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Morphological patterns may vary spatially in response to adaptations to regional environmental conditions. In fossorial animals, edaphic gradients potentially predict intra and interspecific morphological variation because they can limit dispersal and select morphotypes. We measured morphological traits of South American worm lizards, including three Amphisbaena and two Leposternon species, to test whether inter- and intraspecific morphological variation and morphotype distribution are predicted by edaphic gradients measured at different soil profiles. Interspecifically, we found that the variation in head length is mainly predicted by the chemical edaphic structure, which can be explained by spatial variation in prey availability. The variation in the width of the prefrontal and frontal shields was mainly predicted by clay and sand-soil contents, which can be explained by relationships of force and friction associated with digging. Intraspecifically, we found that the effects of edaphic gradients on morphological variation reflects the level of specialization to dig, since the most adapted species were mainly affected by variables measured in deeper soil profiles. We also show that soil-mediated intraspecific morphological variation shape morphotype distribution across different biomes that form the South American dry diagonal, which are discussed in light of biological, ecological, and biogeographic premises. [ABSTRACT FROM AUTHOR]