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IntroductionThe astonishing spectrum of scarabaeine lifestyles makes them an attractive group for studies in entomology and evolutionary biology. As a result of adaptions to specific food substrates and textures, the mouthparts of dung beetles, particularly the mandible, have undergone considerable evolutionary changes and differ distinctly from the presumptive ancestral conditions of the Coleoptera and Polyphaga. The possible functions of dung beetle mouthparts and the evolution of dung feeding have been controversial for decades.ResultsIn this study, 187 scarabs representing all tribes of the Scarabaeinae and the major lineages within the Scarabaeoidea, along with three major feeding types within the Scarabaeoidea (omnivory, phytophagy and coprophagy), were studied. Based on geometric morphometric and three-dimensional (3D) reconstruction approaches, morphological differences in mandibles among the three feeding types were identified. The ancestral forms of the mandible within the Scarabaeinae were reconstructed and compared with those of modern species. The most recent common ancestor of the Scarabaeinae fed on soft materials, and the ancestor of the Scarabaeinae and the Aphodiinae was in an evolutionary transition between processing more solid and softer substrates.ConclusionsCoprophagy originated from omnivorous ancestors that were very likely saprophagous. Furthermore, phytophagy may also have originated from omnivory. In addition, our study addresses the integration and modularity of geometric morphometric data in a phylogenetic context.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-015-0123-z) contains supplementary material, which is available to authorized users.

Stag beetles (Coleoptera, Scarabaeoidea, Lucanidae) have received extensive attention from researchers in behavioral ecology and evolutionary biology. There have been no previous quantitative analyses, particularly using a geometric morphometric approach based on a large sample of data, to shed light on the morphological diversity and evolution of Lucanidae. Thoracic adaptation and ecological differentiation are intimately related, and the pronotum bears important muscles and supports the locomotion of prothoracic legs. The elytron is an autapomorphy of the Coleoptera. To reconstruct and visualize the patterns of evolutionary diversification and phylogenetic history of shape change, an ancestral groundplan can be reconstructed by mapping geometric morphometric data onto a phylogenetic tree. In this study, the morphologies of the pronotum and elytron in 1303 stag beetles (Lucanidae), including approximately 99.2% of all globally described species, were examined, thus revealing several aspects of morphological diversity and evolution. First, on the basis of geometric morphometric analysis, we found significant morphological differences in the pronotum or elytron between any two Lucanidae subfamilies. And we subsequently reconstructed the ancestral groundplans of the two structures in stag beetles and compared them with those of extant species (through cladistic and geometric morphometric methods). The ancestral groundplan of Lucanidae was found to be most similar to extant Nicagini in both the pronotum and elytron, according to Mahalanobis distances. Furthermore, we analyzed species richness and morphological diversity of stag beetles and the relationships between them and found that the two parameters were not always correlated. Aesalinae was found to be the most diverse subfamily in both the pronotum and elytron, despite its poor species richness, and the diversity of the pronotum or elytron was not superior in Lucaninae, despite its high species richness. Our study provides insights into the morphological variations and evolutionary history of the pronotum and elytron in four subfamilies of stag beetles, and it illuminates the relationship between morphological diversity and species richness. Intriguingly, our analysis indicates that morphological diversity and species richness are not always correlated. These findings may stimulate further studies in this field.

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