postorbital bone postorbital bone the postorbital is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. in some vertebrates, the postorbital is fused with the postfrontal to create a postorbitofrontal. birds have a separate postorbital as an embryo, but the bone fuses with the frontal before it hatches. the postorbital of a duck embryo fusing with the frontal during development. a skull diagram of dromaeosaurus, a dromaeosaurid dinosaur. the postorbital is colored dark blue.