The evolution of morphological integration in the mammalian skull

The correlated evolution of traits may be a principal factor in morphological evolution, but it typically has been studied only in genetic or developmental systems. Most previous studies examining phenotypic trait correlations, through analysis of morphological integration, considered only a small number of taxa, and thus had limited ability to test hypotheses of the influence of trait integration on morphological variation and diversity. In this dissertation, I examined morphological integration and phenotypic modularity in 107 species of mammals (98 extant and 9 fossil), including representatives of all three extant clades (monotremes, placentals, and marsupials), and encompassing a broad range of morphological, ecological, and developmental diversity.; This dissertation analyzed the conservation of phenotypic modularity across Mammalia, and applied tests of specific hypotheses to test the relationship of morphological integration to four potential major influences on cranial morphology: phylogeny, ecology, encephalization, and heterochrony. Phenotypic modularity is demonstrated to be conservative across therians, but not monotremes. Differences among modules may reflect developmental complexity, but does not simply follow hypothesized patterns of functional integration. Both phylogeny and diet are significantly correlated with similarity in morphological integration, but often in different clades. In contrast, analyses did not support encephalization or heterochrony as significant influences on trait integration in the mammalian cranium. This study is the first broad, comparative analysis of integration and modularity, and the results should prove useful in suggesting future productive avenues of study.; The interrelationships among morphology, development, function, and phylogeny have been a rich topic of study for centuries. The perspective of modularity is a relatively new and untapped source of information with great potential for elucidating these relationships in a way that is applicable to the fossil record and relevant to many aspects of evolutionary biology. This study not only examines characters as parts of larger regions, but it also considers the independence or integration of characters as an evolving trait, rather than a static feature. Future extensions of this study may cover a broad array of topics, from explicit comparisons of microevolutionary and macroevolutionary scales of evolution to examination of the relationship between morphological diversity and modularity.