Pelvic girdle in tetrapods is fused to
Pelvic girdle in tetrapods evolved robust attachment to vertebral column via sacral region where ilium fuses or articulates tightly to transverse processes of sacral vertebrae forming sacroiliac joint stabilized by dorsal sacroiliac and sacrotuberous ligaments transmitting propulsive thrust from powerful hindlimb musculature to axial skeleton essential for terrestrial locomotion bearing weight against gravity. Contrast aquatic ancestors where both pectoral and pelvic girdles floated within musculature without bony vertebral attachment. Pectoral girdle remains muscularly sling suspended for mobility except some reptiles, fusion to backbone distinct pelvic adaptation.
Ref: Romer Vertebrate Body 6th ed. Appendicular Skeleton; Campbell Biology 12th ed. Chapter 34 Limb Girdles