Modern interpretations of Aztec legends say Tlatelolco was built around A.D. 1358, the same year as Tenochtitlán, although archaeological evidence has cast doubt on that date in recent years, experts say.
"If true, the date of this pyramid fits with many other archaeological finds that reveal evidence of Aztec occupation earlier than the traditional dates," said Susan Gillespie, a University of Florida anthropologist.
The find could also shed light on the poorly understood early relationship between Tlatelolco—a massive market province—and Tenochtitlán, the Aztec capital and one of Mesoamerica's largest cities.
"The nature of Tlatelolco and Tenochtitlán's entwined origins and histories remains one of the underexplored mysteries of the Aztec era," Gillespie said.
The new discovery could challenge the notion that Tenochtitlán was the dominant twin during the early, entangled development of the two Aztec provinces, said Michael Smith, an Aztec expert at Arizona State University.
"There are vague traces in the historical sources that Tlatelolco may have been more powerful than Tenochtitlán in its early decades," he said.
"If there was indeed a large pyramid in Tlatelolco in the Early Aztec period, given that no such find exists in Tenochtitlán, it may suggest that Tlatelolco was indeed the dominant city in their early years. That would be significant."
What's Inside the Pyramid?
Guilliem and his colleagues believe that an offering to Tezcatlipoca Black, the Aztec god of commerce, will be found inside the newly discovered pyramid.
Guilliem theorizes that workers intentionally broke into the smaller pyramid in 1368 while building a subsequent phase.
"When they broke it, it is very probable that they deposited a deity that's likely to be Tezcatlipoca," he said.
"They most likely deposited an offering to the deity [Tezcatlipoca Black], conducted a ceremony, [and] then closed it again."
The team also wants to determine if the Grand Temple at Tenochtitlán has a similar stage of construction—a key to untangling the early power balance between the two city-states, Guilliem said.
Gillespie, the University of Florida anthropologist, said such a comparison could yield crucial clues to the dynamics of the ancient Aztec cities.
"The great temple at Tenochtitlán similarly had many cached offerings as part of the different building phases," Gillespie said.
"It will be interesting to see how the Tlatelolcan corpus of offerings compares to that of Tenochtitlán."
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