title: Perinatal Specimens of Saurolophus angustirostris (Dinosauria: Hadrosauridae), from the Upper Cretaceous of Mongolia.
abstract: BACKGROUND
  The Late Cretaceous Nemegt Formation, Gobi Desert, Mongolia has already yielded abundant and complete skeletons of the hadrosaur Saurolophus angustirostris, from half-grown to adult individuals.
  
  
  RESULTS
  Herein we describe perinatal specimens of Saurolophus angustirostris, associated with fragmentary eggshell fragments. The skull length of these babies is around 5% that of the largest known S. angustirostris specimens, so these specimens document the earliest development stages of this giant hadrosaur and bridge a large hiatus in our knowledge of the ontogeny of S. angustirostris.
  
  
  CONCLUSIONS
  The studied specimens are likely part of a nest originally located on a riverbank point bar. The perinatal specimens were buried by sediment carried by the river current presumably during the wet summer season. Perinatal bones already displayed diagnostic characters for Saurolophus angustirostris, including premaxillae with a strongly reflected oral margin and upturned premaxillary body in lateral aspect. The absence of a supracranial crest and unfused halves of the cervical neural arches characterize the earliest stages in the ontogeny of S. angustirostris. The eggshell fragments associated with the perinatal individuals can be referred to the Spheroolithus oogenus and closely resemble those found in older formations (e.g. Barun Goyot Fm in Mongolia) or associated with more basal hadrosauroids (Bactrosaurus-Gilmoreosaurus in the Iren Dabasu Fm, Inner Mongolia, China). This observation suggests that the egg microstructure was similar in basal hadrosauroids and more advanced saurolophines.
  One of the authors (FE) is employed by the commercial organization Eldonia. Eldonia provided support in the form of a salary for FE, but did not have any additional role or influence in the study design, data collection and analysis, decision to publish, or preparation of the manuscript and it does not alter the authors' adherence to all the PLoS ONE policies on sharing data and materials.
authors.affiliation: Directorate 'Earth and History of Life', Royal Belgian Institute of Natural Sciences, rue Vautier 29, B-1000, Brussels, Belgium; Research Unit Palaeontology, Department Geology and Soil Sciences, Ghent University, Krijgslaan 281, 9000, Ghent, Belgium.
authors.forename: Leonard
authors.initials: L
authors.lastname: Dewaele
authors.affiliation: Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaanbaatar, 210-351, Mongolia.
authors.forename: Khishigjav
authors.initials: K
authors.lastname: Tsogtbaatar
authors.affiliation: Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaanbaatar, 210-351, Mongolia.
authors.forename: Rinchen
authors.initials: R
authors.lastname: Barsbold
authors.affiliation: Université de Poitiers, IPHEP, UMR CNRS 7262, 6 rue M. Brunet, 86073, Poitiers cedex 9, France.
authors.forename: Géraldine
authors.initials: G
authors.lastname: Garcia
authors.affiliation: Earth System Science, AMGC, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium.
authors.forename: Koen
authors.initials: K
authors.lastname: Stein
authors.affiliation: Eldonia, 9 Avenue des Portes Occitanes, 3800, Gannat, France.
authors.forename: François
authors.initials: F
authors.lastname: Escuillié
authors.affiliation: Directorate 'Earth and History of Life', Royal Belgian Institute of Natural Sciences, rue Vautier 29, B-1000, Brussels, Belgium.
authors.forename: Pascal
authors.initials: P
authors.lastname: Godefroit
mesh_terms: D000367:Age Factors; D000818:Animals; D050281:Bones of Lower Extremity / Q000033:anatomy & histology*; D001133:Bones of Upper Extremity / Q000033:anatomy & histology*; D025061:Dinosaurs / Q000033:anatomy & histology* / Q000145:classification; D004528:Egg Shell / Q000033:anatomy & histology; D005580:Fossils; D019015:Geologic Sediments; D008986:Mongolia; D010802:Phylogeny; D012886:Skull / Q000033:anatomy & histology*; D013131:Spine / Q000033:anatomy & histology*