Early Paleocene Mammal Succession In the Nacimiento Formation, San Juan Basin, New Mexico

Following the terminal Cretaceous mass extinction, mammals underwent a “Paleocene Evolutionary Radiation.” However, the detailed pattern of mammal succession during this interval has not been well-documented, especially at fine temporal scales. The San Juan Basin of New Mexico contains among the best records of terrestrial vertebrate succession for the early Paleocene, spanning most of the Puercan and Torrejonian Land Mammal Ages. Moreover, this record can be tied directly to the geomagnetic polarity timescale, giving good age control. This gives an unparalleled opportunity to examine mammal succession through much of the early Paleocene at high temporal resolution.

Previous work correlating vertebrate fossil localities within the Paleocene Nacimiento Formation across the southern San Juan Basin resulted in the construction of a biostratigraphic framework and the recognition of eight mammal biostratigraphic zones. Recently, intensive collecting of selected vertebrate fossil horizons has greatly enhanced this record. This has at least partly addressed previous collecting biases. Significantly, the pattern of early Paleocene mammal succession in the San Juan Basin is not one of smoothly increasing diversity, but instead is marked by several distinct turnover events. The boundary between the two Puercan biostratigraphic zones, within subchron 29n, is marked by the disappearance of numerous taxa, but also by the appearance of probable immigrants. The “Puercan – Torrejonian boundary interval,” within subchron 28r, remains relatively poorly constrained. However, it is marked by the greatest faunal change observed within the entire sequence. Within the Torrejonian interval, the most significant faunal change is found approximately coincident with the base of subchron 27n. It is marked by the disappearance of several long-ranging mammal taxa, but is accompanied by few appearances. The overall pattern of mammal succession within the Nacimiento Formation suggests a number of different factors influenced mammal composition and diversity during the early Paleocene.