Age of Jurassic basal sauropods in Sichuan, China 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 ABSTRACT appeared sporadically in Early to Middle Ju- Thus, our inductively coupled plasma–mass rassic localities of China, India, Europe, South spectrometry (ICP-MS) U-Pb zircon data pro- Sauropoda are the largest terrestrial ani- America and Africa (e.g., Phillips, 1871; Raath, vide novel geochronologic constraints on the mals to have ever lived and represent the 1972; Weishampel, 1992; Monbaron et al., evolutionary history of early sauropods. dominant herbivorous dinosaurs of the Me- 1999; Buffetaut et al., 2000; Allain et al., 2004; sozoic Era. The Lower Shaximiao Formation Stumpf et al., 2015) (Fig. 1). They became more GEOLOGICAL BACKGROUND of the Sichuan Basin, Southwest China, hosts diversified phylogenetically and flourished in abundant Jurassic basal sauropods including both hemispheres from the Middle Jurassic to The intracratonic SCB is tectonically situ- the Shunosaurus-Omeisaurus Fauna. This the end of the Late Jurassic (Sereno, 1999a), ated in the northwestern portion of the Yangtze formation was previously hypothesized to be possibly due to the breakup of Pangea (Chatter- block, surrounded by orogenic belts (Zhang et Middle Jurassic based on biostratigraphic jee and Zheng, 2002). al., 2004; Liu et al., 2006) (Fig. 2). Numerous interpretations, but the exact depositional In China, Early to Middle Jurassic fossils of studies have described the widely-distributed age is uncertain. Here we report the youngest basal sauropod only occur in Sichuan and Yun- Mesozoic stratigraphy of this basin (CC- inductively coupled plasma–mass spectrom- nan provinces (Dong et al., 1983; Dong, 1992). MSPSB, 1982; BGMRSP, 1991; Meng et al., etry (ICP-MS) detrital zircon U-Pb age of In Sichuan Province of southwest China, basal 2005). Upper Triassic to Quaternary terrestrial 159 ± 2 Ma for fossil-bearing strata from this sauropod fossils unearthed from the Ziliujing facies reach thicknesses of 2000–6000 m and formation as the maximum depositional age. Formation (ZLF) and Lower Shaximiao For- overlie carbonate-dominated Sinian to Middle This age falls very close to the Oxfordian age mation (LSF) in the Sichuan Basin (SCB) are Triassic marine facies (BGMRSP, 1991). Up- interpreted for the Shunosaurus-Omeisaurus part of the Zizhongosaurus and Shunosaurus- per Triassic and Jurassic SCB sedimentary units Fauna and is younger than previously pro- Omeisaurus faunas (Figs. 2 and 3). These fau- consist mostly of typical lacustrine and fluvial posed. We suggest that when the widely dis- nas are traditionally assigned Early and Middle facies, which include reddish conglomerates, tributed basal sauropods of the Early-Middle Jurassic ages, respectively, based on biostrati- sandstones and mudstones, referred to as red Jurassic were mostly replaced by the phylo- graphic correlation of assemblage biozones beds (Wang et al., 2008). Based on lithologic, genetically more-derived neosauropods in (Dong et al., 1983; Dong, 1992; Li et al., 1997, paleontologic, and sedimentologic characteris- the Late Jurassic in other regions of Laurasia 2011; Peng et al., 2005; Ye, 2006; Wang et al., tics; these sequences are divided into the follow- and Gondwana, some more basal members 2008). Even though many studies have been ing formations, in ascending order: the Upper survived and diversified in the Sichuan Basin conducted on the basal sauropods from the SCB Triassic Xujiahe Formation, the Lower Jurassic of southwestern China. in the past decades (e.g., Dong, 1992; He et al., Zhenzhuchong Formation and Ziliujing For- 1988, 1998; Li, 1998), few have included geo- mation (ZLF), the Middle Jurassic Xintiangou INTRODUCTION chronologic constraints due to the general lack Formation (XTF), Lower Shaximiao Forma- of igneous bodies within Mesozoic sedimentary tion (LSF) and Upper Shaximiao Formation The earliest known sauropods were discov- rocks. Here we report new detrital zircon U-Pb (USF), and the Upper Jurassic Suining Forma- ered in Late Triassic units—Norian stage—of