Muscle Diversity of the Sacroiliac Joint in Scinaxini (Anura: Hylidae)

Abstract

The musculature associated with the sacroiliac articulation in anurans is essential to locomotor behaviors such as burrowing, swimming, jumping, and walking by enabling pelvic rotation and sliding. While the osteological components—sacral diapophyses, sesamoids, and ilia—have been extensively studied, the corresponding musculature remains comparatively underreported. Muscles such as the m. longissimus dorsi, m. coccygeosacralis, m. coccygeoiliacus, and m. iliolumbaris are fundamental to sacroiliac function, yet detailed anatomical descriptions, particularly beyond m. iliolumbaris are lacking. This gap is especially evident in the hylid tribe Scinaxini, which includes 133 neotropical treefrog species across the genera Julianus, Ololygon, and Scinax, and where some species of the first genus shows distinctive osteological features—such as medially elongated sesamoids and reduced sacral diapophyses. We analyzed in detail the sacroiliac musculature in nine Scinaxini species, including J. camposseabrai and J. pinimus. Our findings reveal that muscle origin and insertion sites are generally conserved across species; however, notable interspecific differences exist in the fiber orientation of the m. coccygeosacralis and in the degree of separation between branches of the m. iliolumbaris. These variations may reflect functional adaptations linked to the unique osteological configurations in this clade.