MEXICO CITY, MEXICO—Excavations for a new extension of Mexico City's subway system have unearthed some 100 Aztec burials, as well as an unusual offering of human skulls placed on a display rack alongside a dog skull. Dating to between 1350 and 1521 A.D., the display rack, known as a tzompantli, would have been used by Aztecs to showcase the severed heads of enemy warriors who had been sacrificed to the gods. Researchers were surprised to find that one of the skulls belonged to a woman, since women were not typically taken as war captives. But the presence of the dog skull was even more puzzling. During the Spanish Conquest, Aztecs were known to place the skulls of horses on display racks, but until now no other animals were known to be associated with a tzompantli. It is possible the dog played a ritual role in death rites, since some Mesoamerican people were known to believe dogs accompanied their owners to the underworld. “Perhaps there are dogs associated with these altars in other sites and we don't know it,” said National Institute of Anthropology and History archaeologist Maria de Jesus Sanchez.