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Age of Jurassic basal sauropods in Sichuan, China
Geological Society of America Bulletin, v. 130, no. 9/10 1493
ABSTRACT
Sauropoda are the largest terrestrial ani-
mals to have ever lived and represent the
dominant herbivorous dinosaurs of the Me-
sozoic Era. The Lower Shaximiao Formation
of the Sichuan Basin, Southwest China, hosts
abundant Jurassic basal sauropods including
the Shunosaurus-Omeisaurus Fauna. This
formation was previously hypothesized to be
Middle Jurassic based on biostratigraphic
interpretations, but the exact depositional
age is uncertain. Here we report the youngest
inductively coupled plasma–mass spectrom-
etry (ICP-MS) detrital zircon U-Pb age of
159 ± 2 Ma for fossil-bearing strata from this
formation as the maximum depositional age.
This age falls very close to the Oxfordian age
interpreted for the Shunosaurus-Omeisaurus
Fauna and is younger than previously pro-
posed. We suggest that when the widely dis-
tributed basal sauropods of the Early-Middle
Jurassic were mostly replaced by the phylo-
genetically more-derived neosauropods in
the Late Jurassic in other regions of Laurasia
and Gondwana, some more basal members
survived and diversified in the Sichuan Basin
of southwestern China.
INTRODUCTION
The earliest known sauropods were discov-
ered in Late Triassic units—Norian stage—of
Zimbabwe (McIntosh, 1990; Raath, 1972; Yates
and Kitching, 2003) and Late Norian or Rhae-
tian units of Thailand (Buffetaut et al., 2000,
2002) (Fig. 1). Basal sauropods, here we mean
sauropods more basal than Neosauropoda, also
GSA Bulletin; September/October 2018; v. 130; no. 9/10; p. 1493–1500; https://doi.org/10.1130/B31910.1; 5 figures; Data Repository item 2018117;
published online 3 April 2018 .
†Corresponding author: suchin@hku.hk.
Age of Jurassic basal sauropods in Sichuan, China:
A reappraisal of basal sauropod evolution
Jun Wang1,2,3, Yong Ye4, Rui Pei1, Yamin Tian2, Chongqin Feng1, Daran Zheng1,3, and Su-Chin Chang1,†
1Department of Earth Sciences, The University of Hong Kong, Hong Kong
2State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology),
Chengdu 610059, China
3Key Laboratory of Economic Stratigraphy and Palaeogeography, Chinese Academy of Sciences (Nanjing Institute of Geology
and Palaeontology), Nanjing 210008, China
4Zigong Dinosaur Museum, Zigong 643013, China
appeared sporadically in Early to Middle Ju-
rassic localities of China, India, Europe, South
America and Africa (e.g., Phillips, 1871; Raath,
1972; Weishampel, 1992; Monbaron et al.,
1999; Buffetaut et al., 2000; Allain et al., 2004;
Stumpf et al., 2015) (Fig. 1). They became more
diversified phylogenetically and flourished in
both hemispheres from the Middle Jurassic to
the end of the Late Jurassic (Sereno, 1999a),
possibly due to the breakup of Pangea (Chatter-
jee and Zheng, 2002).
In China, Early to Middle Jurassic fossils of
basal sauropod only occur in Sichuan and Yun-
nan provinces (Dong et al., 1983; Dong, 1992).
In Sichuan Province of southwest China, basal
sauropod fossils unearthed from the Ziliujing
Formation (ZLF) and Lower Shaximiao For-
mation (LSF) in the Sichuan Basin (SCB) are
part of the Zizhongosaurus and Shunosaurus-
Omeisaurus faunas (Figs. 2 and 3). These fau-
nas are traditionally assigned Early and Middle
Jurassic ages, respectively, based on biostrati-
graphic correlation of assemblage biozones
(Dong et al., 1983; Dong, 1992; Li et al., 1997,
2011; Peng et al., 2005; Ye, 2006; Wang et al.,
2008). Even though many studies have been
conducted on the basal sauropods from the SCB
in the past decades (e.g., Dong, 1992; He et al.,
1988, 1998; Li, 1998), few have included geo-
chronologic constraints due to the general lack
of igneous bodies within Mesozoic sedimentary
rocks. Here we report new detrital zircon U-Pb
age determined by laser ablation–inductively
coupled plasma–mass spectrometry (LA-ICP-
MS) constraints for four dinosaur-bearing LSF
sandstones found on the grounds of the Zigong
Dinosaur Museum, Sichuan Province, China.
