in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
The Vertebrate Assemblage from Shaximiao Formation of Sichuan
Basin, China and Discussions on Its Age
Kui Li 1 , Chunyan Yang 1 , Jian Liu 1 , Tao Jiang 2
1. The Museum of Chengdu University of Technology, Chengdu 610059, China (E-mail: likui9988@sina.com.cn)
2. Institute of Sedimentary Geology of Chengdu University of Technology, Chengdu 610059, China
The Shaximiao Formation, a continental Mesozoic
stratum in Sichuan Basin with a thickness of 650 to
2500 meters, is in much advance of others at its large
distributing area and abundant fossils. It has a con-
formable contact with underlying strata (The Xin-
tiangou Formation or The Qianfoya Formation) and
overlying strata (The Suining Formation). And there
also exists a parallel unconformity between the Sha-
ximiao Formation and the underlying Ziliujin For-
mation. Usually, the Shaximiao Formation is divided
into two members by “Shale Containing Concho-
stracans”, which is located in the middle of the
Shaximiao Formation and contains a lot of concho-
stracans.
The aubergine mudstones and silty mudstones
compose the Lower member of the Shaximiao For-
mation, and also less fine sandstones and siltstones are
mixed. At the bottom of this lower member, a bed of
medium grain sandstone (Guankou Sandstone) with
large oblique beddings has a conformable contact with
the Xintiangou Formation (or The Qianfoya Formation).
As to the Lower member of the Shaximiao Formation,
there is a prominent feature of being thin in the west and
thick in the east. Fossils are mainly from the “Shale
Containing Conchostracans”, including bivalves,
ostracods, a few plants and abundant of conchostracans.
The famous Shunosaurus-Omeisaurus Fauna is located
in Dashanpu of Zigong and Jinji of Kaijiang, which
includes Shunosaurus lii, Omeisaurus tianfuensis, and
additionally with fishes, amphibians, chelonians, cro-
codilians, plesiosaurs, pterosaurs and mammal-like
reptiles.
The Upper member of the Shaximiao Formation
consists of rhythmic layers, composed of dark-
aubergine mudstones and grey-green, aubergine
feldspar-quartz sandstones. The Upper member of the
Shaximiao Formation has prominent features, such as
dark colour of rocks, mudstones being rich of cal-
careous concretion, sandstones varying significantly,
rhythmicity of the sandstones and mudstones, and so on.
The fossils include ostracods, bivalves, conchostracans,
gastropods, chara and the famous Mamenchisaurus
Fauna. The Mamenchisaurus Fauna contains abundant
vertebrates such as fishes, amphibians, chelonians,
crocodilians, plesiosaurs, mass of dinosaurs and a few
mammalians.
For a long time, the age of the Upper member of
the Shaximiao Formation has been in dispute.
Invertebrate paleontologists advocated that the total
Shaximiao Formation belonged to the Middle Jurassic,
whereas vertebrate paleontologists suggested that the
Upper member of the Shaximiao Formation belonged
to the Late Jurassic. The vertebrate paleontologists also
considered the age of Mamenchisaurus Fauna to be
Late Jurassic.
Here the age of Shaximiao Formation would be
discussed in three aspects as follows.
Firstly, vertebrate fossil assemblage of both Upper
and Lower Shaximiao Formation have been analysed
and compared, and the result shows: the age of some
vertebrates such as fishes in Mamenchisaurus Fauna
belongs to the Middle Jurassic; the Upper and Lower
members of the Shaximiao Formation have the same
composition of main vertebrate fossils, coelurosaurs,
carnosaurs, sauropods, ornithopods, stegosaurs, plesio-
saurs, crocodilians, chelonians, chondrichthians, lung-
fishes, actinopterygians and so on; the dinosaurs are
different in the Upper and Lower members of the
Shaximiao Formation, and both primitive sauropods as
Shunosaurus and progressive sauropods as Omeisaurus
occurred in the Shunosaurus-Omeisaurus Fauna of the
Lower member of the Shaximiao Formation, whereas
Mamenchisaurus, a kind of progressive dinosaurs with
long neck, are the primary sauropods in Mamen-
chisaurus Fauna of the Upper member of the
Shaximiao Formation. Omeisaurus are considered to be
the ancestor of Mamenchisaurus. So it is thought that
the Mamenchisaurus Fauna inherits and develops from
the Shunosaurus-Omeisaurus Fauna.
Secondly, the Shaximiao Formation is compared
with the Middle Jurassic stratum of the Chuanjie Basin
in Yunnan province. They have similar lithological
features, and both are composed of the rhythmic layers
of yellow, purplish-grey feldspar-quartz sandstones and
aubergine, purplish-grey mudstones/shales. The Upper
member of the Shaximiao Formation has the similar
vertebrate assemblage to the Chuanjie Formation,
especially Chuanjiesaurus and Mamenchisaurus have
resembled skeletons which are totally in the same
evolutionary level. Therefore, it is thought that the
Chuanjiesaurus Fauna of the Chuanjie Basin in Yunnan
province is coetaneous or at least close to the
Mamenchisaurus Fauna of the Sichuan Basin.
In the third, a contrastive analysis between
Mamenchisaurus Fauna and abroad vertebrate fauna of
Late Jurassic has been carried out. There are two
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