The colossal heads cannot be precisely dated . However , the San Lorenzo heads were buried by 900 BC , indicating that their period of manufacture and use was earlier still . The heads from Tres Zapotes had been moved from their original context before they were investigated by archaeologists and the heads from La Venta were found partially exposed on the modern ground surface . The period of production of the colossal heads is therefore unknown , as is whether it spanned a century or a millennium . Estimates of the time span during which colossal heads were produced vary from 50 to 200 years . The San Lorenzo heads are believed to be the oldest , and are the most skillfully executed . All of the stone heads have been assigned to the Preclassic period of Mesoamerican chronology , generally to the Early Preclassic ( 1500 – 1000 BC ) , although the two Tres Zapotes heads and the La Cobata Head are attributed to the Middle Preclassic ( 1000 – 400 BC ) .