Reliable age determinations for the LSF’s fossil
bearing beds are critical to understanding evo-
lution and paleobiogeography of sauropods and
other Early to Late Jurassic fossil assemblages.
Thus, our inductively coupled plasma–mass
spectrometry (ICP-MS) U-Pb zircon data pro-
vide novel geochronologic constraints on the
evolutionary history of early sauropods.
GEOLOGICAL BACKGROUND
The intracratonic SCB is tectonically situ-
ated in the northwestern portion of the Yangtze
block, surrounded by orogenic belts (Zhang et
al., 2004; Liu et al., 2006) (Fig. 2). Numerous
studies have described the widely-distributed
Mesozoic stratigraphy of this basin (CC-
MSPSB, 1982; BGMRSP, 1991; Meng et al.,
2005). Upper Triassic to Quaternary terrestrial
facies reach thicknesses of 2000–6000 m and
overlie carbonate-dominated Sinian to Middle
Triassic marine facies (BGMRSP, 1991). Up-
per Triassic and Jurassic SCB sedimentary units
consist mostly of typical lacustrine and fluvial
facies, which include reddish conglomerates,
sandstones and mudstones, referred to as red
beds (Wang et al., 2008). Based on lithologic,
paleontologic, and sedimentologic characteris-
tics; these sequences are divided into the follow-
ing formations, in ascending order: the Upper
Triassic Xujiahe Formation, the Lower Jurassic
Zhenzhuchong Formation and Ziliujing For-
mation (ZLF), the Middle Jurassic Xintiangou
Formation (XTF), Lower Shaximiao Forma-
tion (LSF) and Upper Shaximiao Formation
(USF), and the Upper Jurassic Suining Forma-
tion (SNF) and Penglaizhen Formation (PLF)
(BGMRSP, 1991; Wang et al., 2010).
The SCB hosts a range of dinosaur body fos-
sils representing 30 genera and 43 species (Wang
et al., 2010) as well as trace fossils—footprints—
representing 20 genera and 24 species (Ye et al.,
2012). Given this diversity, the SCB is consid-
ered a classic locality for Jurassic dinosaurian
research. The Middle Jurassic LSF and USF crop
For permission to copy, contact editing@geosociety.org
© 2018 Geological Society of America
Jun Wang et al.
1494 Geological Society of America Bulletin, v. 130, no. 9/10
out in Zigong City where the main dinosaur-
bearing units occur on the grounds of the Zigong
Dinosaur Museum. Dinosaur fossils from Zigong
City represent the majority of Mesozoic dinosaur
specimens in the SCB (Peng et al., 2005; Wang et
al., 2008). The LSF yields 10 genera and 12 spe-
cies of saurischian dinosaurs (Wang et al., 2010),
including the basal sauropods Shunosaurus lii
(Dong et al., 1983; Zhang, 1988; Li, 1998) and
Omeisaurus tianfuensis (He et al., 1988), which
are two of the most iconic fossil taxa of the SCB’s
Shunosaurus-Omeisaurus Fauna.
SAMPLING AND METHODOLOGY
We collected four sandstones from the lower
part of the fossil-bearing LSF for geochronolog-
ical analyses. The LSF outcrop was accessible
as part of a large, indoor paleontological exhibit
curated by the Zigong. Figure 3 shows lithol-
ogy of the LSF and sampling localities. Sample
ZG-1 was a gray-green, intermediate sandstone
from the lowermost LSF (layer no. 1) found in
the main hall of the museum. Sample ZG-2 is
also a gray-green, intermediate sandstone from
the same layer that occurs ~50 cm above sam-
ple ZG-1. Sample ZG-3 and ZG-4 are yellow-
green, intermediate sandstones from the layer
nos. 3 and 7, respectively.
Zircon separation and U-Pb dating were con-
ducted at the Department of Earth Sciences, The
University of Hong Kong. The four sandstone
samples were crushed and sieved by standard
methods. Grains having lengths of 60–200 mm
were retained and washed with distilled water.