age determined by laser ablation–inductively tion (SNF) and Penglaizhen Formation (PLF) Zimbabwe (McIntosh, 1990; Raath, 1972; Yates coupled plasma–mass spectrometry (LA-ICP- (BGMRSP, 1991; Wang et al., 2010). and Kitching, 2003) and Late Norian or Rhae- MS) constraints for four dinosaur-bearing LSF The SCB hosts a range of dinosaur body fos- tian units of Thailand (Buffetaut et al., 2000, sandstones found on the grounds of the Zigong sils representing 30 genera and 43 species (Wang 2002) (Fig. 1). Basal sauropods, here we mean Dinosaur Museum, Sichuan Province, China. et al., 2010) as well as trace fossils—footprints— sauropods more basal than Neosauropoda, also Reliable age determinations for the LSF’s fossil representing 20 genera and 24 species (Ye et al., bearing beds are critical to understanding evo- 2012). Given this diversity, the SCB is consid- lution and paleobiogeography of sauropods and ered a classic locality for Jurassic dinosaurian †Corresponding author: [email protected]. other Early to Late Jurassic fossil assemblages. research. The Middle Jurassic LSF and USF crop 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 . For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 130, no. 9/10 1493 © 2018 Geological Society of America Jun Wang et al. 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). out in Zigong City where the main dinosaur- the main hall of the museum. Sample ZG-2 is Nd:YAG laser. The analyses were performed bearing units occur on the grounds of the Zigong also a gray-green, intermediate sandstone from with a 30 or 22 mm beam diameter, 6 Hz repeti- Dinosaur Museum. Dinosaur fossils from Zigong the same layer that occurs ~50 cm above sam- tion rate and an energy of 0.6–1.3 mJ per pulse. City represent the majority of Mesozoic dinosaur ple ZG-1. Sample ZG-3 and ZG-4 are yellow- Other instrumental settings and procedural specimens in the SCB (Peng et al., 2005; Wang et green, intermediate sandstones from the layer details used here were described by Xia et al. al., 2008). The LSF yields 10 genera and 12 spe- nos. 3 and 7, respectively. (2004). The standard zircon 91500 was used as a cies of saurischian dinosaurs (Wang et al., 2010), Zircon separation and U-Pb dating were con- primary calibration standard and GJ-1 as a sec- including the basal sauropods Shunosaurus lii ducted at the Department of Earth Sciences, The ondary reference. Euhedral zircon grains with (Dong et al., 1983; Zhang, 1988; Li, 1998) and University of Hong Kong. The four sandstone zoning structures that indicate magmatic origins Omeisaurus tianfuensis (He et al., 1988), which samples were crushed and sieved by standard were selected for dating. We used the Isoplot/ are two of the most iconic fossil taxa of the SCB’s methods. Grains having lengths of 60–200 mm Ex 3.0 software package (Ludwig, 2003) for Shunosaurus-Omeisaurus Fauna. were retained and washed with distilled water. U-Pb age calculation and the Microsoft Excel Zircons were then separated by magnetic and macro developed by Andersen (2002) for com- SAMPLING AND METHODOLOGY heavy liquid methods. Euhedral zircon grains mon Pb correction. U-Pb age data with 1s er- were hand-picked under binocular microscope rors are shown in Supplementary Table DR11 We collected four sandstones from the lower and mounted in epoxy resin. Epoxy mounts part of the fossil-bearing LSF for geochronolog- were polished to expose grain midsections at ical analyses. The LSF outcrop was accessible approximately two-thirds of their thickness. 1GSA Data Repository item 2018117, Table DR1, as part of a large, indoor paleontological exhibit Zircon U-Pb data were obtained using a VG ICP-MS U-Pb isotopic data for zircon grains sepa- curated by the Zigong. Figure 3 shows lithol- PQ Excel ICP-MS equipped with a New Wave rated from the dinosaur-bearing sandstones of the ogy of the LSF and sampling localities. Sample Research LUV213 laser-ablation system (LA- Late Jurassic Lower Shaximiao Formation, Sichuan, SW China, is available at http://www.geosociety ZG-1 was a gray-green, intermediate sandstone ICP-MS).