Palynological assemblages from the Tehuacán Formation (TF), geochronologically dated as Middle Miocene (15.6 ± 0.4 Ma), provide evidence of a highly diverse flora that, at the generic level, is similar to the extant flora in the Tehuacán Valley. We propose that, during Miocene times, plant communities may have been formed of similar botanical elements to those seen today in the region, with some taxa adapted to semiarid conditions. While major temperate floristic elements of Pinus, Quercus, Juniperus, cloud forest and mexical vegetation can be recognized, components of tropical deciduous forests, such as Burseraceae, Leguminosae and Cactaceae, are also present, indicating semiarid conditions. Semiarid local conditions are also inferred from the geological record, consisting of lacustrine and alluvial fan deposits, which contain abundant evaporites beds. This lithology was formed under high evaporation and moderate precipitation conditions, as usually occurs in small basins fed by a seasonal input of water in semiarid environments. Important differences in the vegetation from the TF palynoflora as compared to older associations from south-central Mexico can be inferred, such as an increase in the diversity of semiarid taxa, belonging to Leguminosae and Burseraceae, and the oldest abundant occurrence of the Cactaceae.

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