MEXICO CITY (AP).- Archaeologists found a round Aztec ceremonial platform studded with stone carvings of serpent heads at Mexico City’s Templo Mayor ruin, raising hopes in the search for an emperor’s tomb, authorities said Thursday. No Aztec ruler’s tomb has ever been located and researchers have been on a five-year quest to find a royal tomb in the area of the Templo Mayor, a complex of two huge pyramids and numerous smaller structures that contained the ceremonial and spiritual heart of the pre-Hispanic Aztec empire. Mexico’s National Institute of History and Anthropology said the stone platform is about 15 yards (meters) in diameter and probably built around A.D. 1469. The site lies in downtown Mexico City, which was built by Spanish conquerors atop the Aztec capital of Tenochtitlan. “The historical records say that the rulers were cremated at the foot of the Templo Mayor, and it is believed to be