Zircons were then separated by magnetic and
heavy liquid methods. Euhedral zircon grains
were hand-picked under binocular microscope
and mounted in epoxy resin. Epoxy mounts
were polished to expose grain midsections at
approximately two-thirds of their thickness.
Zircon U-Pb data were obtained using a VG
PQ Excel ICP-MS equipped with a New Wave
Research LUV213 laser-ablation system (LA-
ICP-MS). The LA system generates a 213 nm
UV light beam with a frequency-quintupled
Figure 1. Global distribution of Late Triassic-Middle Jurassic basal sauropods and the sampling location (red star). Late
Triassic (red dinosaur markers): 1—Isanosaurus from Thailand (Buffetaut et al., 2000) and 2—Melanorosaurus (Haugh-
ton, 1924) from South Africa. Early Jurassic (blue dinosaur markers): 3—Barapasaurus (Jain et al., 1975) from India;
4—Ohmdenosaurus (Wild, 1978) and Gravisauria (Stumpf et al., 2015) from Germany; 5— Tazoudasaurus from Mo-
rocco (Allain et al., 2004); 6—Vulcanodon from Zimbabwe (Raath, 1972; Yates and Kitching, 2003); 7— Zizhongosaurus
(Dong et al., 1983) and Gongxianosaurus (He et al., 1998) from China; 8—Antetonitrus (Yates and Kitching, 2003)
and Pulanesaura (McPhee et al., 2015) from South Africa. Middle Jurassic (yellow dinosaur markers): 9—Jobaria
(Sereno et al., 1999) and Spinophorosaurus (Remes et al., 2009) from Niger; 10—Patagosaurus (Bonaparte, 1979) from
Argentina; 11—Cetiosaurus (Phillips, 1871) from England; 12—Shunosaurus-Omeisaurus Fauna (Dong et al., 1983;
Dong, 1992; Li et al., 1997) from China (dated at 159 ± 2 Ma in this contribution). The world map was downloaded
from: www.alternatehistory.com/wiki/lib/exe/detail.php?id=blank_map_directory%3Aworld_gallery_6&media=new
_world_map_glow_old_colo.png. We used CorelDRAW (version X7) to create this figure (www.coreldraw.com/en/
product/technical-suite/?topNav=en).
1GSA Data Repository item 2018117, Table DR1,
ICP-MS U-Pb isotopic data for zircon grains sepa-
rated from the dinosaur-bearing sandstones of the
Late Jurassic Lower Shaximiao Formation, Sichuan,
SW China, is available at http://www.geosociety
.org/ datarepository/2018 or by request to editing@
geosociety.org.
Nd:YAG laser. The analyses were performed
with a 30 or 22 mm beam diameter, 6 Hz repeti-
tion rate and an energy of 0.6–1.3 mJ per pulse.
Other instrumental settings and procedural
details used here were described by Xia et al.
(2004). The standard zircon 91500 was used as a
primary calibration standard and GJ-1 as a sec-
ondary reference. Euhedral zircon grains with
zoning structures that indicate magmatic origins
were selected for dating. We used the Isoplot/
Ex 3.0 software package (Ludwig, 2003) for
U-Pb age calculation and the Microsoft Excel
macro developed by Andersen (2002) for com-
mon Pb correction. U-Pb age data with 1s er-
rors are shown in Supplementary Table DR11
Age of Jurassic basal sauropods in Sichuan, China
Geological Society of America Bulletin, v. 130, no. 9/10 1495
and plotted on concordia diagrams with 1s
uncertainties calculated at the 95% confidence
level. Given a 15% discordance range for all the
U-Pb ages, we interpret 206Pb/238U ages for zir-
cons younger than 1000 Ma and 207Pb/206Pb ages
for older grains.
RESULTS
We analyzed 97, 100, 95, and 96 zircon
grains from sandstone samples ZG-1, ZG-2,
ZG-3, and ZG-4, respectively, collected on the
grounds of the Zigong Dinosaur Museum. Sam-
ples came from horizons representing basal-
sauropod-bearing units in the LSF. As shown
in Supplementary Table DR1 (see footnote 1)
and Figure 4, we obtained 76, 99, 81, and 69
concordant ages for each sample. Uncertainties
for individual analyses are given at the 1-sigma
level, whereas calculated ages are presented at
the 2-sigma level.
