The "death's head" resting on crossed thigh bones, more commonly called the "skull and crossbones" is a classic symbol of mortality. Yet mortality may be understood in more than one sense. In each instance mortality indicates a type of freedom.
In its most common sense, mortality refers to physical death. The skull and crossbones remind us that death is the great leveler. In death, worldly wealth and honor are meaningless. Regard for a person based on race, class, income level, or other outward distinctions comes to an end.
All that is born must die. "Man that is born of a woman is of few days, and full of trouble. He cometh forth like a flower and is cut down: he fleeth also as a shadow, and continueth not." (Job 14:1, 2) The skull and crossbones remind us that life is short and death is inevitable. We should therefore seek to live a life of virtue while we are able.
Mortality can also be understood as referring to the state of being dead in regard to sin. "Anyone who has died has been set free from sin." (Romans 6:7) "In the same way, count yourselves dead to sin but alive to God in Christ Jesus." (Romans 6:11) It is death in regard to sin that frees one to live virtuously. In the context of Romans 6, this freedom is seen as proceeding from a mystical union with Christ's death that takes place in baptism.
It is also worthy of note that the crucifixion of Jesus took place on a hill outside Jerusalem known as Golgotha, which means, "The place of the skull." Ancient legend has it that Golgotha was the place where the skull of Adam, the first man, was buried.