1. History

"Jane" (BMRP 2002.4.1) specimen in the Burpee Museum. https://handwiki.org/wiki/index.php?curid=1811817

Nanotyrannus is based on CMNH 7541, a skull collected in 1942 by David Hosbrook Dunkle and described by Charles W. Gilmore in 1946, who classified it as a new species in the tyrannosaur genus Gorgosaurus as G. lancensis.^[1] In 1988, the specimen was re-described by Robert T. Bakker, Phil Currie, and Michael Williams, then the curator of paleontology at the Cleveland Museum of Natural History, where the original specimen was housed and is now on display. Their initial research indicated that the skull bones were fused, and that it therefore represented an adult specimen. In light of this, Bakker and colleagues assigned the skull to a new genus, named Nanotyrannus for its apparently small adult size. The specimen is estimated to have been around 5.2 metres (17 ft) long when it died.^[2] However, a detailed analysis of the specimen by Thomas Carr in 1999 showed that the specimen was, in fact, a juvenile, leading Carr and many other paleontologists to consider it a juvenile specimen of Tyrannosaurus rex.^[3][4]

In 2001, a more complete juvenile tyrannosaur (nicknamed "Jane", catalog number BMRP 2002.4.1), belonging to the same species as the original Nanotyrannus specimen, was uncovered. This discovery prompted a conference on tyrannosaurs focused on the issues of Nanotyrannus validity, held at the Burpee Museum of Natural History in 2005. Several paleontologists who had previously published opinions that N. lancensis was a valid species, including Currie and Williams, saw the discovery of "Jane" as a confirmation that Nanotyrannus was, in fact, a juvenile T. rex.^[5][6][7] Peter Larson continued to support the hypothesis that Nanotyrannus lancensis was a separate but closely related species, and also argued that Stygivenator, which is generally considered to be a juvenile of Tyrannosaurus rex, could be a younger specimen of Nanotyrannus.^[8][9] Jane probably died at around 11 years of age and still growing based on bone histology, and weighed roughly 600–900 kg (1,300–2,000 lb).^[10]

In late 2011, news reports about a 2006 discovery of a new, virtually complete tyrannosaurid specimen found along with a ceratopsid were made.^[11] The specimens were studied by Robert Bakker and Larson on-site, who identified the ceratopsian as Triceratops and the tyrannosaurid as Nanotyrannus. The specimen, nicknamed "Bloody Mary", allegedly has arms almost 3 feet in length, with the bones of the hand said to be one and a half times longer than those of the T. rex specimen "Sue". However, the truth to those claims, and whether the specimen is distinct from T. rex, are currently impossible to determine, as the "Bloody Mary" specimen remains in private hands and away from scientific access.^[10] Paleontologist Jack Horner has gone as far as to claim that the "Bloody Mary" specimen is "scientifically useless" as a result of it being collected privately, and not by academics trained to preserve data from the surrounding environment.^[12] Ownership of the specimen, which is disputed, will be decided by the Montana Supreme Court.^[13]

2. Classification and Validity

Restoration. https://handwiki.org/wiki/index.php?curid=1454293

The primary difference that some scientists have used to argue for Nanotyrannus lancensis's validity concerns the number of teeth. Specimens referred to N. lancensis had 14–15 teeth in each side of the upper jaw (maxilla) and 17 teeth in each side of the lower jaw (dentary), whereas adult T. rex specimens had 11–12 tooth positions in the upper jaw and 11–14 in the lower. The exact implications of this difference in tooth count have not been universally accepted, however. In his 1999 study of tyrannosaurid growth patterns, Carr showed that, in Gorgosaurus libratus, the number of teeth decreased as the animal grew, and he used this data to support the hypothesis that N. lancensis is simply a juvenile T. rex.^[3] Tsujihi et al., who studied growth in the related Tarbosaurus bataar, found little to no decrease in tooth count during growth, even though they had juvenile specimens much younger than the Nanotyrannus specimens. These researchers also noted, however, that both Tyrannosaurus and Gorgosaurus show significant differences in tooth count between individuals of the same age group, and that tooth count may vary on an individual basis not related to growth.^[4]

Replica of the skull of "Jane", Burpee Museum of Natural History. The specimen shows the small foramen in the quadratojugal. https://handwiki.org/wiki/index.php?curid=1961948

Larson has also contended that, along with skull features, Nanotyrannus can also be distinguished from Tyrannosaurus by proportionally larger hands with phalanges on the third metacarpal and in the furcula morphology.^[14] 3D models of the brain cavities show that blood vessel positions and those of the optic nerve attachments do not match between CMNH and Tyrannosaurus, possibly suggesting a distinction.^[10] Another difference cited in support of N. lancensis validity is the presence of a small foramen in the quadratojugal. Both the holotype and the "Jane" specimen have this feature, which is also seen in the adult specimen of Daspletosaurus horneri and an isolated quadratojugal from Alberta indicating that this feature is seen in other tyrannosaurs.^[15]

Limb proportion analysis published in 2016 suggested that Nanotyrannus specimens have differing levels of cursoriality, cited as a potential difference between N. lancensis and T. rex.^[16] However, paleontologist Manabu Sakomoto has commented that this conclusion is likely impacted by low sample size, and the discrepancy does not necessarily reflect taxonomic distinction.^[17]

A 2020 study by Woodward et al. shows the specimens referred to Nanotyrannus are ontogenetically immature and found it probable that these specimens belonged to Tyrannosaurus rex.^[18] All of the differences claimed to support Nanotyrannus have turned out to be individually or ontogenetically variable features or products of distortion of the bones.^[19][20]