Samples ZG-1, ZG-2, and ZG-3 yielded three
major age subpopulations including one Juras-
sic to Permian subpopulation (ca. 300–159 Ma)
and two Paleoproterozoic subpopulations (1.9–
1.8 Ga and 2.5–2.4 Ga). Only one major Jurassic
subpopulation appeared in sample ZG-4 (197–
159 Ma). The sample ZG-1 yielded only three
sporadic Jurassic ages, the youngest of which
was 167 Ma. The sample ZG-2 gave a sub-
stantial Early Jurassic subpopulation of 9 ages
ranging from 179 to 175 Ma and a minor Late
Jurassic subpopulation of 7 ages ranging from
161 to 159 Ma. The sample ZG-3 age distribu-
tion included a minor Early Jurassic subpopu-
lation of 8 ages ranging from 188 to 171 Ma.
Figure 2. Topographic map of Sichuan Basin and geological map of the sampling locality. T—Triassic sedimentary rocks;
J1z—Lower Jurassic Zhenzhuchong Formation; J1-2z—Lower-Middle Jurassic Ziliujing Formation; J2x—Middle Juras-
sic Xintiangou Formation; J2xs—Middle Jurassic Lower Shaximiao Formation (LSF); J2s—Middle Jurassic Upper Shaxi-
miao Formation (USF); J3s—Upper Jurassic Suining Formation (SNF); J3p—Penglaizhen Formation (PLF); K1—Lower
Cretaceous; K2—Upper Cretaceous; and Q—Quaternary sediments. The digital relief map was produced using the Generic
Mapping tools software package (GMT-5.3.1) (Wessel et al., 2013). The geological map was drawn by ourselves with the
software CorelDRAW (version X7), as mentioned above.
Figure 3. Stratigraphic column showing lithology of the Lower Shaximiao Formation as it occurs on the Zigong Dinosaur
Museum grounds and other sample locations. LSF—Lower Shaximiao Formation; USF—Upper Shaximiao Formation.
Jun Wang et al.
1496 Geological Society of America Bulletin, v. 130, no. 9/10
Figure 4. Concordia and probability density diagrams for detrital zircon U-Pb ages and cathodoluminescence images for
representative zircon grains analyzed by this study (right). Maximum depositional age was interpreted from concordant ages
indicated by red circles in the concordia diagrams. These ages are the single youngest ages in the youngest age population (as
shown by the gray circles).
Figure 5. Biostratigraphic range of fossils known from Jurassic sedimentary rocks of the Sichuan Basin, marked with
biostratigraphic constraints (purple bars) from previous research. Blue bar represents the extended time interval for the
Shunosaurus-Omeisaurus Fauna constrained by the zircon U-Pb ages in this contribution. Dashed purple bar indicates that
the minimum age of the bivalve Martinsonella? (CCMSPSB, 1982) is unknown. Triangles show the decrease on abundance of
sporopollen and estheria species assemblages. LSF—Lower Shaximiao Formation; SNF—Suining Formation; USF—Upper
Shaximiao Formation, XTF—Xintiangou Formation.
Age of Jurassic basal sauropods in Sichuan, China
Geological Society of America Bulletin, v. 130, no. 9/10 1497
The sample ZG-4 included a major Late Juras-
sic (162–159 Ma) subpopulation of 14 ages
and a major Early Jurassic (197–171 Ma) sub-
population composed of 33 ages. Given that
the youngest detrital zircon ages from samples
ZG-1 and ZG-3 were sporadic and much older
than those found in the other two samples, and
given the consistency of the youngest subpopu-
lations found in samples ZG-2 and ZG-4, the
age distributions from samples ZG-1 and ZG-3
cannot precisely constrain the age of the fossil-
bearing LSF. All Jurassic to Permian zircon
grains show relatively high Th/U ratios (>0.3)
and oscillatory zoning patterns, indicative of
igneous origins. As shown in Supplementary
Table DR1 (see footnote 1), a few Proterozoic
grains gave Th/U lower than 0.1, indicative of
metamorphic alteration. Ages falling within
10% of concordance were plotted on concordia,
cumulative probability and probability density
diagrams (Fig. 4). We interpret the youngest
single analyses of 159 ± 2 Ma from both of the
samples ZG-2 and ZG-4 as the maximum depo-
sitional age for the LSF.
