ago. The event may have been associated with climate change or with the rapid development of a river system across the San Juan basin, which caused a temporary pause in sediment deposition that separates the Nacimiento Formation from the San Jose Formation. History of investigation Edward Drinker Cope described the fossils during the Wheeler Survey. Workers in the early 1900s divided the rocks of the Nacimiento Formation into two formations, the lower Puerco Formation and the upper Torrejon Formation. This was rejected on the grounds that there were no lithological differences between the two, only differences in fossil faunas, making determination of which formation was present in a given area impossible if fossils could not be found. The Puerco and Torrejon were retained as zones within the Nacimiento Formation, and their faunas became the basis of the Puercan and Torrejonian North American Land Mammal Ages. Footnotes References Paleogene formations of New Mexico Natural history of New Mexico Paleocene Series of North America Paleontology in New Mexico