"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 Rhamphorhynchusand as many as 10 in Pteranodon) (Wellnhofer, 1978, 1987)"
See page 10
"The hip joint of pterosaurs is more mobile and profoundly different from that of theropods but is reminiscent of that of mammals, especially of humans, allowing a wide range of adduction and abduction in the vertical plane."
"The femoral component [of the Anhanguera pterosaur femur] is a well defined spherical head which is distinctly separated from the shaft by a narrow non-articular neck at an obtuse angle of 160 degrees. The head forms a ball and socket joint with the close-fitting, shallow and imperforate acetabulum."
"In theropods [dinosaurs], the femoral component is cylindrical without any distinctive head and neck. It projects medially at a right angle [90 degrees] from the shaft and fits into a perforated acetabulum of up to 1.5 times its diameter. As a result, the hip joint is stable and fully congruent during parasagittal motion, permitting a wide range of flexion and extension but very little abduction and adduction."
Two pterodactylid pterosaurs:
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.
Note the pterosaur prepubis:
The “dark wing” specimen of Rhamphorhynchus muensteri JME SOS 4785 (Tischlinger and Frey 2002) has one overlooked oddity worth mentioning. It had an incredibly deep prepubis (Figure 1.)
So much so normal, but pterosaurs also have a fourth pelvic bone in the form of the pre-pubis.
The Auk 124(3):789–805, 2007
Abstract.—The antitrochanter is a uniquely avian osteological feature of the
pelvis that is located lateral to the postero-dorsal rim of the acetabulum. This feature
makes the avian hip joint unique among all vertebrates, living and fossil, in
that a significant portion of the femoral–pelvic articulation is located outside of the
acetabulum. This additional acetabular articulation occurs between the neck of the
femur and the antitrochanter, and operates as a hinge joint or ginglymus. It is complementary
to the articulation of the head of the femur with the acetabulum, which
is a pivot joint or trochoides. The size, location, and spatial orientation of the antitrochanter
were determined for 77 species of birds representing a variety of hindlimb
functions (e.g., highly cursorial, vertical clinging, foot-propelled diving) and spanning
a wide range of body sizes (swifts to rheas). The area of the antitrochanter is
a good predictor of body mass in birds; its position and orientation are reasonably
consistent within hindlimb morphofunctional groups, but not among all birds. The
antitrochanter serves as a brace to prevent abduction of the hindlimb and to absorb
stresses that would otherwise be placed on the head of the femur during bipedal
locomotion. The drum-in-trough-like form of the antitrochanter–femur articulation
tends to assist in the transfer of long-axis rotational movements of the femur to the
pelvis. The avian antitrochanter is a derived feature of birds that evolved as an aid
in maintaining balance during bipedal terrestrial locomotion.
Pterosaurs' hip sockets are oriented facing slightly upwards, and the head of the femur (thigh bone) is only moderately inward facing, suggesting that pterosaurs had a semi-erect stance. It would have been possible to lift the thigh into a horizontal position during flight as gliding lizards do.