Abstract

The evolution of the Jurassic sedimentary basins is a topic that has recently begun to be studied in Mexico. The aspects investigated to date involve the formation, size, origin and weathering of the sediments through the deposit, amongst others. However, so far, they have not been related to the plant communities that were established during the phases of basinal evolution. In this paper we compare the paleofloristic listings and vegetation structure of two communities that developed in the Ayuquila and Otlaltepec basins, with the characteristics of the deposit environments reported for each basin that were deposited during the Bajocian-Bathonian. For the Ayuquila basin, alluvial fans are reported combined with flood plains and isolated lakes within a braided river system. While for the Otlaltepec basin the presence of braided river is inferred. The moisture available for each plant community was estimated by the use of seven foliar classes. The vegetation structure was obtained through the identification of leaves, reproductive structures and fossilized wood at the species level; these data allowed the quantification of plant diversity of the communities, which in the Ayuquila basin amounts to 14 genera and 22 species, while in the Otlaltepec basin nine genera and 22 species were identified, which formed plant communities with three well-defined strata. The herbaceous stratum in both basins was made up of Pteridophytes. In the Ayuquila basin they are represented by the genera Equisetum and Cladophlebis, while in the Otlaltepec basin only by Filicales. In the Ayuquila basin, Cladophlebis sp and Otozamites hespera are the most abundant species, while in the Otlaltepec basin it was Zamites lucerensis. In both basins the foliar area exhibited by the floristic elements mostly correspond to microphyll I or smaller leaves, which correspond to Brachyphyllum and to different genera of Bennettitales. Of these latter plants only Otozamites hespera and Zamites lucerensis are considered generalists as they are present in the four localities that comprise the study area. Finally, the lithological characteristics of the watersheds of Ayuquila and Otlaltepec indicate the presence of ephemeral or permanent bodies of water that vary in volume according to the season of the year these provided the necessary humidity for the establishment of ferns and members of Pelourdea, organisms considered ripary or seasonal. While in the dry season the members of Coniferales (Brachyphyllum), Bennettitales and perhaps Caytoniales (Sagenopteris) were favored.