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Ultrastructure of enamel and dentine in extant dolphins (Cetacea: Delphinoidea and Inioidea)

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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.

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Fig. 1
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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

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Acknowledgments

The authors are very grateful to Karen Stockin and Wendi Roe (Massey University—New Zealand) and Danielle Lima (IEPA–Brazil) for providing some of the samples for this study. Ludwig Jansen van Vuuren (Oral Rehabilitation, University of Otago) kindly helped in the sample embedding process. Thanks are also extended to Liz Girvan (OCEM, University of Otago), who provided technical assistance with SEM facilities; and Robert Boessenecker (Department of Geology, University of Otago) for designing Fig. 1. Michael V. Swain provided thoughtful discussions and valuable comments in early drafts of this manuscript. Three anonymous reviewers also contributed with important improvements during the review process. C. Loch was supported by a University of Otago PhD Scholarship.

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Authors and Affiliations

  1. Department of Geology, University of Otago, Dunedin, 9054, New Zealand

    Carolina Loch & R. Ewan Fordyce

  2. Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, 9054, New Zealand

    Carolina Loch, Warwick Duncan & Jules A. Kieser

  3. Laboratório de Mamíferos Aquáticos, Universidade Federal de Santa Catarina, Florianópolis, 88040-970, Brazil

    Carolina Loch & Paulo C. Simões-Lopes

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  1. Carolina Loch
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  2. Warwick Duncan
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  3. Paulo C. Simões-Lopes
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  4. Jules A. Kieser
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  5. R. Ewan Fordyce
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Correspondence to Carolina Loch.

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Communicated by A. Schmidt-Rhaesa.

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Loch, C., Duncan, W., Simões-Lopes, P.C. et al. Ultrastructure of enamel and dentine in extant dolphins (Cetacea: Delphinoidea and Inioidea). Zoomorphology 132, 215–225 (2013). https://doi.org/10.1007/s00435-012-0180-1

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