DISCUSSION
Depositional Age of the LSF
The SCB does not appear to have experi-
enced significant magmatic activity making dat-
able material scarce in the LSF (Zhang, 1988).
Geochronologic data have not been reported
for the LSF and the lack of chronostratigraphic
constraints limits paleoclimatic and paleoeco-
logical correlations for the SCB. Chronostrati-
graphic assignments of fossil assemblages
from the SCB’s red beds are primarily based
on their lateral correlations (BGMRSP, 1997).
However, some of the fossils, such as the spo-
ropollen, are unevenly distributed and poorly
preserved (Wang et al., 2008), so that depo-
sitional ages remain uncertain (CCMSPSB,
1982). The unique intracontinental setting of
the SCB and its paleogeographic isolation in the
Mesozoic makes lateral correlation of its fossil
assemblages difficult. The Jurassic LSF mainly
consists of fluvial and lacustrine interbedded
sandstones and mudstones that host abundant
fossil animals (summarized in Fig. 5) including
bivalves belonging to the Lamprotula (Eolam-
protula) cremeri-Undulatula sichuanensis as-
semblage (BGMRSP, 1991) or the Lamprotula
(Eolamprotula) cremeri-Psilunio jiangyouen-
sis freshwater assemblage (BGMRSP, 1997),
estheria belonging to the Euestheria ziliujin-
gensis assemblage (BGMRSP, 1991), ostra-
cods belonging to Darwinula sarytirmenensis-
Metacypris (BGMRSP, 1997) or Ovaticythere
reticulate-Metacypris hechuanensis-Darwinula
sarytirmenensis assemblages (BGMRSP, 1991)
and sporopollen belonging to Callialaaporites-
Cerebropollenites (GBSP, 1980; BGMRSP,
1991) or Classopollis-Klukiporites assemblages
(Wang et al., 2008). Additionally, four species
of plant fossils occur in the LSF (Yang, 1978)
(Fig. 5). Several plant species from this forma-
tion, such as Coniopteris (Yang, 1978; Duan
and Peng, 1998), have flourished on a global
scale since the Middle Jurassic (Harris, 1961).
The LSF also hosts well-known vertebrate as-
semblages, the Shunosaurus-Omeisaurus Fauna
(Dong et al., 1983; Dong, 1992; Ye, 2006; Wang
et al., 2008; Li et al., 2011). Among other for-
mations in the SCB, the LSF is exemplary for
its dinosaur fossils, which include 10 genera and
12 species of basal saurischian dinosaurs (Wang
et al., 2010).
Lower to Upper Jurassic fossil assemblages,
especially the plant and invertebrate fossils,
show temporal variation in inheritance and con-
tinuity but also some noteworthy similarities.
For instance, the most recent studies on sporo-
pollen (Wang et al., 2008) and estheria assem-
blages (BGMRSP, 1991) show matching spe-
cies composition between the LSF and USF, but
a decrease in the abundance of each species in
younger units. Other fossil assemblages exhibit
significant differences, for example, bivalve
fossils are distinctly different in the LSF, USF
and SNF (CCMSPSB, 1982) (Fig. 5). Bivalve
assemblages in the XTF, LSF and USF contain
Eolamprotula, which is regarded as a middle to
late Middle Jurassic index fossil in South China
(CCMSPSB, 1982; Gu, 1982). Correlations be-
tween Eolamprotula and other invertebrate fos-
sils also indicate a Middle Jurassic age for the
LSF (Wang et al., 2010).
Our ICP-MS results showed detrital zircon
age subpopulations in two of the LSF’s dino-
saur-bearing sandstones (ZG-2 and ZG-4). The
youngest subpopulation consisted of 7 analyses
ranging from 161 to 159 Ma (ZG-2) and 14
analyses ranging from 162 to 159 Ma, (ZG-4)
(Fig. 4). Zircon grains that gave these ages ex-
hibited euhedral morphologies, oscillatory zon-
ing patterns and high Th/U ratios (0.4–1.7 for
ZG-2 and 0.4–0.5 for ZG-4) indicating igneous
origins. Consistent variation in observed length/
width ratios and color/contrast of zircon grains
from each sample suggests crystallization from
different igneous bodies. We thus interpret the
youngest single grain age of 159 ± 2 Ma (2 Ma
as the analytic error, also the same age as de-
tected in both samples) as the maximum depo-
sitional age for the LSF. Outdated version of the
geologic time scale (Harland et al., 1990) placed
the absolute age boundary between the Middle
and Late Jurassic at 157 Ma. However, this
boundary has been updated to 163.5 ± 1.0 Ma
most recently (Ogg et al., 2016). We thus inter-
pret a Late Jurassic and specifically Oxfordian
depositional age for the LSF.
