High-density three-dimensional morphometric analyses support conserved static (intraspecific) modularity in caecilian (Amphibia: Gymnophiona) crania

Analyses of phenotypic integration and modularity seek to quantify levels of covariation among traits to identify their shared functional, developmental and genetic underpinnings ('integration'), which may delineate semi-independent subsets of highly integrated traits ('modules'). Existing studies have focused mainly on mammals or model organisms, limiting our understanding of factors that shape patterns of integration and modularity and their importance for morphological evolution. We present the first study of static (intraspecific) integration and modularity in caecilian crania, using dense surface sliding semi-landmarks to quantify cranial morphology in Boulengerula boulengeri and Idiocranium russeli. Eleven hypotheses of modular organization were compared with a likelihood approach and best-fitting models confirmed with covariance ratio analysis. Allometric corrections and subsampling analyses demonstrated the robustness of results. Allometry had a substantially larger influence on the results of landmark-only analyses relative to analyses incorporating semi-landmarks. Idiocranium russeli displayed significantly higher variation than B. boulengeri, but they had similar 12- and 13-module patterns, respectively, suggesting that caecilian crania are highly modular and that the modularity pattern is largely conserved across these species, despite their divergent morphologies and > 175 Myr of independent evolution. As in previous mammalian studies, our caecilian study suggests that cranial modularity patterns are conserved within major vertebrate clades.