"Birds are the only living vertebrate whose hind limb includes three long bones in sequence. The innermost of the two long bones are similar to those found in most vertebrates. At the hip, the femur, is held more or less parallel to the ground and is bound to the hipbones by the massive thigh muscles. In effect, the femur is an addition to the hipbones and its rotation contributes little to the length of the bird’s stride. The second long bone in the leg is the tibiotarsus. As the name implies, it is the fused product of the tibia and some tarsal bones. In other vertebrates, the tibia lies parallel to the fibula, another long bone of similar size. The two bones act together to facilitate rotation at the ankle. In birds there is no rotation at the ankle and the fibula no longer reaches the ankle joint and no longer qualifies as a major long bone."
Here is an additional reference:
"The tarsus of Triassic pterosaurs consists of two proximal tarsals, which fuse to the tibia during ontogeny, forming a tibiotarsus, and two distal tarsals." (Fabio M. Dalla Vecchia)
Also see the post entitled Leg Similarities (1) demonstrating that the pterosaur and modern bird tibiotarsus are near identical in terms of the tibia and the fibula.