Dental integration and modularity in pinnipeds

Mieczyslaw Wolsan,Satoshi Suzuki,Masaharu Motokawa,Masakazu Asahara

doi:10.1038/s41598-019-40956-1

Abstract

Morphological integration and modularity are important for understanding phenotypic evolution because they constrain variation subjected to selection and enable independent evolution of functional and developmental units. We report dental integration and modularity in representative otariid (Eumetopias jubatus, Callorhinus ursinus) and phocid (Phoca largha, Histriophoca fasciata) species of Pinnipedia. This is the first study of integration and modularity in a secondarily simplified dentition with simple occlusion. Integration was stronger in both otariid species than in either phocid species and related positively to dental occlusion and negatively to both modularity and tooth-size variability across all the species. The canines and third upper incisor were most strongly integrated, comprising a module that likely serves as occlusal guides for the postcanines. There was no or weak modularity among tooth classes. The reported integration is stronger than or similar to that in mammals with complex dentition and refined occlusion. We hypothesise that this strong integration is driven by dental occlusion, and that it is enabled by reduction of modularity that constrains overall integration in complex dentitions. We propose that modularity was reduced in pinnipeds during the transition to aquatic life in association with the origin of pierce-feeding and loss of mastication caused by underwater feeding.

Highlights

Morphological integration and modularity are important for understanding phenotypic evolution because they constrain variation subjected to selection and enable independent evolution of functional and developmental units
Consistent with this observation, as expected, were values of integration indices, SDrel(λ) and Ir, which were, respectively, 0.776 and 0.767 for Eumetopias jubatus, 0.660 and 0.643 for Callorhinus ursinus, 0.549 and 0.535 for Phoca largha, and 0.510 and 0.500 for Histriophoca fasciata. These results indicated the strongest overall integration in Eumetopias jubatus, followed in descending order by those in Callorhinus ursinus, Phoca largha and Histriophoca fasciata
This study found that dental integration was positively related to dental occlusion across four representative pinniped species, and that integration was stronger between occluding teeth than between non-occluding ones in each of these species

Summary

Introduction

Morphological integration and modularity are important for understanding phenotypic evolution because they constrain variation subjected to selection and enable independent evolution of functional and developmental units. We report dental integration and modularity in representative otariid (Eumetopias jubatus, Callorhinus ursinus) and phocid (Phoca largha, Histriophoca fasciata) species of Pinnipedia This is the first study of integration and modularity in a secondarily simplified dentition with simple occlusion. Tension between the relative independence and the coordination of organismal parts is expressed in concepts of morphological integration[1] and modularity[2,3] Both concepts are closely related and concern the degree of covariation or correlation between different parts of an organism or other biological entity. Integration constrains the variability of individual traits, and modularity enables modules to vary and evolve independently of each other whilst still maintaining the integrity of the functional or developmental unit[4,10,11]. At lower levels of dental organisation, individual teeth[17] or tooth cusps[16] can be separate modules

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