Evolution of Cranial Shape in Caecilians (Amphibia: Gymnophiona)

Abstract

Insights into morphological diversification can be obtained from the ways the species of a clade occupy morphospace. Projecting a phylogeny into morphospace provides estimates of evolutionary trajectories as lineages diversified information that can be used to infer the dynamics of evolutionary processes that produced patterns of morphospace occupation. We present here a large-scale investigation into evolution of morphological variation in the skull of caecilian amphibians, a major clade of vertebrates. Because caecilians are limbless, predominantly fossorial animals, diversification of their skull has occurred within a framework imposed by the functional demands of head-first burrowing. We examined cranial shape in 141 species, over half of known species, using X-ray computed tomography and geometric morphometrics. Mapping an existing phylogeny into the cranial morphospace to estimate the history of morphological change (phylomorphospace), we find a striking pattern: most species occupy distinct clusters in cranial morphospace that closely correspond to the main caecilian clades, and each cluster is separated by unoccupied morphospace. The empty spaces in shape space are unlikely to be caused entirely by extinction or incomplete sampling. The main caecilian clades have different amounts of morphological disparity, but neither clade age nor number of species account for this variation. Cranial shape variation is clearly linked to phyletic divergence, but there is also homoplasy, which is attributed to extrinsic factors associated with head-first digging: features of caecilian crania that have been previously argued to correlate with differential microhabitat use and burrowing ability, such as subterminal and terminal mouths, degree of temporal fenestration (stegokrotaphy/ zygokrotaphy), and eyes covered by bone, have evolved and many combinations occur in modern species. We find evidence of morphological convergence in cranial shape, among species that have eyes covered by bone, resulting in a narrow bullet-shaped head. These results reveal a complex history, including early expansion of morphospace and both divergent and convergent evolution resulting in the diversity we observe today.

Lithic morphological organisation: Gahagan bifaces from the Southern Caddo Area

“…The shape changes described by each principal axis are commonly visualised using thin-plate spline warping of a reference 3D 185 mesh (Klingenberg, 2013;Sherratt et al, 2014). A residual randomisation permutation procedure (RRPP; n=1000 permutations) was used for all Procrustes ANOVAs , which has higher statistical power and a greater ability to identify patterns in the data should they be present (Anderson & Ter Braak, 2003).…”

Analyzing Fluctuating Asymmetry with Geometric Morphometrics: Concepts, Methods, and Applications

“…The symmetric component of variation is what many biologists informally think of as "shape" and is therefore the optimal choice for further analyses addressing a wide range of questions in ecology, evolution or ontogeny [224][225][226]. Alternative approaches, such as using various ad-hoc procedures for identifying a median axis from just some landmarks [215][216][217][218][219][220][221] or ignoring the symmetry of the structure altogether [227][228][229][230], seem clearly inferior to the Procrustes approach for object symmetry.…”

Mating behaviour and its relationship with morphological features in the millipede Pachyiulus hungaricus (Karsch, 1881) (Myriapoda, Diplopoda, Julida)

“…Apart from linear measurements, we analysed size and shape of the walking legs and gonopod promeres using geometric morphometrics, a method for examination of morphological variability (e.g., Drake and Klingenberg, 2008;Jojić et al, 2012;Sherratt et al, 2014;Lazić et al, 2015). For this procedure, we used leg pairs from the 31st body ring in 28 mating pairs and 21 nonmated individuals (11 females and 10 males); and gonopods of 26 mated males and 8 nonmated males.…”