The discovery of around 150 skulls of human sacrifice victims in a central Mexico field has baffled experts.

It is rare that such a large number of severed heads has been found outside of a major pyramid or temple complex. Archaeologists are puzzled by the unexpected find of such a large number of skulls at what appears to have been a small, unremarkable shrine.

The heads were carefully deposited in rows or in small mounds, mostly facing east towards the rising sun, some time between 660 and 860 AD, a period when the nearby city-state of Teotihuacan had already declined, but the Aztec empire, founded in 1325, was still centuries in the future.

Georgia State University archaeologist Christopher Morehart, who found the skulls last year in Xaltocan, a farming village just north of Mexico City, said that so far, between 150 and 200 adult skulls or their equivalent in bone parts had been excavated from fields that stand on a former lake bed.

Experts were not expecting to find anything of this kind in the flat, undistinguished pasture land and corn fields. The site is near, but not immediately adjacent to, Teotihuacan, one of the biggest pre-Hispanic cities. It reached its height between 100 BC and AD 750 and was abandoned by the time the Aztecs arrived in the area in the 1300s.

Mr Morehart was conducting a study of ancient agricultural patterns and human landscape uses in the northern part of the Mexico Valley in 2007, when during a walking survey of the site he started noticing looters' pits that had turned up human bones. A subsequent season of excavations in 2012 turned up dozens more skulls. The results of the 2007 dig have just been published in the academic journal Latin American Antiquity.

While the Teotihuacan culture and the Aztecs were known to practise human sacrifice and remains of hundreds of victims have been found in their pyramids or other large structures, the Xaltocan mound "is like a bump in the landscape that you could really easily walk over and not know you're standing on it", Mr Morehart said.

Physical anthropologist Abigail Meza Penaloza of the Institute of Anthropology at Mexico's National University said her team was still cleaning and assembling the skulls, but had a confirmed count of about 130 so far, all of which appear to be of adult males. Ms Meza Penaloza said it was the first find of its kind, both because of the location - a small, artificial mound built in the middle of an agricultural field - and the kind of decapitations carried out there. She said mass sacrifices had been documented at temple inaugurations of temple closings, but not in the middle of fields.

The skulls were also found with a shorter length of vertebrae attached to the skulls than is the case of other such finds, suggesting the decapitation cut was made closer to the base of the skull.

More strange details have emerged. Mr Morehart said some of the skulls were found with finger bones inserted into the eye sockets. "It was common enough that it was intentionally placed there in the eye socket," he said, though the ritual significance of that remains unclear. Dr Michael Smith, professor of anthropology at Arizona State University, who was not involved in the project, said it was "certainly an impressive and very puzzling find", adding: "I am not aware of any other finds of mass burials or mass sacrifices outside of major settlements."

PA Media