The influence of character correlations on phylogenetic analyses: a case study of the carnivoran cranium

Character independence is a major assumption in many morphologybased phylogenetic analyses (Felsenstein, 1973; Emerson and Hastings, 1998). However, the fact that most studies of modularity and morphological integration have found significant correlations among many phenotypic traits worryingly calls into question the validity of this assumption. Because gathering data on character correlations for every character in every taxon of interest is unrealistic, studies of modularity are more tractable for assessing the impact of character non-independence on phylogenetic analyses in a real system because modules summarise broad patterns of trait correlations. In this study, we use empirically derived data on cranial modularity and morphological integration in the carnivoran skull to assess the impact of trait correlations on phylogenetic analyses of Carnivora. Carnivorans are a speciose clade of over 270 living species, with an extremely broad range of morphological and dietary diversity, from social insectivores to folivores to hypercarnivores (Nowak, 1999; Myers, 2000). This diversity offers many opportunities to isolate various potential influences on morphology, and, in this case, to study the effects of trait correlations on cranial morphology. Carnivorans also have an excellent fossil record, providing the opportunity to examine morphologies not represented in extant species, such as in the sabretoothed cat Smilodon. Perhaps most importantly, several recent molecular and morphological studies of carnivoran phylogeny (Hunt and Tedford, 1993; Wyss and Flynn, 1993; Tedford et al., 1995; Flynn and Nedbal, 1998; Flynn et al., 2000, 2005; Flynn and Wesley-Hunt, 2005; Wesley-Hunt and Flynn, 2005; Flynn et al., this volume) provide the necessary resolution to assess the influence of character correlations on morphology-based phylogenetic analyses.