William J. Sanders, assistant research scientist and supervising preparator at the Vertebrate Fossil Preparation Laboratory of the University of Michigan’s Museum of Paleontology, explains:
Living animals have primary sexual characteristics, such as genitalia, that differ between males and females. These are soft-tissue organs that generally do not leave marks on the skeleton and are therefore not preserved in the fossil record. There may be features associated with reproduction that do leave bony traces, however. For instance, in the female human pelvis, the angle of the bony arch beneath the pubis is much wider than it is in males and the sacrum is usually is flatter as well. Both of these features relate to the need for a larger pelvic outlet for birthing infants and are thus reliable sexual markers for forensic scientists and hominid paleontologists.
More here in Scientific American.