The Eocene-Oligocene transition (about 42 to 30 million years ago) was the most significant episode of climatic change and mass extinction since the demise of the dinosaurs. The extinctions during this transition have been attributed to many causes, including meteorite or cometary impacts, volcanism, and climatic cooling due to Antarctic glaciation and oceanographic changes. Recent research has shown that the most dramatic changes occur between the middle and late Eocene. The Trans-Pecos region of Texas is the only place in North America where this change is recorded by superposed mammalian faunas datable by both radiometrics and magnetostratigraphy. There is no place in North American the contains interfingering marine/non-marine sequence of this age, so this area is the key to unlocking the details of the Uintan-Duchesnean-Chadronian transition in land mammals. The Trans-Pecos rocks contain a number of well studied faunas of middle Eocene (Uintan) and late Eocene (Duchesnean and Chadronian) age. Although preliminary paleomagnetic studies have been on some these rocks, many of the key fossil localities, and most of the Uintan and Chadronian parts of the section, have never been sampled. This project will collect and analyzed paleomagnetic samples from most of the key localities, and provide new 40Ar/39Ar dates to improve the geochronology. It will then be possible to correlate Uintan, Duchesnean, and Chadronian faunas from all over western North America to the magnetic polarity timescale. Episodes of extinction and faunal change can then be examined in greater detail, and correlated more precisely with the global record of climatic and biotic change during the Eocene-Oligocene transition.
