in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
232
The Evolvement of Long-necked Sauropods of Sichuan Basin
Chunyan Yang 1 , Kui Li 1 , Jian Liu 1 , Tao Jiang 2 , Zhiguang Li 2
1. The Museum of Chengdu University of Technology, Chengdu 610059, China (E-mail: ycy-y@163.com)
2. Institute of Sedimentary Geology of Chengdu University of Technology, Chengdu 610059, China
Lots of sauropod fossils, including long-necked
type and short-necked type, had been discovered in the
Jurassic of Sichuan Basin. There are three genera of
long-necked sauropods: Tonganosaurus from the
Early Jurassic, Omeisaurus from the Middle
Jurassic, Mamenchisaurus from the Middle and
Late Jurassic.
The Early Jurassic sauropod Tonganosaurus
from Yimen Formation in Huili (in the south of Sichuan
Province) is among the earliest known long-necked
sauropod dinosaur in the Sichuan Basin. Tongano-
saurus hei Li et al., 2010 is the only species of this
genus. According to the original description by Li et al.
(2010), Tonganosaurus is a moderate sauropod (its
length is approximately 12 m), Pleurocoels are
developed in all presacral vertebrae, and the bone
texture is solid. Cervical centra are slender. A ventral
keel is present on each cervical centrum. Posterior
cervical centra have well developed laminae and
cavities. Both laminae and pleurocoels are well
developed in the dorsal vertebrae. Cervical and anterior
dorsal centra are opisthocoelous, middle dorsal centra
are platycoelous, posterior dorsal and anterior caudal
centra are amphicoelous. Centra lack a cancellous
structure. Forelimb length is 80% of hindlimb length.
Humerus is straight and robust, and deltopectoral crest
is well developed. Femur is straight and robust with a
well developed 4 th trochanter (Li et al.,2010).
The Middle Jurassic sauropod Omeisaurus have
six species, they are Omeisaurus junghsiensis Young,
1939 (Young, 1939) from Rongxian County, Omei-
saurus fuxiensis Dong et. al., 1983 (Dong et al., 1983)
from Zigong County,Omeisaurus tianfuensis He et. al.,
1984 (He et al.,1984)from Dashanpu of Zigong
County, Omeisaurus luoquanensis Li, 1988 (He et al.,
1988) from Luoquan of Zizhong County,Omeisaurus
changshouensis Young , 1958 (Young, 1958) from
Changshou County of Chongqing Municipality, Omei-
saurus maoianus Tang et al., 2001 (Tang et al., 2001)
from Huangshikang of Jingyan County . Above all six
species of Omeisaurus have been discovered in the
Shaximiao Formation. Their geographic positions
extend from south to east of Sichuan Basin. Omei-
saurus are middle, 18-m-long sauropods, their
skull moderately high. Teeth spatulate and robust.
The denticles well developed on the anterior edge
of the teeth, poor or absent on its posterior edgr.
The vertebral formula: 17 cervicals, 12 dorsals.
Posterior cervical and anterior dorsal spine simple
and not bifurcated, anterior caudals slightly amphi-
coelous; first caudal rib more less fan-shaped (He
et al., 1984).
Genus Mamenchisaurus had been established in
1954 on the basis of sauropod fossils from the Sha-
ximiao Formation of Yibin County. Mamenchisaurus
constructus Young, 1954 is the type species (Yang,
1954). Beside, there are four species of Mamen-
chisaurus had been found in Sichuan Basin, they are:
Manenchisaurus hochuanensis (Young and Zhao, 1972)
from the Shaximiao Formation of Taihe Town of
Hechuan County (Yang and Zhao, 1972), Manen-
chisaurus anyueensis (He et al.,1996) from the Penglai-
zhen Formation (Late Jurassic) of Longqiao Town,
Anyue county (He et al., 1996), Manenchisaurus
youngi (Pi et al., 1996) from the Shaximiao Formation
of Zigong County, Manenchisaurus jingyanensis
(Zhang et al., 1998) from the Shaximiao Formation of
Jingyan County (Zhang et al., 1998). Mamenchi-
saurus are large size sauropods (their length
approximately 24m long) with exteremely small skull.
Occipital crest is well developed. Mandible slender
with a hight symphyseal region. External mandiblular
foramen is present. Teeth arranged closely. The wear
facets of teeth prominent, and denticles on the posterior
carinae degenerate. Vertebral formula: 18-19 cer-
vicals, 12 dorsals, 4-5 sacrals and more than 50
caudals. Presacrals are opsithocoelous with cacell-
ous structure in varying degree. Posterior cervical
and anterior dorsal spine bifurcated, Cervicals
elongated with long cervical ribs. Anterior caudals
are procoelous, the middle-posterior ones amphi-
coelous,and distal caudals fused and expanded.
There are typical diplodocoid forked chevrons in
the middle tail region. Scapula is longer than femur
with a expanded distal blade. Sternum small and
subcircular. The length ratio of forelimb and
handlimb about 0.75 to 0.80 (Ouyang and Ye,
2002).
Those three long-necked sauropods bear many
homology characteristics: 1) their cervical centra are
slender, the ratio of centrum length to posterior width in
the 3 rd cervical are more than 2.90; They have or more
15 cervical centra; 2) Anterior dorsal centra, in which
both laminae and pleurocoels are well developed, are
opisthocoelous; 3) Forelimb length is 0.75~0.80 of
hindlimb length. Owing to the above resemble
characteristic, we considered that Tonganosaurus,