title: Synchrotron imaging of dentition provides insights into the biology of Hesperornis and Ichthyornis, the "last" toothed birds.
abstract: BACKGROUND
  The dentitions of extinct organisms can provide pivotal information regarding their phylogenetic position, as well as paleobiology, diet, development, and growth. Extant birds are edentulous (toothless), but their closest relatives among stem birds, the Cretaceous Hesperornithiformes and Ichthyornithiformes, retained teeth. Despite their significant phylogenetic position immediately outside the avian crown group, the dentitions of these taxa have never been studied in detail. To obtain new insight into the biology of these 'last' toothed birds, we use cutting-edge visualisation techniques to describe their dentitions at unprecedented levels of detail, in particular propagation phase contrast x-ray synchrotron microtomography at high-resolution.
  
  
  RESULTS
  Among other characteristics of tooth shape, growth, attachment, implantation, replacement, and dental tissue microstructures, revealed by these analyses, we find that tooth morphology and ornamentation differ greatly between the Hesperornithiformes and Ichthyornithiformes. We also highlight the first Old World, and youngest record of the major Mesozoic clade Ichthyornithiformes. Both taxa exhibit extremely thin and simple enamel. The extension rate of Hesperornis tooth dentine appears relatively high compared to non-avian dinosaurs. Root attachment is found for the first time to be fully thecodont via gomphosis in both taxa, but in Hesperornis secondary evolution led to teeth implantation in a groove, at least locally without a periodontal ligament. Dental replacement is shown to be lingual via a resorption pit in the root, in both taxa.
  
  
  CONCLUSIONS
  Our results allow comparison with other archosaurs and also mammals, with implications regarding dental character evolution across amniotes. Some dental features of the 'last' toothed birds can be interpreted as functional adaptations related to diet and mode of predation, while others appear to be products of their peculiar phylogenetic heritage. The autapomorphic Hesperornis groove might have favoured firmer root attachment. These observations highlight complexity in the evolutionary history of tooth reduction in the avian lineage and also clarify alleged avian dental characteristics in the frame of a long-standing debate on bird origins. Finally, new hypotheses emerge that will possibly be tested by further analyses of avian teeth, for instance regarding dental replacement rates, or simplification and thinning of enamel throughout the course of early avian evolution.
authors.affiliation: Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5242, Institut de Génomique Fonctionnelle de Lyon, Equipe évo-dévo de la denture chez les vertébrés, Ecole Normale Supérieure de Lyon, Université Lyon 1, 46 Allée d'Italie, 69364, Lyon cedex 7, France.|UMR CNRS/MNHN 7179, "Mécanismes adaptatifs: des organismes aux communautés", 57 rue Cuvier CP55, 75005, Paris, France.
authors.forename: Maïtena
authors.initials: M
authors.lastname: Dumont
authors.affiliation: ESRF-The European Synchrotron, 71, avenue des Martyrs, CS 40220, F-38043, Grenoble Cédex 9, France.
authors.forename: Paul
authors.initials: P
authors.lastname: Tafforeau
authors.affiliation: Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5242, Institut de Génomique Fonctionnelle de Lyon, Equipe évo-dévo de la denture chez les vertébrés, Ecole Normale Supérieure de Lyon, Université Lyon 1, 46 Allée d'Italie, 69364, Lyon cedex 7, France.
authors.forename: Thomas
authors.initials: T
authors.lastname: Bertin
authors.affiliation: Department of Geology and Geophysics and Peabody Museum of Natural History, Yale University, New Haven, 06520, CT, USA.
authors.forename: Bhart-Anjan
authors.initials: BA
authors.lastname: Bhullar
authors.affiliation: Department of Geology and Geophysics and Peabody Museum of Natural History, Yale University, New Haven, 06520, CT, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Field
authors.affiliation: Natuurhistorisch Museum Maastricht, De Bosquetplein 6-7, NL-6211 KJ, Maastricht, The Netherlands.|Present Address: Naturalis Biodiversity Center, Darwinweg 2, 2333CR, Leiden, The Netherlands.
authors.forename: Anne
authors.initials: A
authors.lastname: Schulp
authors.affiliation: Royal Tyrrell Museum of Palaeontology, P.O. Box 7500, Drumheller, T0J 0Y0, AB, Canada.
authors.forename: Brandon
authors.initials: B
authors.lastname: Strilisky
authors.affiliation: Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5242, Institut de Génomique Fonctionnelle de Lyon, Equipe évo-dévo de la denture chez les vertébrés, Ecole Normale Supérieure de Lyon, Université Lyon 1, 46 Allée d'Italie, 69364, Lyon cedex 7, France.
authors.forename: Béatrice
authors.initials: B
authors.lastname: Thivichon-Prince
authors.affiliation: Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5242, Institut de Génomique Fonctionnelle de Lyon, Equipe évo-dévo de la denture chez les vertébrés, Ecole Normale Supérieure de Lyon, Université Lyon 1, 46 Allée d'Italie, 69364, Lyon cedex 7, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Viriot
authors.affiliation: Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5242, Institut de Génomique Fonctionnelle de Lyon, Equipe évo-dévo de la denture chez les vertébrés, Ecole Normale Supérieure de Lyon, Université Lyon 1, 46 Allée d'Italie, 69364, Lyon cedex 7, France. antoine.louchart@ens-lyon.fr.|Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5553, LECA, Equipe Paléo-Génomique, and Palgene (CNRS/ENS de Lyon), Ecole Normale Supérieure de Lyon, Université de Lyon, 46 Allée d'Italie, 69364, Lyon cedex 7, France. antoine.louchart@ens-lyon.fr.
authors.forename: Antoine
authors.initials: A
authors.lastname: Louchart
keywords: Dental attachment; Dental replacement; Dinosaurs; Fossil birds; Growth; Histology; Microstructure; Paleobiology; Teeth