• West of the Achoque ́n dam (Fig. 3), in an area that we refer to as El Mixto ́n, is exposed a .80-m-thick fossil-bearing stratigraphic succession that begins close to the bottom of an arroyo, where massive (1–5 m thick) beds of green tuffaceous claystone are interlayered with thinner, more indurat- ed strata of calcareous claystone with abundant ostracods and gastropods (Fig. 4). Resting atop this shallow lacustrine succes- sion is a thin bed of unconsolidated white ash-fall material composed of felsic glass shards and small, rounded grains of black obsidian. In this part of the succession, camelid and proboscidean footprints were found. Moving up the section, the green tuffaceous claystone beds alternate with pale red, sometimes with irregular green spots, tuffaceous mudstone, and rare layers of epiclastic-volcanic, fine- to medium-grained, distinctly laminated sandstone that in places displays convoluted and/or graded bedding. The clastic succession above the first sandstone layer in the section lacks beds with ostracods and gastropods, and the proportion of reddish claystone and mudstone steadily increases upward. Higher up in the section exists an approximately 12-m-thick succession composed of several ero- sion-resistant beds of medium- to coarse-grained, slightly con- glomeratic (clasts up to 1 cm) epiclastic-volcanic sandstone that in places displays structures such as convoluted bedding, bioturba- tion, normal graded bedding, cross bedding, and shallow channels (several meters wide and tens of centimeters deep). Associated with these sandstone beds are thin, clay-rich, unconsolidated horizons that are interpreted as paleosols. Both the cemented conglomeratic sandstone beds and loose paleosols contain mammal fossils. Without any apparent break, this coarse-grained interval is followed by ,10 m of finer sediments (mudstones and sandy clays) in thick, massive beds with alternating green and red colors. Near the top of the exposed section, the proportion of conglomeratic sandstone and fine-grained conglomerate notably increases, as does the maximum clast size (up to 5 cm), while the finer-grained, reddish mudstone layers decrease. Close to a dirt road that leads from the paved road (Mexico 54) to this locality and up to a cliff-forming outcrop of felsic ignimbrite, the Pliocene sedimentary succession is covered by talus deposits derived from the Oligocene ignimbrite.