# Xiagou Formation

The Xiagou Formation is the middle strata of the Xinminbao Group. It is named for its type site in Xiagou, in the Changma Basin of Gansu Province, northwestern China and is considered Early Cretaceous in age. It is known outside the specialized world of Chinese geology as the site of a Lagerstätte in which the fossils were preserved of Gansus yumenensis, the earliest true modern bird.

## Description

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.

## Fossil content

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.
| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |

### Invertebrates
#### Arthropods

| Genus | Species | Notes |
| Blattapterix | B. gansu | |
| Cretacechorista | C. qilianshanensis | |
| Eurycoleus | E. arcuatus, E. clypeolatus, E. dimorphocellatus, E. parvus | A carabid. |
| Glottocoleus | G. lenticulata, G. stellatus | |
| Mesoblattina | M. cretacea | |
| Mesocoleus | M. zhonggouense | |
| Mesotricupes | M. reticulatus | |
| Petalocupes | P. arcus | |
| Phyllocoleus | P. striolatus | |
| Planocoleus | P. ensatus | |
| Pleurocoleus | P. catenatus | |
| Spinus | S. yumenense | |
| Tetillopsis | T. parvula | |
| Yumenocoleus | Y. intermedius, Y. lineatus, Y. longus | |

### Theropods
#### Birds

| Genus | Species | Location | Material | Notes |
| Avimaia | A. schweitzerae | Changma Basin | "Partial skeleton with an egg." | An enantiornithean. |
| Brevidentavis | B. zhangi | | Incomplete skull and cervical vertebrae. | An ornithuromorph. |
| Changmaornis | C. houi | Changma Basin | "Partial pelvic girdle with hindlimb." | An euornithian. |
| Dunhuangia | D. cuii | Changma Basin | "Forelimbs." | An enantiornithean. |
| Feitianius | F. paradisi | Changma Basin | "Hindlimbs with pelvic area and vertebrae." | An enantiornithean. |
| Gansus | G. yumenensis | | "Partial hindlimbs." | An euornithian. |
| Jiuquanornis | J. niui | Changma Basin | "Partial pectoral girdle." | An euornithian. |
| Meemannavis | M. ductrix | | Incomplete skull and cervical and thoracic vertebrae. | An ornithuromorph. |
| Novavis | N. pubisculata | Changma Basin | Partial skeleton including tail vertebrae, a pelvis, and the hindlimbs. | An enantiornithean. |
| Qiliania | Q. graffini | Changma Basin | "Partial skeleton from two individuals." | An enantiornithean. |
| Unnamed enantiornithean | Indeterminate | | "Partial forelimb." | An enantiornithean. |
| Yumenornis | Y. huangi | Changma Basin | "Right forelimb." | An euornithian. |

#### Ornithomimosaurs

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

#### Therizinosaurs

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

#### Tyrannosauroids

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

### Cerapods
#### Ceratopsians

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

#### Hadrosauroids

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

### Sauropods
#### Macronarians

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

### Other vertebrates
#### Turtles

| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
| Changmachelys | C. bohlini | Changma Basin | | "Nearly complete skeletons from four individuals." | A macrobaenid. | |

#### Fish

| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
| Shuleichthys | S. brachypteryx | | | "Complete skeleton from four individuals." | An osteoglossomorph. | |

### Plants

| Genus | Species | Location | Stratigraphic member | Material | Notes | Images |
| Xenoxylon | X. meisteri | Hongliuxia | | Wood | A podocarp | |