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    • Acknowledgements

    We thank Z.-Y. Sun and Sanya Museum of Marine Paleontology for access to the specimen, J.-Z. Ding (IVPP) for skilful preparation of the specimen, W. Gao (IVPP) for photographic assistance, Y.-S. Lou (IVPP) for laboratory assistance during the course of the study, L.-T. Wang (Guizhou Geological Survey) for field assistance, and R.R. Schoch, H.-D. Sues, G.S. Bever and T.R. Lyson for providing information or references. X.-C.W., N.F. and O.R. thank the IVPP for hospitality during their visits. This work was supported by the Strategic Priority Research Programs of Chinese Academy of Sciences (XDA19050102 and XDB26000000 to C.L.), the National Science Foundation of China (41772006 to C.L.) together with support from the IVPP and the Canadian Museum of Nature (RCP09 to X.-C.W.).

    Reviewer information

    Nature thanks G. S. Bever, R. R. Schoch and H.-D. Sues for their contribution to the peer review of this work.

    Author information

    Affiliations

    1. Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Chinese Academy of Sciences, Beijing, China

      • Chun Li

    2. Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Beijing, China

      • Chun Li

    3. National Museums Scotland, Edinburgh, UK

      • Nicholas C. Fraser

    4. Field Museum of Natural History, Chicago, IL, USA

      • Olivier Rieppel

    5. Canadian Museum of Nature, Ottawa, Ontario, Canada

      • Xiao-Chun Wu

    Contributions

    All authors conceived the project and participated in the writing of the text and Supplementary Information. C.L. led the field work and acquired the specimen for study. X.-C.W. drafted the figures with input from all authors.

    Competing interests

    The authors declare no competing interests.

    Corresponding authors

    Correspondence to Chun Li or Xiao-Chun Wu.

    Extended data figures and tables

    1. Extended Data Fig. 1 Stratigraphic section of the Lower Wayao Member (LWM) of the Falang Formation.

      The section was found at Heshangjing, Baiyuncun of Xinpuxiang in Guanling of Guizhou Province, near the locality of E. sinensis. The Guanling biota originates in the lower part of the upper unit in the LWM. The LWM is about 14.5 m thick in this section, nearly 2.5 m thicker than that of the Wolonggang quarry section near the village of Xiaowa (Supplementary Information). The block of the black shaly marlstone containing E. sinensis is full of small bivalves, whereas the block of thin marlstones containing O. semitestacea has large bivalves (species of Halobia and Daonella) and also scattered ammonites (species of Trachyceras and Paratrachyceras). The horizon where E. sinensis originated is 7.5 m below the one that contained O. semitestacea but slightly above the dark grey, marly, laminated micritic limestones that produce abundant skeletons of ichthyosaurs, thalattosaurs and placodonts as well as other fossils (Supplementary Information).

    2. Extended Data Fig. 2 Extra information on the skull of E. sinensis (SMMP 000016).

      a, b, X-radiograph and photograph of the skull in ventral view. c, d, Photograph and line drawing of the snout portion of the skull in right-lateral view, showing the ornamentation on the external surface of the edentulous premaxillae, the anterior portions of the right dentary and maxilla; the absence of teeth and the surface ornamentation suggest the presence of a rhamphotheca in E. sinensis in life. e, Close-up photograph of the posterior portion of the left maxilla, showing column-like pleurodont posterior teeth with blunt tips and the teeth deeply inset from the labial margin and ankylosed to the labial base, which differs from the subthecodont implantation in Eunotosaurus africanus and Pappochelys rosinae; the implantation pattern of the teeth is unknown for O. semitestacea, because the lingual side of the dentition is not exposed in the known specimens. New characters: ch. 273 (1), keratinous beak present, as indicated by surface ornamentation; ch. 274 (1), anterior end of the dentary edentulous; ch. 275 (1), dentary tooth number fewer than 30. d, dentary; l, lacrimal; m, maxilla; ob, orbit; pm, premaxilla.

    3. Extended Data Fig. 3 A close-up of the postorbital portion of the skull roof of E. sinensis (SMMP 000016) in dorsal view.

      a, Photograph. b, Line drawing, showing no supratemporal or trace of a supratemporal fenestra between the frontal, postfrontal, postorbital and parietal. Zigzag lines denote broken surfaces. f, frontal; j, jugal; lp, left parietal; ltf, lower temporal fenestra; ob, orbit, po, postorbital; pof, postfrontal; rp, right parietal; sq, squamosal.

