Raúl Barrera, who leads the archaeological project for the National Institute of Anthropology and History, or INAH, said most of the remains belonged to women between the ages of 35 and 40.

Archaeologists have not yet been able to determine which civilization the remains are from, although they know the find dates back to the period 200 B.C. to A.D. 600.

The tombs - located in vertical chambers - were in a type of burial ground found in Nayarit, Colima, Michoacán, Zacatecas and parts of Jalisco, but they have not been found in Mesoamerica.

The tombs have, however, been found in Colombia, Ecuador and Venezuela, Barrera said, noting that there may have been cultural and trade links between the regions.

Artifacts from these countries have been discovered on the coast of Nayarit, he said.

This type of tomb may have signified some type of a "return to the womb of Mother Earth," the archaeologist said.

The content of the tombs varied depending on the social status of the individual, since "they held the belief that in making the crossing to the next life the dead person needed certain items for the journey," Barrera said.

Archaeologists found figurines of warriors, ball players, pregnant women and animals in the tombs, as well as vessels and various types of jewelry.

Barrera said the sites contained only burial grounds and no dwellings, which may have been located on mountains or other locations at higher altitudes.

Of the 29 tombs, 28 were found at the La Playa site and the other was at Las Lagunillas. Only 12 were still intact, the others having been looted at some point.

The actual digging and recovery work lasted four years.