A new type of sacroiliac articulation in Anura: Unveiling the anatomical diversity in Scinaxini (Hylidae: Hylinae)

The sacroiliac articulation in anurans enables locomotion, including burrowing, swimming, jumping, and walking, by facilitating pelvic rotation and sliding. The hylid tribe Scinaxini comprises 134 Neotropical treefrogs divided into three genera: Julianus, Ololygon, and Scinax. The osteological elements of the sacroiliac articulation are well studied within the tribe, with Julianus having distinctive sacral diapophyses and sesamoids. Notably, the species J. camposseabrai has a medially elongated sesamoid, about three times its width in length, along with a short sacral diapophysis-a unique combination among anurans. However, information on the associated musculature in the tribe remains limited, restricting our understanding of this unique morphology. This knowledge gap prompted a detailed investigation of the musculature of the sacroiliac articulation in this clade of treefrogs. We revisited the osteology of the sacroiliac articulation and described its muscles in nine species of Scinaxini, including J. camposseabrai and J. pinimus. Our results showed that the origin and insertion of the muscles are largely conserved across the tribe, but variations exist in the orientation of the m. coccygeosacralis and the degree of separation between slips of the m. iliolumbaris. The species of Julianus have a unique sacroiliac osteo-muscular configuration, particularly J. camposseabrai, which is distinct from any previously described in anurans.