    4. Extended Data Fig. 4 Cervical and dorsal vertebrae of E. sinensis (SMMP 000016).

      a, b, Axis and cervical 3 in dorsal and left-lateral views, showing that the axis is characterized by an anteroposteriorly broadened neural spine that is higher posteriorly than anteriorly, unlike Proganochelys quenstedti and other turtles that show an anterior midline projection on the axis neural spine. c, d, Dorsals 6 and 7 in dorsal view, showing the prezygapophyses and postzygapophyses that are unusually different in size, unlike those of the post-axial cervicals and the first two dorsal vertebrae that are similar in size and robustness. e, An incomplete osteoderm in dorsal view. There is no evidence of extensive dermal armour. The single fragment of an osteoderm with surface ornamentation located between the transverse processes of the fourth and fifth caudal vertebrae is, however, inconsistent with the otherwise excellent preservation and articulation of the skeleton and almost certainly represents a different taxon and individual. Zigzag lines denote broken surfaces. atc, atlantal centrum; ax, axis; cap, capitulum; cav4, caudal vertebra 4; cv3, cervical vertebra 3; dv7, dorsal vertebra 7; ns, neural spine; nt, neural table; os, osteoderm; poz, postzygapophysis; prz, prezygapophysis; r, rib; rh, rib head; tub, tuberculum.

    5. Extended Data Fig. 5 Posterior trunk of E. sinensis (SMMP 000016) in dorsal view.

      a, Photograph. b, Line drawing showing that the dorsal surface of the distinctly broadened dorsal ribs is faintly ornamented by a series of striations. In addition, the stout sacral ribs are fused to the sacral vertebrae (as they are in O. semitestacea and P. rosinae) and their distinctly expanded distal ends are detached from the ilium, in contrast to the tightly fused condition in P. quenstedti. Dorsal ribs 11 and 12 of E. sinensis are not specialized and are very different from dorsal ribs 1–9 in morphology.

    6. Extended Data Fig. 6 Puboischiadic plate in stem turtles.

      a–c, Photograph, line drawing and reconstruction of the puboischiadic plate of E. sinensis (SMMP 000016) in ventral view. d, e, Photograph and line drawing of the puboischiadic plate of O. semitestacea (the paratype, IVPP V 13240) with the ossified hypoischium in ventral view. New characters: ch. 276 (1), lateral process of the ischium present; ch. 277 (1), posterior directed process of the ischium present; ch. 278 (1), bony symphysis (a sutural midventral contact) between pubes and ischia present; ch. 279 (1), the rigid puboischiadic plate with a ventromidline keel present in adults; ch. 280 (1), hypoischium present. fem;, femur; his, hypoischium; is, ischium; lppu, lateral process of the pubis; ltis, lateral tubercle of the ischium; of, obturator foramen; peis; posterior elongation of the ischium; pik, puboischiadic keel; plt, plastron; pu, pubis.

    7. Extended Data Fig. 7 Strict consensus of 16 most parsimonious trees.

      The 16 trees (with a tree length of 1,207, a consistency index of 0.269, and a retention index of 0.582) were generated using a new technology search of TNT v.1.016 based on the large data matrix (52 taxa and 280 characters) derived from previously published datasets3,4, showing that the monophyly of ‘Pantestudines’ with Claudiosaurus and Acerosodontosaurus excluded and the interrelationships of Eorhynchochelys within the clade are the same as produced by the analysis based on the reduced dataset4 (see the clade formed by Eunotosaurus and more derived turtles in Fig. 4). As in the analysis of the reduced dataset, the new technology search based on 100 random addition sequence replicates and 1,000 random seeds was implemented, and the advanced search settings were changed to ensure enough iterations: 100 sectorial search drifting cycles, 100 ratchet iterations, 100 drift cycles and 100 rounds of tree fusion for every replicate. Multistate characters were treated as unordered and all characters were equally weighted. Bremer supporting values (numbers) are shown for each node.

    Supplementary information

    1. Supplementary Information

      This file contains Supplementary Data sections 1-3 and Supplementary References.

    2. Reporting Summary