Olmec colossal heads by Matthew Stirling in 1938 spurred the first archaeological investigations of Olmec culture. Seventeen confirmed examples are known from four sites within the Olmec heartland on the Gulf Coast of Mexico. Most colossal heads were sculpted from spherical boulders but two from San Lorenzo Tenochtitlán were re-carved from massive stone thrones. An additional monument, at Takalik Abaj in Guatemala, is a throne that may have been carved from a colossal head. This is the only known example from outside the Olmec heartland. Dating the monuments remains difficult because of the movement of many from their original contexts prior to archaeological
Olmecs earlier Olmec writing system and Mayan writing, the matter remains unsettled. The Long Count calendar used by many subsequent Mesoamerican civilizations, as well as the concept of zero, may have been devised by the Olmecs. Because the six artifacts with the earliest Long Count calendar dates were all discovered outside the immediate Maya homeland, it is likely that this calendar predated the Maya and was possibly the invention of the Olmecs. Indeed, three of these six artifacts were found within the Olmec heartland. But an argument against an Olmec origin is the fact that the Olmec civilization had ended by