Mosaic Convergence of Rodent Dentitions

Vincent Lazzari,Cyril Charles,Jacques Michaux,Jean-Jacques Jaeger,Monique Vianey-Liaud,Jean-Pierre Aguilar,Laurent Viriot,Paul Tafforeau

doi:10.1371/journal.pone.0003607

Abstract

BackgroundUnderstanding mechanisms responsible for changes in tooth morphology in the course of evolution is an area of investigation common to both paleontology and developmental biology. Detailed analyses of molar tooth crown shape have shown frequent homoplasia in mammalian evolution, which requires accurate investigation of the evolutionary pathways provided by the fossil record. The necessity of preservation of an effective occlusion has been hypothesized to functionally constrain crown morphological changes and to also facilitate convergent evolution. The Muroidea superfamily constitutes a relevant model for the study of molar crown diversification because it encompasses one third of the extant mammalian biodiversity.Methodology/Principal FindingsCombined microwear and 3D-topographic analyses performed on fossil and extant muroid molars allow for a first quantification of the relationships between changes in crown morphology and functionality of occlusion. Based on an abundant fossil record and on a well resolved phylogeny, our results show that the most derived functional condition associates longitudinal chewing and non interlocking of cusps. This condition has been reached at least 7 times within muroids via two main types of evolutionary pathways each respecting functional continuity. In the first type, the flattening of tooth crown which induces the removal of cusp interlocking occurs before the rotation of the chewing movement. In the second type however, flattening is subsequent to rotation of the chewing movement which can be associated with certain changes in cusp morphology.Conclusion/SignificanceThe reverse orders of the changes involved in these different pathways reveal a mosaic evolution of mammalian dentition in which direction of chewing and crown shape seem to be partly decoupled. Either can change in respect to strong functional constraints affecting occlusion which thereby limit the number of the possible pathways. Because convergent pathways imply distinct ontogenetic trajectories, new Evo/Devo comparative studies on cusp morphogenesis are necessary.

Highlights

For decades tooth crown morphology in mammals has provided key characters for taxonomy, phylogeny and reconstruction of diet adaptations of past species [1,2,3]
Understanding mechanisms that guided tooth crown morphological changes during evolution constitute a crucial area of investigation common to both paleontology and developmental biology
The various functional types of occlusion in muroid molars are characterized by two variables; i) the interlocking of corresponding valleys and summits delimited by cusps rows of opposite teeth [11] and ii) the direction of masticatory movements, which can be oblique or propalinal within the horizontal plane

Summary

Introduction

For decades tooth crown morphology in mammals has provided key characters for taxonomy, phylogeny and reconstruction of diet adaptations of past species [1,2,3]. The various functional types of occlusion in muroid molars are characterized by two variables; i) the interlocking of corresponding valleys and summits delimited by cusps rows of opposite teeth [11] and ii) the direction of masticatory movements, which can be oblique or propalinal (longitudinal) within the horizontal plane. Rodents with grade B (Fig. 1A) display oblique chewing movements, cuspidate crowns and cusp interlocking during occlusion [9].

Results

Conclusion