The Shaximiao Formation () is a Middle to Late Jurassic aged geological formation in Sichuan, China, most notable for the wealth of dinosaurs fossils that have been excavated from its strata. The Shaximiao Formation is exposed in and around the small township of Dashanpu (), situated seven kilometres north-east from Sichuan's third largest city, Zigong, in the Da'an District.
Geology The Shaximiao Formation includes two distinct subunits: The upper and lower Shaximiao Formations (), although they are commonly referred to as one, simply being called the "Shaximiao Formation". The upper Shaximiao Formation is also known as the Shangshaximiao Formation, and the lower Shaximiao Formation is also known as the Xiashaximiao Formation, which are direct transliterations of the Chinese names. Both subunits primarily consist of purple-red mudstones, with variable sand inclusion. and siltstones with interbedded sandstones.
Dinosaur finds
The Shaximiao Formation has produced mainly sauropods, but has also held numerous other dinosaur types, such as theropods and stegosaurians amongst others. In total, over 8,000 pieces of bone have been unearthed from the area – amounting to nearly 40 tonnes. The site was unknown until the early 1970s, when a Chinese gas company unearthed Gasosaurus in 1972. It would be the first of the many dinosaurs to be uncovered from the area. Most specimens found are held at the Zigong Dinosaur Museum which has been placed on the area during the mid-1980s.
The site is currently a frequented "dinosaur-quarry", but the Shaximiao Formation once hosted a lush forest, evidenced by fossilised wood found alongside the dinosaur remains. Paleontologists speculate that the area also had a lake that was fed by a large river. Dinosaur remains would have been swept toward the lake over millions of years, thus accounting for the hundreds of specimens found. Based on biostratigraphy, the Lower Shaximiao Formation has been usually seen to date to 168 to 161 million years old, between the Bathonian to Callovian stages of the Mid Jurassic, while the Upper Shaximiao was thought to be Oxfordian in age. A paper by Wang et al. (2018), (recalibrated dates reported in Moore et al. (2020)), reported a zircon U-Pb age of 160.4 ± 0.4 mya for the lower part of the Shaximiao Formation, suggesting that the Shaximiao Formation is younger than previously thought. Contradicting this, U-Pb dates from a tuff bed supports traditional Middle Jurassic age for the lower part of the formation, with an average age of around 166.0 ± 1.5 Ma. The top of the Shaximiao Formation has been believed to be the end of the Tithonian age.
Dong Zhiming's research The paleontologist who has made the largest contribution to the formation and its excavation is Dong Zhiming. He first examined the formation in 1975, after bone fragments were found embedded in rock from the area. The site was being demolished to make way for both a natural gas field facility and a vehicle park when Dong first saw the area. Amongst the extensive clearings, Dong found numerous bone fragments which were exposed.
However, the specimens were being damaged due to bulldozers in the area and there would be little chance of closing the area as the state had invested millions of yuan in the site already. It was not until 1985 that the government finally agreed to close the construction on the site, and by then Dong and his team had already excavated over 100 dinosaurs from the area, including several rare sauropod skulls. A dinosaur found in the Shaximiao Formation, Dashanpusaurus dongi, was named in tribute of both Dashanpu and Dong Zhiming.
Paleobiota In addition to dinosaur finds, many other prehistoric finds have been uncovered from the Shaximiao Formation. Amongst these finds are fishes, amphibians, turtles, marine reptiles such as crocodiles and also pterosaurs. Bienotheroides, a Tritylodont Synapsid has been found there, as well as Sinobrachyops, a Labyrinthodont.
