Skullheads inhabited the world long ago, during the last ice age. Fortunately, they are extinct — which is good news, for they were aggressive barbarians and resistant to steel.
- "Archeological research is the only source of knowledge about skullheads. Skeleton reconstructions make it possible to describe the creature's appearance and movement. Addressing his students, Professor Sendivoy Barren summed up research on the skullhead thusly: 'We are more than lucky that the bastards died out.'"
- According to Abigail during the Epilogue, the skullheads met in the Ice Plains "are what remains of humanity" and our "own great-grandchildren".