By Julianna Kettlewell
BBC News Online science staff
Elaborate ritual objects and carved masks have been uncovered in the ancient ruins of a city in Guatemala.
Professor Estrada-Belli believes the term "pre-classic" is a misnomer
Exploration of the 2,000-year-old site has caused archaeologists to question the established chronology of the enigmatic Maya civilisation.
The city, Cival, thrived in what is generally considered the "pre-classic" period - but it bore the hallmarks of the more advanced "classic" period.
The excavations were supported by the National Geographic Society.
The ancient city of Cival, in Guatemala's Peten region, was first mapped by the explorer Ian Graham in 1984. Since 2001, it has been the focus of an exhaustive excavation, led by Francisco Estrada-Belli, of Vanderbilt University in Tennessee, US.
His team's discoveries have included two monumental carved masks, 120 pieces of polished jade, a ceremonial centre that spanned 800m (2,600ft) and an inscribed stone slab dating to 300 BC.
Cival had pyramids and a large complex surrounding a central plaza. The buildings were carefully positioned so they faced the sunrise in the equinox. According to Professor Estrada-Belli, this suggests they were used to measure time.
"It had an important astronomical function," Professor Estrada-Belli said. "It's not coincidence that the central axis of the main buildings and the plaza is oriented to sunrise at the equinox."
The lead archaeologist said his most exciting find turned up in a dank tunnel dug in the side of a pyramid.
While he was inspecting the tunnel, he reached into a crack in the wall - and felt a curved piece of stucco. Digging to it from the other side, he found a well-preserved giant face of a Maya deity.
The 4.5m by 3m (15ft by 9ft) stucco mask had one eye visible and the mouth squared, with snake's fangs in its centre. "The mask's preservation is astounding," Professor Estrada-Belli said. "It's almost as if someone made this yesterday."
Excavations this April revealed a second, apparently identical, mask on the other side of a set of stairs.
Professor Estrada-Belli believes the masks flanked the staircase of the pyramid that led to the chamber, serving as the backdrop for a ritual involving the Maya king.
Pre-classic or classic?
After several seasons of digging, the researchers believe Cival was one of the largest Maya cities of the time. In its prime - between 150 BC and AD 100 - it had a buzzing population of around 10,000. But it was not just the city's size that made it remarkable.
As the archaeologists learn more about life in the city of Cival, they are finding it does not sit comfortably with existing notions of Mayan civilisation.
Strictly speaking, Cival flourished in the pre-classic period, which stretches from 2000 BC to AD 240. But it was more advanced than pre-classic societies were thought to be.
It had kings, complex iconography, grand palaces, writing and polychrome ceramics: all the hallmarks of the later - and apparently more civilised - classic period.
"It is pretty clear that 'pre-classic' is a misnomer," said Professor Estrada-Belli. "It's very interesting when we reverse some existing ideas. We thought the pre-classic Maya were a relatively simple society - and they were not."
"There was a whole civilization during the pre-classic time we are just beginning to recover," he added.
Professor Fred Valdez, a Maya expert from the University of Texas, Austin, is in strong agreement. "These finds show that Maya civilisation advanced earlier than folks have previously thought," he told BBC News Online. "Classic and pre-classic are unfortunate terms in relation to when civilisation was reached.
The artefacts indicate the Mayan society may have been more advanced than first thought
"There are a significant number of cultural changes that occurred between the two periods - so I don't think the two terms will stop being used - but it needs to be cleared up about when we think Maya society became civilised."
He added: "It needs to be recognised that civilisation was not confined to classic Maya societies. This research has provided a great support to that argument."