Amphibians
Genus Species Stratigraphic position Material Notes Images Sinobrachyops S. placenticephalus Shunosaurus-Omeisaurus assemblage, Lower Shaximiao Formation A brachyopid stereospondyl董枝明. "四川自贡大山铺蜀龙动物群——简报 V 两栖类." 古脊椎动物学报 4 (1985): 008
Fish
Genus Species Stratigraphic position Material Notes Images Ceratodus C. guanganensis Mamenchisaurus assemblage, Upper Shaximiao Formation A ceratodontid lungfish C. minor Mamenchisaurus assemblage, Upper Shaximiao Formation C. youngi Mamenchisaurus assemblage, Upper Shaximiao Formation C. zigongensis Mamenchisaurus assemblage, Upper Shaximiao Formation, Shunosaurus-Omeisaurus assemblage, Lower Shaximiao Formation Chungkingichthys C. tachuensis Mamenchisaurus assemblage, Upper Shaximiao Formation A ptycholepiform fish Hybodus H. sp. Shunosaurus-Omeisaurus assemblage, Lower Shaximiao Formation A hybodont chondrichthyan Isanichthys I. luchowensis Shunosaurus-Omeisaurus assemblage, Lower Shaximiao Formation A ginglymodian fish, originally identified as Lepidotes Yuchoulepis Y. szechuanensis Shunosaurus-Omeisaurus assemblage, Lower Shaximiao Formation A ptycholepiform fish
Sauropterygians
Genus Species Stratigraphic position Material Notes Images Bishanopliosaurus B. zigongensis Shunosaurus-Omeisaurus assemblage, Lower Shaximiao Formation A pliosauroid plesiosaur
Crocodyliformes
Genus Species Stratigraphic position Material Notes Images Sunosuchus S. shunanensis Shunosaurus-Omeisaurus assemblage, Lower Shaximiao Formation A goniopholidid Hsisosuchus H. dashanpuensis Shunosaurus-Omeisaurus assemblage, Lower Shaximiao Formation A goniopholid
Pterosaurs Genus Species Stratigraphic position Material Notes Images Angustinaripterus A. longicephalus Shunosaurus-Omeisaurus assemblage, Lower Shaximiao Formation A rhamphorhynchine rhamphorhynchid with a very elongated and flat snout and an estimated wingspan at
Dinosaurs
Ornithischians Neornithischians Genus Species Stratigraphic position Material Notes ImagesAgilisaurusA. louderbackiShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationGongbusaurusG. shiyiiMamenchisaurus assemblage, Upper Shaximiao FormationTeethHexinlusaurusH. multidensShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationXiaosaurusX. dashanpuensisShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationYandusaurusY. hongheensisMamenchisaurus assemblage, Upper Shaximiao Formation
Stegosaurs
Genus Species Stratigraphic position Material Notes ImagesBashanosaurusB. primitivusLower Shaximiao FormationThe earliest-diverging stegosaur currently known; it shares features with basal thyreophorans and derived stegosaurs.ChialingosaurusC. kuaniMamenchisaurus assemblage, Upper Shaximiao FormationA stegosaurian.ChungkingosaurusC. jiangbeiensisMamenchisaurus assemblage, Upper Shaximiao FormationA stegosaurian.HuayangosaurusH. taibaiiShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationA huayangosaurid with more spike-like plates than Stegosaurus. It was also one of the smallest known stegosaurs, at just in length.GigantspinosaurusG. sichuanensisMamenchisaurus assemblage, Upper Shaximiao FormationA stegosaurian. G. sp.Mamenchisaurus assemblage, Upper Shaximiao FormationA stegosaurian.TuojiangosaurusT. multispinusMamenchisaurus assemblage, Upper Shaximiao FormationA stegosaurian.YingshanosaurusY. jichuanensisUpper Shaximiao FormationNearly complete skeletonA stegosaurian.
