Iren Dabasu Formation

The Iren Dabasu Formation is a Mesozoic geologic formation in the Iren Nor region of Inner Mongolia. Dinosaur remains diagnostic to the genus level are among the fossils that have been recovered from the formation.^[1] The Upper Cretaceous Iren Dabasu Formation in the Iren Nor region of Inner Mongolia is typically referred to continental clastic sediments consisting of light grey fine sandstones, coarse sandstones and glutenites as well as mottled claystones and siltstones. This lithological unit has yielded plentiful dinosaur remains in quantity and group, also including a great deal of microfossil material. The stratigraphic sequence, sedimentary system, and the morphologies and assemblages of the paleontological taxa are summarized on the basis of previous studies on the Iren Dabasu Formation. The fine-grained floodplain sediments and the coarse-grained sediments of the point bar formed a series of repeated frequently binary sedimentary rhythms. The “binary structure” of the sedimentary rhythms strongly indicates meandering stream deposits rather than braided river deposits as previously thought. The ostracod and charophyte assemblages of the Iren Dabasu Formation has suggested a potential correlation with those of the Sifangtai Formation and the basal Mingshui Formation in the Songliao Basin (mid/late Campanian). Charophytes can be notoriously unreliable, however, as the Nemegt was once considered Santonian on the basis of such evidence--a glaring error. Vertebrates point to an older date than middle-late Campanian. The turtle Khunnuchelys is known from both Iren Dabasu and Baynshirenian-equivalent units such as the Bostobe and Bissetky. In addition, a giant Caenagnathid similar to Gigantoraptor is now known from the Baynshirenian beds of Tsagaan Teg. Like the coeval Baynshiree (or possibly Javkhlant) Formation in the Gobi, the dinosaur fauna of the Iren Dabasu Formation includes tyrannosaurs, ornithomimids, therizinosaurs and oviraptors. Gigantism, notably in the case of Gigantoraptor, may have been due the humid climate of the Iren Nor region during the Late Cretaceous. It may also have been due to weak contemporary tyrannosaurs, as predator decline tends to maximize herbivore diversity. ^[2]

Paleofauna

Dinosaur eggs are known from the formation.

Ornithischians

Ornithischians reported from the Iren Dabasu Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Bactrosaurus[3]	B. johnsoni[3]			"Disarticulated, associated skull and postcranial material pertaining to at least [twelve] individuals, juvenile to adult."[4]		Bactrosaurus
Gilmoreosaurus[3]	G. mongoliensis[3]			"Isolated skull material, associated, disarticulated postcrania, at least [eight] individuals."[4]

Saurischians

Saurischians reported from the Iren Dabasu Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Alectrosaurus[3]	A. olseni[3]					Avimimus Mononykus Velociraptor
Archaeornithomimus[3]	A. asiaticus[3]			"Partial manus, metatarsus, vertebrae, limb elements."[5]
Avimimus[3]	A. portentosus[3]
Erliansaurus[3]	E. bellamanus[3]			"Partial skeleton."[6]
Garudimimus[7]	Indeterminate[7]
Gigantoraptor[8]	G. erlianensis[8]
Mononykus[3]	M. olecranus[3]
Neimongosaurus[3]	N. yangi[3]			"[Two] specimens with most of the axial column, many limb and girdle elements, [and a] dentary."[6]
Saurornithoides[3]	Indeterminate[3]
Velociraptor[3]	Indeterminate[3]

See also

• List of dinosaur-bearing rock formations

Footnotes

References

• Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. ISBN 0-520-24209-2.