Ultrastructure of enamel and dentine in extant dolphins (Cetacea: Delphinoidea and Inioidea)
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ORIGINAL PAPER
Ultrastructure of enamel and dentine in extant dolphins (Cetacea: Delphinoidea and Inioidea) Carolina Loch • Warwick Duncan • Paulo C. Simo˜es-Lopes • Jules A. Kieser R. Ewan Fordyce
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Received: 18 July 2012 / Revised: 22 October 2012 / Accepted: 30 October 2012 / Published online: 17 November 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract Longitudinal and cross sections of teeth from 17 species of the Recent dolphins (Delphinoidea and Inioidea) were examined under scanning electron microscope to study the arrangement and ultrastructure of dental tissues with reference to phylogenetic and functional constraints. For most species, enamel had a simple bi-layered structure of radial enamel and an outer layer of prismless enamel. The outer prismless layer varied from 5 to 30 % of enamel thickness. The enamel of Burmeister’s porpoise (Phocoena spinipinnis) was entirely prismless. The prisms had an open sheath; tubules and tuft-like structures were common at the enamel-dentine junction. Cetacean dentine was characterized by irregularly distributed dentinal tubules in a relatively homogenous dentinal matrix. Radial enamel was observed in all Delphinoidea and in the franciscana (Pontoporia blainvillei), whereas the Amazon river dolphin (Inia geoffrensis) had prisms organized in Hunter– Schreger bands. HSB in enamel are regarded as a device for resisting propagation of cracks. These may occur due to increased functional demands, possibly related to the hardness of the species diet. Simplification in tooth shape and reduced biomechanical demands plausibly explain the
primitive radial organization among delphinoids and Pontoporia. The HSB structure in the Amazon river dolphin, similar to those of extinct archaeocetes, seems to have secondary functional implications. However, the distribution of HSB in more-basal odontocetes is too poorly known to judge whether the HSB of Inia are a retained plesiomorphic feature or convergence. Keywords Cetaceans Dental tissues Functional morphology Phylogeny Teeth Abbreviations EDJ Enamel-dentine junction HAP Hydroxyapatite HSB Hunter-Schreger bands IPM Interprismatic matrix OES Outer enamel surface PLEX Prismless external enamel SEM Scanning electron microscope
Introduction Communicated by A. Schmidt-Rhaesa. C. Loch (&) R. E. Fordyce Department of Geology, University of Otago, Dunedin 9054, New Zealand e-mail: [email protected] C. Loch W. Duncan J. A. Kieser Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin 9054, New Zealand C. Loch P. C. Simo˜es-Lopes Laborato´rio de Mamı´feros Aqua´ticos, Universidade Federal de Santa Catarina, Floriano´polis 88040-970, Brazil
Enamel forms the outermost layer of reptilian and mammalian tooth crowns. It is a highly mineralized tissue formed by inorganic crystals of hydroxyapatite surrounded by a protein matrix (Lucas 2004). In most reptiles, the needle-like crystals have a simple organization and are oriented in parallel, extending toward the surface of the tooth. In ma
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