Sauropods
Genus Species Stratigraphic position Material Notes ImagesAbrosaurusA. dongpoiLower Shaximiao FormationA macronarian known from a well-preserved skull and other elements.BashunosaurusB. kaijiangensisLower Shaximiao FormationAn obscure possible macronarianDaanosaurusD. zhangiMamenchisaurus assemblage, Upper Shaximiao FormationDashanpusaurusD. dongi Lower Shaximiao FormationA basal macronarianDatousaurusD. bashanensisShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationA mamenchisaurid.MamenchisaurusM. constructusMamenchisaurus assemblage, Upper Shaximiao FormationA mamenchisaurid.M. hochuanensisMamenchisaurus assemblage, Upper Shaximiao FormationA mamenchisaurid.M. youngiMamenchisaurus assemblage, Upper Shaximiao FormationA mamenchisaurid.M. jingyanensisMamenchisaurus assemblage, Upper Shaximiao FormationA mamenchisaurid. M. sanjiangensis Dai H, Hu XF, Tan C, Ren XX, Ma QY, Wei GB, You HL. A new mamenchisaurid sauropod dinosaur from the upper jurassic of Southwest China reveals new evolutionary evidence from East Asian eusauropods. Sci Rep. 2025 Nov 25. doi: 10.1038/s41598-025-29995-z. Epub ahead of print. PMID: 41291141. Upper Shaximiao Formation A single partial skeleton A mamenchisaurid.OmeisaurusO. luoquanensisShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationA mamenchisaurid.O. tianfuensisShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationA mamenchisaurid.O. junghsiensisShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationA mamenchisaurid.O. changshouensisMamenchisaurus assemblage, Upper Shaximiao FormationA mamenchisaurid.O. maoianusMamenchisaurus assemblage, Upper Shaximiao FormationA mamenchisaurid.O. fuxiensisMamenchisaurus assemblage, Upper Shaximiao FormationA mamenchisaurid.O. jiaoiA mamenchisaurid.O. puxianiA mamenchisaurid.ProtognathosaurusP. oxyodonShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationShunosaurusS. liiShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationA eusauropod estimated with a length at , the weight at 3 metric tons (3.3 short tons). The tail ended in a club, equipped on its top with two successive spikes formed by -long cone-shaped osteoderms probably used to fend off enemies.YuzhoulongY. qurenensisLower Shaximiao FormationA basal macronarian known from a subadult specimenZigongosaurusZ. fuxiensisMamenchisaurus assemblage, Upper Shaximiao FormationPossibly a species of Mamenchisaurus.
Theropods
Genus Species Synonyms Stratigraphic position Material Notes ImagesChienkosaurusC. ceratosauroidesMamenchisaurus assemblage, Upper Shaximiao FormationChuandongocoelurusC. primitivusShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationElaphrosaurinae indet.Shunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationFormerly designated as the paratype of Chuandongocoelurus.GasosaurusG. constructusShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationAn avetheropod that measured between in length, with a weight of around . However, some estimates put its weight as high as , as very little is known about it.KaijiangosaurusK. liniShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationA megalosaurid, early tetanuran, early averostran or basal carnosaur.LeshansaurusL. qianweiensisShunosaurus-Omeisaurus assemblage, Upper Shaximiao formationA megalosaurid.SinocoelurusS. fragilisMamenchisaurus assemblage, Upper Shaximiao Formation A theropod of uncertain affinities SinraptorS. hepingensisMamenchisaurus assemblage, Upper Shaximiao FormationA metriacanthosaurid, formally named Yangchuanosaurus hepingensis.SzechuanosaurusS. campiMamenchisaurus assemblage, Upper Shaximiao FormationA metriacanthosaurid. Remains from the Shaximiao Formation might represent an indeterminate theropod.XuanhanosaurusX. qilixiaensisShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationA metriacanthosaurid.YangchuanosaurusY. shangyouensisY. magnusMamenchisaurus assemblage, Upper Shaximiao formation A metriacanthosaurid Y. zigongensis Szechuanosaurus zigongensisShunosaurus-Omeisaurus assemblage, Lower Shaximiao FormationZDM 9011 (holotype), a partial postcranial skeleton; ZDM 9012, a left maxilla; ZDM 9013, two teeth and ZDM 9014, a right hind limb.
Turtles