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Abstract A snake‐like body plan and burrowing lifestyle characterize numerous vertebrate groups as a result of convergent evolution. One such group is the amphisbaenians, a clade of limbless, fossorial lizards that exhibit head‐first burrowing behavior. Correlated with this behavior, amphisbaenian skulls are more rigid and coossified than those of nonburrowing lizards. However, due to their lifestyle, there are many gaps in our understanding of amphisbaenian anatomy, including how their cranial osteology varies among individuals of the same species and what that reveals about constraints on the skull morphology of head‐first burrowing taxa. We investigated intraspecific variation in the cranial osteology of amphisbaenians using seven individuals of the trogonophidDiplometopon zarudnyi. Variation in both skull and individual skull element morphology was examined qualitatively and quantitatively through three‐dimensional (3D) models created from microcomputed tomography data. Qualitative examination revealed differences in the number and position of foramina, the interdigitation between the frontals and parietal, and the extent of coossification among the occipital complex, fused basioccipital and parabasisphenoid (“parabasisphenoid‐basioccipital complex”), and elements X. We performed 3D landmark‐based geometric morphometrics for the quantitative assessment, revealing shape differences in the skull, premaxilla, maxilla, frontal, and parietal. The observed intraspecific variation may be the result of different stages of ontogenetic development or biomechanical optimization for head‐first burrowing. For example, variation in the coossification of the occipital region suggests a potential ontogenetic coossification sequence. Examination of these areas of variation across other head‐first burrowing taxa will help determine if the variation is clade‐specific or part of a broader macroevolutionary pattern of head‐first burrowing. 

Synopsis Pygopodids are elongate, functionally limbless geckos found throughout Australia. The clade presents low taxonomic diversity (∼45 spp.), but a variety of cranial morphologies, habitat use, and locomotor abilities that vary between and within genera. In order to assess potential relationships between cranial morphology and ecology, computed tomography scans of 29 species were used for 3D geometric morphometric analysis. A combination of 24 static landmarks and 20 sliding semi-landmarks were subjected to Generalized Procrustes Alignment. Disparity in cranial shape was visualized through Principal Component Analysis, and a multivariate analysis of variance (MANOVA) was used to test for an association between shape, habitat, and diet. A subset of 27 species with well-resolved phylogenetic relationships was used to generate a phylomorphospace and conduct phylogeny-corrected MANOVA. Similar analyses were done solely on Aprasia taxa to explore species-level variation. Most of the variation across pygopodids was described by principal component (PC) 1(54%: cranial roof width, parabasisphenoid, and occipital length), PC2 (12%: snout elongation and braincase width), and PC3 (6%: elongation and shape of the palate and rostrum). Without phylogenetic correction, both habitat and diet were significant influencers of variation in cranial morphology. However, in the phylogeny-corrected MANOVA, habitat remained weakly significant, but not diet, which can be explained by generic-level differences in ecology rather than among species. Our results demonstrate that at higher levels, phylogeny has a strong effect on morphology, but that influence may be due to small sample size when comparing genera. However, because some closely related taxa occupy distant regions of morphospace, diverging diets, and use of fossorial habitats may contribute to variation seen in these geckos. 

Pygopodids are limb-reduced, miniaturized geckos found across Australia and New Guinea. Pygopodids are mainly terrestrial; however, Aprasia species are highly fossorial and further miniaturized, converging on similar ecology and morphology to typhlopid snakes. Additionally, Aprasia from eastern/central and western Australia exhibit distinct skull shapes, possibly due to the functional demands of burrowing in different soil types. Another pygopodid genus, Ophidiocephalus, also was described as fossorial with morphology most similar to eastern Aprasia species, and thus may experience a similar pattern of cranial stress when digging. The burrowing mechanics of pygopodids have never been studied; however, we propose that mechanical stress is distributed outwardly as a shell across the expanded nasals, rather than along an anterior-posterior central column as suggested for other head-first burrowing squamates. To test how differences in morphology may be related to differing functional demands, Finite Element Analysis was implemented by applying and comparing both face loads and point loads of 20N onto 3D solid meshes of the skulls of one eastern/central and one western Aprasia, and one Ophidiocephalus. The resulting stress and strain were low in all taxa and appeared to be evenly spread out across each axis; however, Ophidiocephalus experienced slightly higher average stress than either Aprasia. Although anatomically divergent, each lineage appears to have independently converged on a similar level of biomechanical performance.