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Bactrosaurus; Temporal range: Late Cretaceous (Cenomanian – Santonian) 96–85 Ma PreꞒ Ꞓ O S D C P T J K Pg N
	Skeleton on display at the Central Museum of Mongolian Dinosaurs, Ulaanbaatar
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Neornithischia
Clade:	†Ornithopoda
Clade:	†Hadrosauromorpha
Genus:	†Bactrosaurus; Gilmore, 1933
Type species
	†Bactrosaurus johnsoni; Gilmore, 1933
Other species
	†B. kysylkumensis? (Riabinin, 1931 [originally Cionodon]);
Synonyms
	"Bakesaurus" Zhou, 2005 (nomen nudum); Cionodon kysylkumensis? Riabinin, 1931;

Bactrosaurus (/ˌbæktrəˈsɔːrəs/; meaning "Club lizard," "baktron" = club + sauros = lizard) is a genus of herbivorous hadrosauroid dinosaur that lived in Asia during the Late Cretaceous, from about 96 to 85 million years ago. The position Bactrosaurus occupies in the Cretaceous makes it one of the earliest known hadrosauroids, and although it is not known from a full skeleton, Bactrosaurus is one of the best known of these early hadrosauroids, making its discovery a significant finding.

Discovery

Restored skeleton displayed at the Hong Kong Science Museum

The first Bactrosaurus remains recovered from the Iren Dabasu Formation in the Gobi Desert of China were composed of partial skeletons of six individual B. johnsoni. The specimens collected appear to come from a variety of age groups, from individuals that may be hatchlings to full-sized adults. The fossils were described in 1933 by Charles W. Gilmore, who named the new animal Bactrosaurus, or "club lizard", in reference to the large club-shaped neural spines projecting from some of the vertebrae.^[1] The Iren Dabasu Formation has been dated to the Cenomanian stage, around 95.8 ± 6.2 million years ago.^[2]

No complete remains have yet to be uncovered, but Bactrosaurus is still better known than most of the early hadrosaurs. Known parts of the anatomy of Bactrosaurus include the limbs, pelvis, and most of the skull (although the crest is notably absent).^[3]

"Bakesaurus" is an informal name based on a maxilla from the Majiacun Formation of China assigned to Bactrosaurus in 2001.^[4] The nomen nudum was created and pictured in a Chinese-language book by Zhou (2005)^[5] and first surfaced on the Internet during February 2006 when it was mentioned on the Dinosaur Mailing List by Jerry D. Harris.^[6]

Description

A typical Bactrosaurus would have been 6–6.5 m (20–21 ft) long and weighed 1.2 metric tons (1.3 short tons).^[7]^[8] It was an early relative of Lambeosaurus and shows a number of iguanodont-like features, including three stacked teeth for each visible tooth, small maxillary teeth, and an unusually powerful build for a hadrosaur. It shows features intermediate between those of the two main hadrosaurid groups.^[3]^[9] Its femur measured 80 centimetres (2.6 ft) long.^[1]

Bactrosaurus was originally thought to be a lambeosaurine hadrosaurid, supposedly the oldest and most primitive known, and its crestless head was seen as an anomaly. A 1990 popular book suggested that it had an incompletely preserved crest,^[3] but recent studies place Bactrosaurus as a more basal hadrosauromorph.^[10] Basal members do not preserve hollow crests, so Bactrosaurus itself is likely to be crestless.

Paleobiology

In 2003, evidence of tumors, including hemangiomas, desmoplastic fibroma, metastatic cancer, and osteoblastoma was discovered in fossilized Bactrosaurus skeletons. Rothschild et al. tested dinosaur vertebrae for tumors using computerized tomography and fluoroscope screening. Several other hadrosaurids, including Brachylophosaurus, Gilmoreosaurus, and Edmontosaurus, also tested positive. Although more than 10,000 fossils were examined in this manner, the tumors were limited to Bactrosaurus and closely related genera. The tumors may have been caused by environmental factors or genetic propensity.^[11]

References

^ ^a ^b Gilmore, C. W. (1933). "On the dinosaurian fauna of the Iren Dabasu Formation". Bulletin of the American Museum of Natural History. 67 (2): 23–78. hdl:2246/355.
^ Guo, Z. X.; Shi, Y. P.; Yang, Y. T.; Jiang, S. Q.; Li, L. B.; Zhao, Z. G. (2018). "Inversion of the Erlian Basin (NE China) in the early Late Cretaceous: Implications for the collision of the Okhotomorsk Block with East Asia" (PDF). Journal of Asian Earth Sciences. 154: 49–66. Bibcode:2018JAESc.154...49G. doi:10.1016/j.jseaes.2017.12.007. Archived from the original (PDF) on 2020-09-19. Retrieved 2020-05-02.
^ ^a ^b ^c "Bactrosaurus." In: Dodson, Peter & Britt, Brooks & Carpenter, Kenneth & Forster, Catherine A. & Gillette, David D. & Norell, Mark A. & Olshevsky, George & Parrish, J. Michael & Weishampel, David B. The Age of Dinosaurs. Publications International, LTD. p. 131. ISBN 0-7853-0443-6.
^ Li, Z. (2001). "Distribution, burying and classification of dinosaur fossils in Upper Cretaceous strata at Meipu Town, Yunxian County of Hubei Province". Hubei Geology & Mineral Resources. 15 (4): 25–31.
^ Zhou, S.Q.D. (2005). The Dinosaur Egg Fossils in Nanyang, China. China University of Geosciences Press. pp. 1–145. ISBN 978-7-562-52033-7.
^ Harris, Jerry D., 2006. "New Dinosaurs?" Archived 2016-02-22 at the Wayback Machine
^ Paul, G. S. (2016). The Princeton Field Guide to Dinosaurs (2nd ed.). Princeton, New Jersey: Princeton University Press. pp. 151−152. ISBN 9780691167664.
^ Prieto-Marquez, A. (2011). "Cranial and appendicular ontogeny of Bactrosaurus johnsoni, a hadrosauroid dinosaur from the Late Cretaceous of northern China". Palaeontology. 54 (4): 773−792. Bibcode:2011Palgy..54..773P. doi:10.1111/j.1475-4983.2011.01053.x.
^ Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 146. ISBN 1-84028-152-9.
^ Prieto-Márquez, Albert; Carrera Farias, Miguel (2021). "A new late-surviving early diverging Ibero-Armorican duck-billed dinosaur and the role of the Late Cretaceous European Archipelago in hadrosauroid biogeography". Acta Palaeontologica Polonica. 66. doi:10.4202/app.00821.2020.
^ Rothschild, B. M.; Tanke, D. H.; Helbling II, M.; Martin, L. D. (2003). "Epidemiologic study of tumors in dinosaurs" (PDF). Naturwissenschaften. 90 (11): 495–500. Bibcode:2003NW.....90..495R. doi:10.1007/s00114-003-0473-9. PMID 14610645. S2CID 13247222.