Ontogenetic skull variation in a round-headed Trogonophidae amphisbaenian species with inferences on bite force

Amphisbaenia is a group of burrowing fossorial reptiles that uses their heads as the primary tool for digging. The shape of their skulls is influenced by the mechanical needs of the animal, such as digging and feeding. Four basic skull shapes are recognized: rounded, keeled, shovel-shaped, and spade-shaped. Trogonophis wiegmanni is an Amphisbaenia that is the only round-headed representative among Trogonophidae. The species occurs in the Maghreb region in the arid Mediterranean biome. We analyzed the skull and mandible of this species using 3D Geometric Morphometrics (3D-GMM), a technique that uses Cartesian coordinates of homologous landmarks to analyze and compare shape variations through robust statistical methods, enabling separate analysis of size and shape components. Our hypothesis is that the ontogenetic variation present in T. wiegmanni development would follow the general pattern found in other amphisbaenians. We analyzed the cranial morphological variations in an ontogenetic series of T. wiegmanni using 3D-GMM. We examined computed tomographic scans of 11 specimens (7 juveniles and 4 adults). For this, 16 landmarks were placed on the left side of the skull, covering the dorsal, lateral, and ventral surfaces, and 18 other landmarks were placed on the labial and lingual views of the mandible. The landmarks were selected for their ontogenetic stability, meaning they could be identified in both juvenile and adult individuals. Principal components and regression analyses between shape (dependent variable) and size (independent variable) showed clear differences between the cranial morphological patterns of juveniles and adults. For instance, juveniles tend to have a wider intermediate segment, a subtle sagittal crest, and little bone fusion, with evident sutures. As in other Amphisbaenia species with different skull morphotypes, the variation was strongly correlated with the size change from juvenile to adult, indicating a dominant role for ontogenetic allometry in determining skull and mandible shapes. Therefore, at least for the skull, this appears to be a common ontogenetic pattern for skull morphology of Amphisbaenia.