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.

Invertebrates

Arthropods Genus Species Location Stratigraphic position Material Notes Images BlattapterixP. Vršanský. 2003. Umenocoleoidea - an amazing lineage of aberrant insects (Insecta, Blattaria). AMBA Projekty 7(1):1-32 B. gansu CretacechoristaY. C. Hong, D. S. Yan, and D. R. Wang. 1989. Discovery of Early Cretaceous Cretacechorista gen. nov. Insecta: Mecoptera from Jiuquan basin Gansu Province. Memoirs of Beijing Natural History Museum 44:1-9 C. qilianshanensis EurycoleusY. C. Hong. 1982. Mesozoic Fossil Insects of Jiuquan Basin in Gansu Province 1-187 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 Stratigraphic position Material Notes Images Avimaia A. schweitzerae Changma Basin "Partial skeleton with an egg." An enantiornithean. BrevidentavisB. zhangiIncomplete skull and cervical vertebrae.An ornithuromorph. Changmaornis C. houi Changma Basin"Partial pelvic girdle with hindlimb." An euornithian. DunhuangiaM. Wang, D. Li, J. K. O'Connor, Z. Zhou and H. You. 2015. "Second species of enantiornithine bird from the Lower Cretaceous Changma Basin, northwestern China with implications for the taxonomic diversity of the Changma avifauna". Cretaceous Research 55: 56-65 D. cuii Changma Basin "Forelimbs." An enantiornithean. FeitianiusO'Connor, J. K., D.-Q. Li, M. C. Lamanna, M. Wang, J. D. Harris, J. Atterholt, and H.-L. You. 2015. "A new Early Cretaceous enantiornithine (Aves, Ornithothoraces) from northwestern China with elaborate tail ornamentation". Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.1054035 https://www.tandfonline.com/doi/abs/10.1080/02724634.2015.1054035?journalCode=ujvp20 F. paradisi Changma Basin "Hindlimbs with pelvic area and vertebrae." An enantiornithean. GansusL. Hou and Z. Liu. 1984. A new fossil bird from Lower Cretaceous of Gansu and early evolution of birds. Scientia Sinica, series B 27(12):1296-1301 G. yumenensis "Partial hindlimbs." An euornithian. Jiuquanornis J. niui Changma Basin "Partial pectoral girdle." An euornithian.MeemannavisM. ductrixIncomplete 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."You et al. 2005 A new fossil bird from the Early Cretaceous of Gansu Province, northwestern China Historical Biology 17(1-4):7-14 An enantiornithean. Yumenornis Y. huangi Changma Basin "Right forelimb." An euornithian.

Non-avian theropods Genus Species Location Stratigraphic position Material Notes Images Beishanlong B. grandis Yujingzi Basin Gray-variegated beds "Fore and hindlimbs with partial vertebrae." An ornithomimosaur Jian J. changmaensis Changma Basin Pectoral girdle and partial forelimb A microraptorine Suzhousaurus S. megatherioides Yujingzi Basin Gray-variegated beds "Dorsal vertebrae, ribs and partial shoulder girdle." A therizinosauroid 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

Pterosaur Genus Species Location Stratigraphic position Material Notes Images Pterosauria indet. indeterminate Proximal end of the first wing phalanx and fused extensor tendon process Estimated wingspan around 4 meters

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 ShuleichthysA. M. Murray, H. L. You, and C. Peng. 2010. A New Cretaceous Osteoglossomorph Fish from Gansu Province, China. Journal of Vertebrate Paleontology 30(2):322-332 S. brachypteryx "Complete skeleton from four individuals." An osteoglossomorph.

Plants Genus Species Location Stratigraphic memberMaterial Notes ImagesXenoxylonX. meisteriHongliuxiaWoodA podocarp

See also

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, contemporaneous fossiliferous formation of Victoria, Australia Elrhaz Formation, contemporaneous fossiliferous formation of Niger Crato Formation, contemporaneous fossil insect bearing Lagerstätte of Brazil

References