Late Tertiary terrestrial mammals from Central Mexico and their relationship to South American immigrants

Abstract

Many new discoveries of late Tertiary mammals have been made across central Mexico in the past two and one-half decades. Most came from basins located in the Transmexican Volcanic PROVAS Belt. Unlike a majority of earlier finds, these have been carefully placed in a stratigraphic context. Associated volcanic ash layers have permitted a number of radiometric ages and, coupled with recent paleomagnetic studies, allow placement of the Hemphillian – Blancan North American Land Mammal Age boundary at 4.8 Ma. Research on late Tertiary mammals from central Mexico has also provided valuable information on new taxa, significant geographic range extensions for many animals, and environmental changes.Several South American immigrant mammals of late Tertiary age are now known from central Mexico. They add important new information concerning formation of the Panamanian land bridge and the Great American Biotic Interchange. Taxa providing relevant data regarding this event are the ground sloths Megalonyx and Glossotherium, the glyptodont Glyptotherium, the pampathere Plaina, and the capybara Neochoerus. It has been generally accepted that the Panamanian land bridge formed between two and one-half and three million years ago. However, radiometric and paleomagnetic age determinations in central Mexico associated with South American immigrants, indicate that this land bridge might have originated more than one million years earlier. Active plate movement in Middle America during the late Tertiary with attendant volcanism and insular changes, coupled with concurrent eustatic and vegetational changes, make chronologic precision regarding isthmus formation and the resultant biotic interchange difficult at best.