Xiagou Formation

The laminated yellowish mudstones of the Xiagou Formation are the lithified remnants of varves that were laid down as extremely fine silt settled to the bottom of a tranquil freshwater lake. The result was dense anoxic bottom sediment, where the lack of bacteria slowed the processes of decay, preserving uncompressed fossils in details that include feather impressions and remnants of the webbing between the bird's toes. The age of the formation has not yet been confidently determined. The underlying Chijinpu Formation is likely the same age as the Jehol Group due to the presence of similar fossils, meaning that the Xiagou Formation is probably slightly younger than the Jehol biota, dating to around the late Aptian.^[1][2]

The Xiagou Formation is particularly noted for its high diversity of ancient birds. These include both modern birds close to the ancestors of living species, and related lineages now entirely extinct. Other fossils from the Xiagou Formation are characteristic of an Early Cretaceous lake ecology. There are fossils of abundant fish fauna, Charophyta and ostracods.^[3]

Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Avimaia[7]	A. schweitzerae[7]	Changma Basin		"Partial skeleton with an egg."[7]	An enantiornithean.
Brevidentavis[8]	B. zhangi[8]			Incomplete skull and cervical vertebrae.[8]	An ornithuromorph.
Changmaornis[9]	C. houi[9]	Changma Basin		"Partial pelvic girdle with hindlimb."[9]	An euornithian.
Dunhuangia[10]	D. cuii[10]	Changma Basin		"Forelimbs."[10]	An enantiornithean.
Feitianius[11]	F. paradisi[11]	Changma Basin		"Hindlimbs with pelvic area and vertebrae."[11]	An enantiornithean.
Gansus[3]	G. yumenensis[3]			"Partial hindlimbs."[3]	An euornithian.
Jiuquanornis[9]	J. niui[9]	Changma Basin		"Partial pectoral girdle."[9]	An euornithian.
Meemannavis[8]	M. ductrix[8]			Incomplete skull and cervical and thoracic vertebrae.[8]	An ornithuromorph.
Qiliania[12]	Q. graffini[12]	Changma Basin		"Partial skeleton from two individuals."[12]	An enantiornithean.
Unnamed enantiornithean	Indeterminate			"Partial forelimb."[13]	An enantiornithean.
Yumenornis[9]	Y. huangi[9]	Changma Basin		"Right forelimb."[9]	An euornithian.

Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Beishanlong[14]	B. grandis[14]	Yujingzi Basin	Gray-variegated beds	"Fore and hindlimbs with partial vertebrae."[14]	An ornithomimosaur.

Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Suzhousaurus[15]	S. megatherioides[15]	Yujingzi Basin	Gray-variegated beds	"Dorsal vertebrae, ribs and partial shoulder girdle."[15]	A therizinosauroid.

Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Xiongguanlong[16]	X. baimoensis[16]	Yujingzi Basin	Gray-variegated beds	"Skull without lower jaws, partial vertebrae, ilium and femur."[16]	A tyrannosauroid.

Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Archaeoceratops[17]	A. yujingziensis[18]	Yujingzi Basin	Gray-variegated beds	"Caudal vertebrae and a partial hindlimb."[18]	A neoceratopsian.

Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Jintasaurus[19]	J. meniscus[19]	Yujingzi Basin	Gray-variegated beds	"Partial skull."[19]	A hadrosauroid.
Xuwulong[20]	X. yueluni[20]	Yujingzi Basin	Gray-variegated beds	"Articulated skeleton lacking limbs."[20]	A hadrosauroid.

Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Qiaowanlong[21]	Q. kangxii[21]	Yujingzi Basin	Gray-variegated beds	"Cervical vertebrae and right pelvic girdle."[21]	A somphospondylan.

• List of fossil sites (with link directory)
• Lianmuqin Formation, contemporaneous fossiliferous formation of Xinjiang, western China
• Jiufotang Formation, contemporaneous fossiliferous formation of Liaoning, northeastern China
• Eumeralla Formation, contemporaenous fossiliferous formation of Victoria, Australia
• Elrhaz Formation, contemporaenous fossiliferous formation of Niger
• Crato Formation, contemporaneous fossil insect bearing Lagerstätte of Brazil