Lateral correlation of the LSF’s Euesthe-
ria fossils with those found in the Lanqi For-
mation in western Liaoning, China, supports
our Oxfordian age interpretation. Euestheria
ziliujingensis found in the LSF occur widely
throughout China, appearing in the Haifanggou
and Lanqi formations of western Liaoning and
in the Jiulongshan and Tiaojishan formations of
northern Hebei (Wang, 1998; Chen and Hud-
son, 1991). The Lanqi for example has yielded
tuff samples from its uppermost units recently
dated by high-precision 40Ar/39Ar methods at
160.7 ± 0.4 Ma and 158.7 ± 0.4 Ma (Chang et
al., 2009). An andesite dated from the Lanqi’s
lowermost unit gave 40Ar/39Ar ages of 160.7 ±
0.4 Ma and 158.7 ± 0.4 Ma (Chang et al., 2014).
Age constraints for the lowermost (165 Ma) and
uppermost (156–153 Ma) units in Tiaojishan
Formation of western Liaoning and northern
Hebei provinces (Zhang et al., 2008) also sup-
port a Late Jurassic age interpretation for the
correlative LSF.
Age of Basal Sauropods in the SCB
The Shunosaurus Fauna (Dong et al., 1983)
or Shunosaurus-Omeisaurus Fauna (Li et al.,
1997) were previously interpreted as represen-
tative Middle Jurassic basal sauropods (Dong et
al., 1983; Dong, 1992; Zhang, 1988; Li, 1998;
Sereno, 1999b; Buffetaut et al., 2000; Chat-
terjee and Zheng, 2002). The Chuanjiesaurus
Fauna also occurs in the lower Middle Juras-
sic Chuanjie Formation of Yunnan Province,
south of Sichuan Province. Chuanjiesaurus is
proposed to be chronostratigraphically compa-
rable to Omeisaurus, based on morphological
similarities in the postcranial skeletons (e.g.,
cervicals, caudals, the shoulder girdle, and the
hindlimbs) (Fang et al., 2008). Li et al. (2011)
argued that the Chuanjiesaurus is more derived
than Shunosaurus and Omeisaurus, while the
Chuanjiesaurus Fauna is different from the
Shunosaurus-Omeisaurus Fauna in its fossil
composition, and therefore may correspond
instead to the Mamenchisaurus Fauna, which
is suggested Middle to Late Jurassic in age (Li
and Cai, 1997; Ye, 2008). This lack of chro-
nostratigraphic constraints on basal sauropods
limits lateral correlation and interpretation of
their Middle Jurassic geographic range. Even
though invertebrate fossil assemblages gener-
ally indicate a Middle Jurassic age for the LSF
and USF, Shunosaurus-Omeisaurus and Mam-
enchisaurus faunas appear distinct from each
other in these two formations and may represent
faunas at different evolutionary stages (Li et
Jun Wang et al.
1498 Geological Society of America Bulletin, v. 130, no. 9/10
al., 2011). Dinosaurs from the Shunosaurus-
Omeisaurus Fauna show more phylogenetically
derived characteristics than those observed in
the Zizhongosaurus Fauna from the underly-
ing ZLF (Wang et al., 2010). The Shunosaurus-
Omeisaurus Fauna however shows more ple-
siomorphic characteristics than those observed
in the Mamenchisaurus Fauna (McPhee et al.,
2017). Previous studies therefore assigned the
Shunosaurus-Omeisaurus Fauna a Middle Ju-
rassic age (Dong et al., 1983; Dong, 1992; Li
and Cai, 1997; Li et al., 2011). Jurassic sauro-
pod dinosaurs in the SCB in fact appear to show
a high diversity associated with both temporal
differentiation and strong provincialism, pos-
sibly indicating rapid radiation during this
time (Li et al., 2011). Li et al. (1997) also in-
terpreted a middle to late Middle Jurassic age
for the LSF and USF from invertebrate fossils
and from a 178–165 Ma age measured by elec-
tron spin resonance (ESR) dating of samples
from the Shaximiao Formation in western areas
of the SCB (Gou et al., 2000). Thermal stabil-
ity constraints, however, place the upper age
limit of ESR dating at around 1–2 Ma (Grün,
1989). ESR therefore does not give reliable age
estimates for rocks formed before the Cenozoic
(Laurent et al., 1998; Zhao et al., 2006).
