GEOLOGIC/GEOGRAPHIC CONSTRAINTS IN UNDERSTANDING THE BIOTIC EVOLUTION OF NORTHERN MEXICO

The geologic/geographic history of Northern Mexico allows to trace the major features of its biotic development. This region’s Recent geographic position has remained the same since at least the Late Cretaceous, hence the distributional patterns of atmospheric factors related to solar energy input (temperatures, humidity, rainfall, and wind circulation), became increasingly locked in place with the development of this territory’s Recent physiography during the Cenozoic.

To this broad regional climate constancy corresponded some degree of territorial uniformity during the Late Cretaceous, which became episodically complicated and more diverse through the Tertiary. By Middle-Late Eocene time, intense tectonic activity generated the Sierra Madre Oriental, the fold-ranges and high plateaus of Chihuahua and Coahuila, the regression of the sea that covered much of Northern Mexico, and the development of the Gulf Coastal Plain.

By Early Oligocene time, magmatic activity in the west generated the Sierra Madre Occidental. Such activity is related to extensional deformation ultimately responsible in the Late Neogene, for the detachment of Baja California, and for producing the basin and range structural pattern, so characteristic of southwestern North America. By Middle Miocene time, extensive magmatic activity across central Mexico generated the Trans-Mexican Volcanic Belt, thus completing the major geographic features included in or bounding Northern Mexico’s landscape.

The biota that lived in Northern Mexico during the Late Paleogene-Early Neogene adapted to varying local/regional conditions, and became differentiated into 13 regional biotas, as terrestrial diversity episodically increased, providing newer habitats, barriers and corridors. Other environmental factors also played a role in this differentiation, but to trace it precisely, requires a data-base that does not exist at present. However, it remains clear, that the current biotic differentiation of Northern Mexico could not be older than Middle Miocene time (circa 15 Ma).

The Pleistocene glaciations that affected the Northern Hemisphere must have exerted an important effect of the climatic and biotic distribution in Northern Mexico; however, the detailed description of such effect largely remains to be done.