The [Rhamphorhynchus] pelvic and pectoral girdles fused as the animals aged, with full pectoral fusion attained by one year of age.http://www.bioone.org/doi/full/10.4202/app.2011.1109
Some [pterosaur] pelves are fused along their ventral margins to form a sealed pelvic canal that, in anterior or posterior view, gives the pelvic girdle a U- or V-shaped profile (Fig. 1D, E) (Bennett 1990, 2001). Other pelves appear to remain unfused along their ventral margins; a feature that has been suggested as exclusive to females (Lü et al. 2011). Other putative females, however, possess fused ventral pelvic margins while retaining relatively broad pelvic canals (Bennett 1992).
Several aspects of the pelvic girdle suggest that pterosaurs were specialized for pelvic aspiration during flight. As in birds, the three pelvic bones of pterosaurs were solidly fused into a single unit (Fig. 9), and an increased number of dorsal vertebrae were incorporated into the sacrum (3 to 5 in Rhamphorhynchus and as many as 10 in Pteranodon) (Wellnhofer, 1978, 1987)http://en.wikipedia.org/wiki/Bird_anatomy
The hips [of modern birds] consist of the pelvis which includes three major bones: Illium (top of the hip), Ischium (sides of hip), and Pubis (front of the hip). These are fused into one (the innominate bone). Innominate bones are evolutionary significant in that they allow birds to lay eggs. They meet at the acetabulum (the hip socket) and articulate with the femur, which is the first bone of the hind limb.http://archosaurmusings.wordpress.com/2009/12/24/back-to-that-pterosaur-sacrum-pelvis/
For all that pterosaurs are lightly built, they do tend to have a very robust pelvis and while I’m not aware of any explicit studies the assumption for this has long been, rather reasonably, that it’s to do with the forces of landing, which can be pretty hard going. Plenty of birds have very robust pelves for the same reason,.
Dinosaurs are not like birds:
Theropod hips and hindlimbs show marked morphological changes (Fig. 3) that are consilient with functional changes during their evolution:
(1) The antitrochanter repositioned from its primitive archosaurian location on the ischium, facing craniodorsally, to a more craniolateral orientation on the ischium and ilium in dinosaurs and their closest relatives. The antitrochanter then enlarged and re-oriented to face cranioventrally in birds.
(2) The femoral head shifted from a craniomedial orientation in basal theropods to a more offset medial orientation in avetheropods, especially birds.
(3) The ectocondylar tuber of the distal femur enlarged in “theropods” and moved distally from the proximal popliteal region onto the distal lateral condyle in birds.
(4) The main weight-bearing axis of the crus shifted medially in theropods onto the tibia as the fibula and calcaneum were reduced, and elements of the knee and ankle joint became more rigidly appressed.
(5) The fibular tubercle, the insertion of the knee flexor M. ilio-fibularis (Müller and Streicher, 1989), moved from a plesiomorphic craniolateral position on the proximal fibula in “theropods” to a caudolateral position in birds, consistent with a change in the action of this muscle related to increased knee flexion.