindsay E; Horner, John R; Myhrvold, Nathan (January 1, 2020). "Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy "Nanotyrannus" and supports ontogenetic niche partitioning in juvenile Tyrannosaurus". Science Advances. 6 (1): eaax6250. Bibcode: 2020SciA....6.6250W. doi: 10.1126/sciadv.aax6250. PMC 6938697. PMID 31911944. 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. [70] [71] [72] Their criticism was subsequently published in a technical paper. [73] 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 natural variation within a species. Both Carr and O'Connor expressed concerns about the study's inability to determine which of the proposed species several well-preserved specimens belonged to. Another paleontologist, Philip J. Currie, originally co-authored the study but withdrew from it as he did not want to be involved in naming the new species. [70] Paul rejected the objections raised by critics, insisting that they are unwilling to consider that Tyrannosaurus might represent more than one species. [74] In a subsequent paper awaiting publication, Paul maintained the conclusion that Tyrannosaurus consists of three species. He pointed out that the criticism of the study naming T. imperator and T. regina only focused on two of the features used to distinguish the two new species (the number of small incisiform teeth and femur robustness), while the original study also compared the robustness of other bones as well (the maxilla, dentary, humerus, ilium and metatarsals). Furthermore, Paul argued that Tyrannosaurus can be separated into three different species based on the shape of knob-like bumps ('postorbital bosses') behind the eyes. Paul also argued that past research concluding that Tyrannosaurus only consists of one species ( T. rex) has simply assumed that all Tyrannosaurus skeletons are a single species, and that many new dinosaur species have been named on the basis of fewer differences than he and his colleagues used when proposing T. imperator and T. regina. [75] Nanotyrannus Former holotype of Nanotyrannus lancensis, now interpreted as a juvenile Tyrannosaurus

a b Xing, X.; Norell, M. A.; Kuang, X.; Wang, X.; Zhao, Q.; Jia, C. (October 7, 2004). "Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids" (PDF). Nature. 431 (7009): 680–684. Bibcode: 2004Natur.431..680X. doi: 10.1038/nature02855. PMID 15470426. S2CID 4381777. Main article: Feeding behavior of Tyrannosaurus Tyrannosaurus tooth marks on bones of various herbivorous dinosaurs A Tyrannosaurus mounted next to a Triceratops at the Los Angeles Natural History Museum Thomas Holtz Jr. would note that high depth perception of Tyrannosaurus may have been due to the prey it had to hunt, noting that it had to hunt ceratopsians such as Triceratops, ankylosaurs such as Ankylosaurus, and hadrosaurs. He would suggest that this made precision more crucial for Tyrannosaurus enabling it to, "get in, get that blow in and take it down." In contrast, Acrocanthosaurus had limited depth perception because they hunted large sauropods, which were relatively rare during the time of Tyrannosaurus. [102] Paul, Gregory S.; Persons IV, W. Scott; van Raalte, Jay (2022). "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". Evolutionary Biology. 49 (2): 156–179. doi: 10.1007/s11692-022-09561-5. S2CID 247200214.A study by Grant R. Hurlburt, Ryan C. Ridgely and Lawrence Witmer obtained estimates for Encephalization Quotients (EQs), based on reptiles and birds, as well as estimates for the ratio of cerebrum to brain mass. The study concluded that Tyrannosaurus had the relatively largest brain of all adult non-avian dinosaurs with the exception of certain small maniraptoriforms ( Bambiraptor, Troodon and Ornithomimus). The study found that Tyrannosaurus's relative brain size was still within the range of modern reptiles, being at most 2 standard deviations above the mean of non-avian reptile EQs. The estimates for the ratio of cerebrum mass to brain mass would range from 47.5 to 49.53 percent. According to the study, this is more than the lowest estimates for extant birds (44.6 percent), but still close to the typical ratios of the smallest sexually mature alligators which range from 45.9–47.9 percent. [170] Other studies, such as those by Steve Brusatte, indicate the encephalization quotient of Tyrannosaurus was similar in range (2.0–2.4) to a chimpanzee (2.2–2.5), though this may be debatable as reptilian and mammalian encephalization quotients are not equivalent. [171] Social behavior Mounted skeletons of different age groups (skeleton in lower left based on the juvenile formerly named Stygivenator), Natural History Museum of Los Angeles County Let's add some color now to a story many millions of years old - the story of the dinosaurs. Scientists have been able to learn a lot about dinosaurs by studying fossils. How well do you know SUE?". Field Museum of Natural History. August 11, 2016 . Retrieved December 31, 2018.

Even if T. rex does exhibit evidence of homeothermy, it does not necessarily mean that it was endothermic. Such thermoregulation may also be explained by gigantothermy, as in some living sea turtles. [137] [138] [139] Similar to contemporary crocodilians, openings (dorsotemporal fenestrae) in the skull roofs of Tyrannosaurus may have aided thermoregulation. [140] Soft tissue T.rex femur (MOR 1125) from which demineralized matrix and peptides (insets) were obtained Walton, T. (2016). "Forget all you know from Jurassic Park: For speed, T. rex beats velociraptors". USA Today . Retrieved March 13, 2016.D. Carr, Thomas (September 15, 2013). "Nanotyrannus isn't real, really". Tyrannosauroidea Central . Retrieved May 28, 2019.