Clasificación del área foliar de las gimnospermas fósiles de la zona norte de Oaxaca, México
The foliar area of different genera of fossil gymnosperms from seven Jurassic localities in Mexico was classified with the purpose of being used in the inference of the environmental conditions that favored the establishment and domination of this group in the Early and Middle Jurassic in the Mixteco terrane. After getting the foliar area of 186 complete specimens and analyzing the data obtained using Kernel density estimation (Stata), seven categories of the foliar area were obtained. In the study area, the microphyll leaves were the most plentiful (87.08 %), however the Microphyla I category (0.08241–1.3660 cm2; 58.03 %) is the dominate size and is the only found in all localities. On the other hand, a lithological analysis carried out in the study area revealed alternating strata of sandstone with variable grain size, and coal layers of variable thickness in all the localities. In order to infer the paleoclimate, the foliar area of the current cycada species distributed in Mexico was obtained. The smallest leaves found in this sample are included in the Microphyla II category (1.3661–3.4835 cm2), and come from organisms distributed in zones with dry to humid climate, and scarce rainfall. This foliar area category of the current species is the closest to what is found in the fossil leaves of the Mixteco terrane, which let us propose that the climate in the Early Jurassic was drier than in the Middle Jurassic, with localities subjected to hydric stress at least during some seasons of the year, and more humid swamp areas. The difference between the localities in the percentage of microphyll leaves, the variable thickness of the coal strata and the texture of the sedimentary rocks indicate that the climatic conditions during the Jurassic in the Mixteco terrane were not homogeneous.
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