Mexican archaeologists discovered a group of millennium-old basalt tombs in the Pacific coast state of Nayarit, the National Institute of Anthropology and History, or INAH, said.
They are the first pre-Columbian stone tombs catalogued in that part of Mexico, INAH-Nayarit archaeologists Lourdes Garcia and Jose Beltran said.
What appear to be complete skeletons were found inside and around the 12 burial vaults, INAH said in a statement.
Experts from INAH-Nayarit came across the tombs amid efforts to safeguard archaeological sites in the area where crews are building a new highway to the resort town of Puerto Vallarta.
The structures lay beneath a lay of volcanic rock deposited at the time of the largest-known eruption of the nearby Ceboruco volcano, in A.D. 1000.
Besides the human remains, the tombs contain ceramics, tools and other artifacts that appear to date from A.D. 900-1100, the Late Postclassical period, which was marked by a migration of Nahua people from interior Mexico to the Pacific coast.
The tombs in Nayarit may have been the work of Nahua settlers, INAH said.
The Nahua peoples included the Aztecs and Toltecs.