Question: How long and in what capacity did Joachim Rittstieg study a system of calenders used in pre-Columbian Mesoamerica?
Sentence (from "Obsidian use in Mesoamerica"): Obsidian was a highly integrated part of daily and ritual life, and its widespread and varied use may be a significant contributor to Mesoamerica's lack of metallurgy.
Relevant evidence: No