The detrital zircon U-Pb ages from the LSF’s
dinosaur-bearing beds indicate a maximum dep-
ositional age of 159 ± 2 Ma for the Shunosaurus-
Omeisaurus Fauna, younger than the previous
estimate of Middle Jurassic for the fauna (e.g.,
Dong et al., 1983; Dong, 1992; Zhang, 1988;
Buffetaut et al., 2000; Sereno, 1999b; Chatter-
jee and Zheng, 2002; Upchurch et al., 2004).
Omeisaurus tianfuensis however is generally
assigned a Late Jurassic age (McIntosh, 1990).
Radiation and Migration of Sauropods
Prior to the Late Jurassic
The basal sauropodomorphs, including the
“prosauropods” and Early Jurassic sauropods,
also have a presence in western China (e.g.,
Yunnanosaurus, Lufengosaurus, Gongxiansau-
rus), yet phylogenetic positions of these western
Chinese taxa are embedded with other coeval
taxa from a global distribution (Brusatte et al.,
2010; McPhee et al., 2017). The probable ear-
liest reported sauropod, Isanosaurus attavipa-
chi, was discovered from Late Triassic units in
Thailand indicating that sauropods originated
as early as Middle Triassic from a small region
of Southeast Asia, part of Pangea, (Buffetaut et
al., 2000) as predicted by Wilson and Sereno
(1998). Basal sauropods dispersed to other parts
of Laurasia and Gondwana in the Early Juras-
sic. Evidence of this expansion includes Mela-
norosaurus from the Late Triassic of South
Africa (Haughton, 1924), Antetonitrus (Yates
and Kitching, 2003) and Pulanesaura (McPhee
et al., 2015) from the Early Jurassic of South
Africa, Barapasaurus from the Early Jurassic
of India (Jain et al., 1975), Vulcanodon from
the Early Jurassic of Zimbabwe (Raath, 1972),
Tazoudasaurus from the Early Jurassic of Mo-
rocco (Allain et al., 2004), Zizhongosaurus
(Dong et al., 1983) and Gongxianosaurus (He
et al., 1998) from the Early Jurassic of China,
Jobaria (Sereno et al., 1999) and Spinophoro-
saurus (Remes et al., 2009) from the Middle Ju-
rassic of Niger, Patagosaurus from the Middle
Jurassic of Argentina (Bonaparte, 1979), Ce-
tiosaurus from the Middle Jurassic of England
(Phillips, 1871) and Ohmdenosaurus from the
Early Jurassic of Germany (Wild, 1978) (Fig.
1). Osteological descriptions of these basal sau-
ropods are generally based on fragmentary and
incomplete skeletons precluding accurate phy-
logenetic analyses and taxonomic assignments.
Uncertainties persist regarding phylogenetic
relationships among sauropods (Gillette, 2003;
Apaldetti et al., 2011). Regardless of these on-
going debates, Middle and Late Jurassic sau-
ropods clearly thrived throughout Laurasia and
Gondwana (McIntosh, 1990; Upchurch, 1998;
Wilson and Sereno, 1998; Gillette, 2003; Up-
church et al., 2004). Sauropods however did not
appear in North America until the Late Juras-
sic (Gillette, 1996a, 1996b), and all reported
sauropod fossils from North America belong to
the clade Neosauropoda. Except for Antarctica,
which hosts only a putative sauropod (Smith
and Pol, 2007), all the other major land masses
hosted clear examples of Late Jurassic and Cre-
taceous sauropods. Along with the SCB, Tibet
hosts an additional Asian example of the Shu-
nosaurus-Omeisaurus Fauna (He et al., 1988).
Rich et al. (1999) described the resemblance
between the neosauropod Tehuelchesaurus from
the late Middle Jurassic/Late Jurassic of Argen-
tina (Rich et al., 1999; Cúneo et al., 2013; Rauhut
et al., 2015) and the Late Jurassic Omeisaurus
from the SCB. Those workers suggested that
South American and Chinese sauropods were
not isolated but rather enjoyed a continuous and
broad geographic range in the Middle to Late
Jurassic. However, Russell (1993) and Li et al.
