Freidel, David A. (1979). «Culture Areas and Interaction Spheres:». Contrasting Approaches to the Emergence of Civilization in the Maya Lowlands. The Society for American Archaeology. American Antiquity (em inglês). 44 (1): 49. doi:10.2307/279188
Freidel, David A. (1979). «Culture Areas and Interaction Spheres:». Contrasting Approaches to the Emergence of Civilization in the Maya Lowlands. The Society for American Archaeology. American Antiquity (em inglês). 44 (1): 49. doi:10.2307/279188