Abstract

Interactions among morphological traits, or phenotypic integration, reflect genetic, developmental, and functional relationships among traits and can significantly bias morphological evolution. Simulations using theoretical and empirical trait covariance matrices confirm that integration can result in both more and less disparate organisms, and most often the latter, than would be expected under unconstrained evolution. However, high rates can persist even when morphological disparity is constrained by trait integration. Similar to a “fly in a tube”, trait integration may restrict evolution to particular regions of possible morphospace, but it doesn’t necessarily limit the pace of evolution within those regions. Importantly, high evolutionary rates within restricted regions of morphospace would be expected to result in a high degree of convergence and homoplasy.

Here, I discuss the patterns of cranial phenotypic integration and morphological evolution in a dataset spanning over 1000 species of living and extinct tetrapod species and representing over 300 million years of evolution. While most large-scale studies of phenotypic integration and morphological evolution utilise relatively limited descriptors of morphology, hindering comparisons across clades, surface sliding semi-landmark analysis allows for detailed quantification of complex 3D shapes, even across highly disparate taxa. Our dense 3D morphometric dataset of 700-1500 landmarks and sliding semi-landmarks demonstrates that patterns of cranial modularity are generally conserved across large clades (e.g., within mammals, birds, squamates, caecilians), there are clear shifts in patterns of integration across these clades. Tempo and mode are similarly highly variable across cranial regions and clades, with high variance concentrated in neural crest-derived regions, in particular the jaw joint and facial regions. While some clades show evidence that high integration constrains morphological evolution, there is not a consistent pattern of constraint across tetrapods.