Singer, Ben (Mayo 2005). Una breve historia del caballo en América(en inglés). Canadian Geographic Magazine. Archivado desde el original el 19 de agosto de 2014. «Multiple factors including hunting by early Natives, climate change, and disease are thought to have helped contribute to their demise».
Singer, Ben (Mayo 2005). Una breve historia del caballo en América(en inglés). Canadian Geographic Magazine. Archivado desde el original el 19 de agosto de 2014. «Multiple factors including hunting by early Natives, climate change, and disease are thought to have helped contribute to their demise».
Talaat Shehata; Harald Haarmann (2011). «13. The Toltecs and Maya developed wheels for religious reasons, but not for wheelbarrows or other practical uses. The reason is that they had sufficient slave labor». En Steven L. Danver, ed. Popular Controversies in World History, Tomo II: The Ancient World to the Early Middle Ages(en inglés). Santa Barbara, Denver, y Oxford: ABC-CLIO. pp. 281-300. ISBN978-1-59884-077-3.