Eocene Mequitongo Formation palynoflora from the intertropical Tehuacán–Cuicatlán Valley, Mexico

Abstract New Eocene Mequitongo Formation (MF) pollen–spore data suggest evolution of flora as a response to climatic and geological changes. During the early Eocene, conditions were more humid than those that exist today in the botanically diverse Tehuacan–Cuicatlan Valley (TCV), in south-central Mexico. The most representative floristic elements in the palynological assemblages were boreal taxa such as Alnipollenites spp., Betulaceoipollenites sp., Cupressacites sp., Fraxinoipollenites sp., Liquidambarpollenites sp., Momipites group taxa, Pinuspollenites sp. and Quercoidites sp. among others. Additionally, Brossipollis sp., Rhuspollenites spp., Tricolporoidites spp. and Tubulifloridites sp. were frequent throughout the MF sections. The overall MF palynoflora suggests that climatic conditions led to the development of temperate Pinus forest, Pinus – Quercus forest, deciduous forest and cloud forest, which grew in mountain ranges. Deciduous forest was widespread during the Eocene, and its presence in the Mexican TCV represents the first record of this ecosystem within the southernmost extension of North America. Additionally, palynofloristic taxa of gallery and tropical deciduous forests, both lowland vegetation types, were identified. The palynoflora of the Mequitongo Formation (Eocene) and the Tehuacan Formation (Miocene), exposed in the same valley, were compared in order to study the evolution of the vegetation through time. Our findings strengthen the hypothesis that the Tehuacan–Cuicatlan Valley has acted as a centre of biodiversity, with a mixture of elements that evolved during the Paleogene and Neogene, leading to its exceptional extant diversity with a large number of endemic plants.