Endocranial morphology in worm lizards (Amphisbaenia, Squamata): multiple neuroanatomical solutions to a fossorial lifestyle

Amphisbaenians are fossorial squamates characterized by extreme cranial specialization, particularly in snout morphology. Whether variation in external rostral form predicts internal cranial anatomy remains poorly understood. We present the first large-scale comparative analysis of amphisbaenian endocranial morphology, using three-dimensional landmark data from endocasts of 50 species representing all major families and snout morphotypes. Endocasts within Amphisbaenia share general squamate features typical of fossorial taxa. Phylogeny shows a weak but significant signal in endocast shape, while allometry shows no detectable effect. Snout morphology contributes modestly to endocranial variation, and substantial differences among taxa sharing the same snout shape indicate that anterior cranial modifications influence the braincase locally. Convergence and disparity analyses reveal limited, case-specific similarities across morphotypes, with similar endocranial configurations arising only sporadically in different lineages. Overall, amphisbaenian endocasts display considerable morphological variability and extreme snout modifications linked to digging performance do not produce a single, predictable endocranial configuration. These results suggest that endocranial shape reflects the interaction of multiple evolutionary factors. Future studies should explore additional sources of variation-such as ontogeny, sexual dimorphism, sensory biology, external forces or cranial musculature-to fully understand endocranial diversity in this clade.