(2011) proposed a paleogeographic isolation in
the Late Jurassic SCB based on the unique mor-
phologies in Mamenchisaurus (e.g., the elon-
gated cervical vertebrae, though an elongated
neck is also found in Omeisaurus) as well as
the differences in the dinosaurian assemblage of
the Mamenchisaurus Fauna from the contempo-
rary world. The chronostratigraphic constraints
on fossil beds described here support this latter
hypothesis. Although phylogenetically more
primitive, the Shunosaurus-Omeisaurus Fauna
is younger than the neosauropod Tehuelchesau-
rus. Given that all non-Chinese basal sauropods
appear in units older than Late Jurassic, basal
sauropods from the SCB such as Shunosaurus,
Omeisaurus and even Mamenchisaurus may
represent isolated faunas. While other basal sau-
ropods went extinct, basal sauropods from the
SCB survived into the Late Jurassic, existing si-
multaneously with neosauropods in other parts
of Laurasia and Gondwana, as the neosauropods
became the dominant sauropod clade beginning
in the Late Jurassic. Sauropods appear to have
greatly expanded their geographic range from
the late Middle Jurassic to early Late Juras-
sic, a time frame during which they appeared
on all continents except Antarctica (Gillette,
2003). Comprehensive geochronologic and pa-
laeobiogeographic studies on early sauropods
can further constrain their evolutionary history
following the breakup of Pangea. The new and
reappraised age interpretations for the LSF and
associated Shunosaurus-Omeisaurus Fauna
presented here reveals the spatial complexity of
early sauropod evolution.
CONCLUSIONS
Our new and robust detrital zircon U-Pb geo-
chronology by ICP-MS provide the maximum
depositional age for the lower Lower Shaxim-
iao Formation, where the famous Shunosaurus-
Omeisaurus Fauna was established. One hun-
dred and ninety-six detrital zircon grains from
two out of four sandstones collected from the
dinosaur-bearing Lower Shaximiao Formation
yielded 7 and 14 concordant analyses with ages
ranging between 162–159 Ma, respectively.
Combining with the zircon age data and the geo-
biological age of the invertebrate fossil assem-
blages, we interpret the single youngest analy-
ses, 159 ± 2 Ma, as the maximum depositional
age of the Lower Shaximiao Formation and the
age of the Shunosaurus-Omeisaurus Fauna.
According to the latest version of the geologic
time scale, given the ICP-MS age provided
here, the Lower Shaximiao Formation and the
Shunosaurus-Omeisaurus Fauna should be as-
signed to the Oxfordian stage of the Late Juras-
sic, rather than the Middle Jurassic as previously
proposed. Since the Shunosaurus-Omeisaurus
Fauna was geographically isolated, our new and
reappraised age interpretations for the LSF may
lead to insights on the geographic expansion of
the Middle–Late Jurassic sauropods in terms
of temporal distribution and diversity through
Laurasia and Gondwana.
ACKNOWLEDGMENTS
This research was supported by an open fund
from the Key Laboratory of Economic Stratigraphy
Age of Jurassic basal sauropods in Sichuan, China
Geological Society of America Bulletin, v. 130, no. 9/10 1499
and Palaeogeography, Chinese Academy of Sciences
(Nanjing Institute of Geology and Palaeontology)
(2016KF05), an open fund from the State Key Labo-
ratory of Oil and Gas Reservoir Geology and Exploi-
tation (Chengdu University of Technology, China)
(PLC201703), The University of Hong Kong (HKU)
General Research Fund (17300515 and 17308316),
and the HKU Seed Funding Program for Basic Re-
search (201411159057). We thank Dr. Jean Wong and
Dr. Helen Geng for assistance with the zircon U-Pb
dating. Professor Hai-Chun Zhang assisted with field-
work. We thank Dr. Brad Singer, Dr. Kaori Tsukui,
and Dr. Michelle Stocker for their helpful comments
and suggestions.
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Science editor: Bradley S. Singer
aSSociate editor: Matthew hurtgen
ManuScript received 11 auguSt 2017
reviSed ManuScript received 12 January 2018
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