Nanotyrannus
Temporal range: Late Maastrichtian 68.5–66 Ma
Nanotyrannus lancensis skull.jpg
CMNH 7541 (holotype) skull
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Family: Tyrannosauridae
Subfamily: Tyrannosaurinae
Genus: Nanotyrannus
Bakker, Currie & Williams, 1988
Species:
N. lancensis
Binomial name
Nanotyrannus lancensis
Gilmore, 1946
Synonyms

Nanotyrannus ("dwarf tyrant") is a controversial genus of tyrannosaurid dinosaur. It is known from only a single certain specimen, CMNH 7541, that was originally proposed to be a distinct genus based on a handful of cranial and postcranial features.[1] It has since been suggested to be juvenile of the contemporary species Tyrannosaurus rex, based on a lack of mature specimens and the proposed diagnostic traits being variable within the species,[2][3] a conclusion that is not universally accepted.[4]

History

"Jane" (BMRP 2002.4.1) specimen in the Burpee Museum

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.[5] 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.[6] 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.[2][7]

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.[8][9][10] 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.[1][4] 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).[11]

In late 2011, news reports about a 2006 discovery of a new, virtually complete tyrannosaurid specimen found along with a ceratopsid were made.[12] 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.[11] 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.[13] Ownership of the specimen, which is disputed, will be decided by the Montana Supreme Court.[14]

Classification and validity

Restoration

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.[2] 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.[7]

Replica of the skull of "Jane", Burpee Museum of Natural History. The specimen shows the small foramen in the quadratojugal.

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.[15] 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.[11] 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.[16]

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.[17] However, paleontologist Manabu Sakomoto has commented that this conclusion may be impacted by low sample size, and the discrepancy does not necessarily reflect taxonomic distinction.[18]

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.[19] 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.[20][21]

See also

References

  1. ^ a b Larson (2005). "A case for Nanotyrannus." In "The origin, systematics, and paleobiology of Tyrannosauridae", a symposium hosted jointly by Burpee Museum of Natural History and Northern Illinois University.
  2. ^ a b c Carr, T.D. (1999). "Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria)". Journal of Vertebrate Paleontology. 19 (3): 497–520. doi:10.1080/02724634.1999.10011161.
  3. ^ Woodward, Holly N.; Tremaine, Katie; Williams, Scott A.; Zanno, Lindsay E.; Horner, John R.; Myhrvold, Nathan (2020). "Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy "Nanotyrannus" and supports ontogenetic niche partitioning in juvenile Tyrannosaurus". Science Advances. 6 (1). doi:10.1126/sciadv.aax6250. ISSN 2375-2548.
  4. ^ a b Larson P (2013), "The validity of Nanotyrannus Lancensis (Theropoda, Lancian - Upper Maastrichtian of North America", Society of Vertebrate Paleontology: 73rd annual meeting, Abstracts with Programs, p. 159.
  5. ^ Gilmore, C.W. (1946). "A new carnivorous dinosaur from the Lance Formation of Montana". Smithsonian Miscellaneous Collections. 106: 1–19.
  6. ^ Bakker, R.T.; Williams, M.; Currie, P.J. (1988). "Nanotyrannus, a new genus of pygmy tyrannosaur, from the latest Cretaceous of Montana". Hunteria. 1: 1–30.
  7. ^ a b Tsuihiji, T.; Watabe, M.; Tsogtbaatar, K.; Tsubamoto, T.; Barsbold, R.; Suzuki, S.; Lee, A.H.; Ridgely, R.C.; Kawahara, Y.; Witmer, L.M. (2011). "Cranial osteology of a juvenile specimen of Tarbosaurus bataar from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia". Journal of Vertebrate Paleontology. 31 (3): 497–517. doi:10.1080/02724634.2011.557116.
  8. ^ Currie, P.J. (2003a). "Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada". Acta Palaeontologica Polonica. 48: 191–226.
  9. ^ Currie, Henderson, Horner and Williams (2005). "On tyrannosaur teeth, tooth positions and the taxonomic status of Nanotyrannus lancensis." In "The origin, systematics, and paleobiology of Tyrannosauridae", a symposium hosted jointly by Burpee Museum of Natural History and Northern Illinois University.
  10. ^ Henderson (2005). "Nano No More: The death of the pygmy tyrant." In "The origin, systematics, and paleobiology of Tyrannosauridae", a symposium hosted jointly by Burpee Museum of Natural History and Northern Illinois University.
  11. ^ a b c "Dino Death Match" (2015) Part of Jurassic Week. National Geographic Channel (UK). 13/06/2015
  12. ^ Switek B, "Montana’s "Dueling Dinosaurs", blogs.smithsonianmag.com, 10-11-2011.
  13. ^ https://cosmosmagazine.com/palaeontology/the-breaking-of-america-s-fossil-monument
  14. ^ https://www.the-scientist.com/news-opinion/who-owns-the-dueling-dinos--montana-supreme-court-to-decide-66141
  15. ^ Larson P (2013), "The validity of Nanotyrannus Lancensis (Theropoda, Lancian - Upper Maastrichtian of North America", Society of Vertebrate Paleontology: 73rd annual meeting, Abstracts with Programs, p. 159.
  16. ^ Yun, C (2015). "Evidence points out that "Nanotyrannus" is a juvenile Tyrannosaurus rex"". PeerJ PrePrints. 3: e1052. doi:10.7287/peerj.preprints.852v1.
  17. ^ Persons, W. S.; Currie, P. J. (2016). "An approach to scoring cursorial limb proportions in carnivorous dinosaurs and an attempt to account for allometry". Scientific Reports. 6. 19828. Bibcode:2016NatSR...619828P. doi:10.1038/srep19828. PMC 4728391. PMID 26813782. open access
  18. ^ "Hind limb proportions do not support the validity of Nanotyrannus". mambobob-raptorsnest.blogspot.com.
  19. ^ Woodward, Holly N.; Tremaine, Katie; Williams, Scott A.; Zanno, Lindsay E.; Horner, John R.; Myhrvold, Nathan (2020). "Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy "Nanotyrannus" and supports ontogenetic niche partitioning in juvenile Tyrannosaurus". Science Advances. 6 (1). doi:10.1126/sciadv.aax6250. ISSN 2375-2548.
  20. ^ Yun, C (2015). "Evidence points out that "Nanotyrannus" is a juvenile Tyrannosaurus rex"". PeerJ PrePrints. 3: e1052. doi:10.7287/peerj.preprints.852v1.
  21. ^ D. Carr, Thomas (2013-09-15). ""NANOTYRANNUS ISN'T REAL, REALLY"". Tyrannosauroidea Central. Retrieved 2019-05-28.

External links

  • TEDx talk by Jack Horner on shape-shifting dinosaur skulls and dinosaur misclassification.
  • "Jane" juvenile tyrannosaur on Cleveland Museum of Natural History
  • BBC website article