Cranial anatomy of the spade-headed amphisbaenian Diplometopon zarudnyi (Squamata, Amphisbaenia) based on high-resolution X-ray computed tomography

The skull of the trogonophid amphisbaenian Diplometopon zarudnyi is described from high-resolution X-ray computed tomographic (HRXCT) imagery of a whole museum specimen preserved in ETOH. The skull was digitally resliced and disarticulated into individual elements, producing novel visualizations that allow detailed morphological analysis of its three-dimensionally complex structure. The prefrontal and jugal are absent in Diplometopon. The septomaxilla is present but hidden entirely from superficial view. In contrast to previous studies, we recognize a splenial fused to the compound bone of the mandible and a squamosal fused to the otic-occipital complex. Comparison of Diplometopon to the two other amphisbaenians previously described in comparable detail, Rhineura hatcherii and Amphisbaena alba, reveals a mosaic of cranial similarities and differences. Both Diplometopon and Rhineura exhibit a craniofacial angulation and expanded rostral blade related to use of the head as a digging tool, but the detailed architecture of these features is quite different. Additionally, whereas the snout of Rhineura exhibits a high degree of sculpturing and sensory innervation, this is not the case in Diplometopon. Unlike in Rhineura and Amphisbaena, the cranial elements of Diplometopon do not exhibit an extensive degree of overlap or complex interlocking sutures; instead, most of the cranial elements lie in loose apposition to each other. The degree to which this mosaic of features reflects functional demands, shared ancestry, and/or convergence is unclear in the absence of a stable hypothesis of amphisbaenian phylogeny.