The Archeology Magazine Web site has up a spiffy post exploring the phenomenon of "screaming mummies" – the gaping open mouth common to mummies that appears, on initial observation, to be a sign of agony and pain.
For well over a century, the contorted features of ancient mummies have led to speculation of untold pain and horrible deaths. The examples quoted above are from the examination of Egyptian mummies more than 120 years ago. Today, similar descriptions can still be found in television programs and academic writings. "Is this the face of a queen? What kind of terrible end did she meet?" and "a terrible head wound, an agonized scream," intones the narrator of "Secrets of Egypt's Lost Queen," a 2007 documentary. A photo caption in the scholarly volume Mummies and Death in Egypt (2006) reads "mummy of a boy five years of age, fixed in agony." And the widely covered 2007 discovery of Chachapoya mummies in Peru prompted this newspaper headline "Moment 600 years ago that terror came to Mummies of the Amazon" and copy "Hands over her eyes and her face gripped with terror, the woman's fear of death is all too obvious."
Author Mark Rose suggests a different explanation: Ancient funerary practices that did not account for postmortem changes in the jaw.
Your jaw bone ascends toward the back (almost at a right angle to the horizontal line of the teeth), ending in a rounded protuberance (the condyloid process), which fits into a shallow groove in your temporal bone on the lower part of your skull.
The nature of this joint is a key to understanding why mummies scream. Physician Trisha Macnair explains in "Human decomposition after death" on the BBC Health website.
"This temporo-mandibular joint is fairly loose.... Unlike the tight ball-and-socket linking the leg and the hip, the jaw and cranium are held together only by ligaments and muscles. If unimpeded--by the position of the body, wrappings, or very fast desiccation--the jaw will drop down as the muscles relax and decompose after rigor mortis."
For the curious, we keep modern corpses from opening their mouths by tying the mouth shut. Morticians run a thread up through the bottom of the chin, loop it through the nostrils, cross over in the mouth, out the chin again, tighten, and tie.