The two heads at Tres Zapotes , with the La Cobata head , are stylistically distinct from the other known examples . Beatriz de la Fuente views them as a late regional survival of an older tradition while other scholars argue that they are merely the kind of regional variant to be expected in a frontier settlement . These heads are sculpted with relatively simple headdresses ; they have squat , wide proportions and distinctive facial features . The two Tres Zapotes heads are the earliest known stone monuments from the site . The discovery of one of the Tres Zapotes heads in the nineteenth century led to the first archaeological investigations of Olmec culture , carried out by Matthew Stirling in 1938 .