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Tooth occlusal morphology in the durophagous marine reptiles, Placodontia (Reptilia: Sauropterygia)

Published online by Cambridge University Press:  14 September 2016

Stephanie B. Crofts ,
James M. Neenan ,
Torsten M. Scheyer  and
Adam P. Summers
Stephanie B. Crofts
Affiliation:
Department of Biological Sciences, New Jersey Institute of Technology, Newark, New Jersey 07102-1982, U.S.A. E-mail:crofts@njit.edu
James M. Neenan
Affiliation:
Oxford University Museum of Natural History, Oxford, OX1 3PW, U.K., and Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, U.K. E-mail: james.neenan@oum.ox.ac.uk
Torsten M. Scheyer
Affiliation:
Universität Zürich, Paläontologisches Institut und Museum, Zürich CH-8006, Switzerland
Adam P. Summers
Affiliation:
Department of Biology, University of Washington, Seattle, Washington 98195-1800, U.S.A., and University of Washington, Friday Harbor Laboratories, Friday Harbor, Washington 98250, U.S.A
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Abstract

Placodontia were a group of marine reptiles that lived in shallow nearshore environments during the Triassic. Based on tooth morphology it has been inferred that they were durophagous, but tooth morphology differs among species: placodontoid placodonts have teeth described as hemispherical, and the teeth of more highly nested taxa within the cyamodontoid placodonts have been described as flat. In contrast, the sister taxon to the placodonts, Palatodonta bleekeri, like many other marine reptiles, has tall pointed teeth for eating soft-bodied prey. The goals of this paper are to quantify these different tooth morphologies and compare tooth shape among taxa and with a functionally “optimal” tooth. To quantify tooth morphology we measured the radius of curvature (RoC) of the occlusal surface by fitting spheres to 3D surface scans or computed microtomographic scans. Large RoCs correspond to flatter teeth, while teeth with smaller RoCs are pointier; positive RoCs have convex occlusal surfaces, and a negative RoC indicates that the occlusal surface of the tooth is concave. We found the placodontoid taxa have teeth with smaller RoCs than more highly nested taxa, and palatine teeth tend to be flatter and closer to the optimal morphology than maxillary teeth. Within one well-nested clade, the placochelyids, the rearmost palatine teeth have a more complex morphology than the predicted optimal tooth, with an overall concave occlusal surface with a small, medial cusp. These findings are in keeping with the hypothesis that placodonts were specialized durophagous predators with teeth modified to break hard prey items while resisting tooth failure.

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Articles
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Paleobiology , Volume 43 , Issue 1 , February 2017 , pp. 114 - 128
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Copyright © 2016 The Paleontological Society. All rights reserved 

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