Gymnosperms Through Time. Their History in Mexico

It is difficult to synthesize the evolution of this group from their appearance (310 million years ago) to the present day. During this period, several tectonic processes and climatic changes occurred, which acted as forcing factors on their distribution. Their history begins with the appearance of the Cordaitales during the Carboniferous, reaching their climax in the Triassic with the orders Bennettitales and Cycadales, and their subsequent decline in abundance and dominance at the end of the middle Cretaceous. In this chapter, we attempt to provide a general overview of this group through its global fossil record, with emphasis on the Mexican territory. We also briefly mention the evolutionary trends of this group, which is characterized by the presence of naked seeds, wood produced by the bifacial cambium, and the presence of megaphyllous leaves with linear, dichotomous, and reticulate vein patterns. This group represents an evolutionarily successful and long-lived division, since it is present from the Upper Paleozoic to the present day.