The Mural of the 4 Eras is pretty gruesome, like typical Toltec art at Tula and Chichen Itza.  It felt very negative and warlike.  I did not care for it.

According to the LONELY PLANET: Guide to Mexico, 9th Edition,  (2004), ISBN 1-74059-686-2, page 811, the Mayans created this mural sometime between 790 and 840 A.D, when a wave of destruction was running through the Mayan world.  The people of Tonina believed themselves to be living in the 4th Sun-that of Winter, mirrors, the direction North, and the end of human life.  This stucco relief consists of 4 panels; the 1st panel, from the left end, is lost.  It represents the 4 Eras of human history.  Each panel’s center has a decapitated prisoner’s upside down head.  Blood spurting from the prisoner’s head forms a ring of feathers and, at the same time, a sun.