By JOHN NOBLE WILFORD

A discovery of monumental carved masks and elaborate jade ritual objects in 2,000-year-old ruins of a city in Guatemala is raising serious questions about the chronology of the enigmatic Mayan civilization. In many respects, the city appeared to be ahead of its time.

The leader of excavations there, Dr. Francisco Estrada-Belli of Vanderbilt University in Nashville, said yesterday that the city, Cival, appeared to have been one of the earliest and largest in what is generally regarded as the preclassic period. But it has been found to have all the hallmarks of a classic Mayan city: kings, complex iconography, grand palaces, polychrome ceramics and writing.

"It's pretty clear that 'preclassic' is a misnomer," Dr. Estrada-Belli said in a telephone interview. But he added, "It may be too late to change the names" in the established framework of Maya history.

Archaeologists have long dated the start of the classic Mayan civilization at A.D. 250, which had seemed to be the time of the earliest written inscriptions in city plazas and temples. The period ended around 900 with the mysterious collapse of the largest Mayan cities in Guatemala, Honduras, Belize and parts of Mexico. The postclassic period of general decline continued until the arrival of the Spanish in the early 16th century.

The preclassic period may have begun as early as 2000 B.C. Cival reached its prime about 150 B.C. and was abandoned shortly before A.D. 100.

The new findings from Cival were announced by the National Geographic Society, which was a supporter of the research. Besides the two huge stucco masks, the discoveries included 120 pieces of polished jade, a ceremonial center that spanned a half mile and an inscribed stone slab dating to 300 B.C.

It is perhaps the earliest such monument ever found in the Mayan lowlands, Dr. Estrada-Belli said.

Other archaeologists not involved in the research said they were amazed by the size of the city but not surprised to learn that the preclassic Maya were capable of such advanced architecture, art and other classic-type culture.

Previous discoveries had already overturned the former model of the preclassic Maya as a culture of simple farming villages, Dr. David Webster, a Pennsylvania State University archaeologist, wrote in his book "The Fall of the Ancient Maya" (Thames & Hudson, 2002).

The ruins of El Mirador, also in Guatemala, have revealed a preclassic city with a highly developed culture as early as 500 B.C., a pyramid that rivaled in size those of Egypt and a population that may have reached 100,000. Cival may have had 10,000 inhabitants at its peak.

Two years ago a Harvard researcher, Dr. William Saturno, discovered a 1,900-year-old mural at San Bartolo, Guatemala, that experts hailed as a masterpiece and as fine as any wall painting ever found in Mayan ruins.

Dr. Ian Graham, a Harvard archaeologist who specializes in Maya inscriptions, said he accepted the interpretation of the Cival discovery because it seemed to corroborate other evidence of an unexpected flowering of preclassic culture.

"Extraordinary things are emerging from preclassic sites," he said. "They are simply mind-boggling."

Dr. Graham said that when he mapped the Cival site two decades ago, the jungle concealed all but some outlines of the stone buildings and pyramids that once stood there. The central plaza appeared to have been less than half the size of what has now been uncovered.

The Harvard team did not linger for extensive excavations.

Dr. Estrada-Belli's painstaking investigation began paying off with spectacular results a year ago. He was inspecting a dank tunnel in the main pyramid. Reaching into a fissure in the wall, his hand met a piece of carved stucco. Later, he saw before him the mask of an anthropomorphic face, 15 feet by 9 feet, with snake fangs in its squared mouth.

"The mask's preservation is astounding," Dr. Estrada-Belli said in a statement about the discoveries.

Last week, the archaeologist said, a second mask, apparently identical, was excavated from the same pyramid. The second mask is made of carved stone overlaid with thick plaster. Its eyes appear to be adorned with corn husks, suggesting the Mayan maize deity.

A study of ceramics associated with the mask, Dr. Estrada-Belli said, indicated that the two artifacts were part of the backdrop for elaborate rituals in about 150 B.C., plus or minus 100 years.

Other evidence suggested that Cival was occupied as early as 600 B.C. and that the broad plaza was being used for important ceremonies and ritual offerings by 500 B.C. The central axis of the main buildings and the plaza is oriented to sunrise at the equinox, presumably for solar rituals associated with the agricultural cycle.

The remains of a hastily erected defensive wall around the city attest to Cival's probable fate. Overwhelmed by an invading enemy, the city was abandoned, apparently for good.