Age and Correlation of Fossiliferous Late Paleocene–Early Eocene Strata of the Erlian Basin, Inner Mongolia, China

The Asian continent preserves a rich and diverse record of Paleogene mammal faunas and their evolution through time. The sequence of faunal succession is of key importance to our understanding of the origin and diversification of modern mammal groups, as phylogenetic data suggest that many major modern clades may be rooted in Asia. By calibrating the Asian fauna sequence within a chronostratigraphic framework, we can begin to compare patterns of succession on a global scale and constrain models for the origination and dispersal of modern mammal groups in the early Paleogene. The Erlian Basin of Inner Mongolia preserves Early Paleogene strata and mammal fossils assignable to the Gashatan, Bumbanian, and Irdin Manhan Asian Land Mammal Ages (ALMAs). We measured stratigraphic sections and analyzed the stable isotope composition of paleosol carbonates and paleomagnetic directions of rocks at three localities in the Erlian Basin. The data document patterns in lithology, carbon isotope composition, and magnetic polarity that are consistent at all three localities and allow us to present two constrained hypotheses for the correlation of the local stratigraphic sections. Within the resulting composite section, we are able to identify a secular decrease in the carbon isotope composition of paleosol carbonate that can be equated to a multimillion-year trend preserved in late Paleocene and 1 Current address: Department of Biology, University of Utah, Salt Lake City, UT, 84112 ([email protected]. edu). 2 Department of Earth Sciences, University of California, Santa Cruz, CA 95064 ([email protected]). 3 Division of Paleontology, American Museum of Natural History ([email protected]). 4 Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, P.O. Box 643, Beijing 100044, P.R. China ([email protected]). 5 Museum of Natural Science, Louisiana State University, Baton Rouge, LA 70803 ([email protected]). 2 NO. 3474 AMERICAN MUSEUM NOVITATES early Eocene terrestrial and marine records. Using this trend and previously documented constraints on the age of the Bumbanian ALMA, the composite section is shown to correlate within the interval of time represented by chrons C26n–C24n of the Geomagnetic Polarity Timescale (GPTS). We outline three possible correlations of the sequence of magnetic polarity zones in our composite section to the GPTS and explore the biostratigraphic implications of these. All three possible correlations show that Gashatan faunas in Inner Mongolia occur within chron C24R, and the preferred correlation suggests that the Gashatan taxa may have persisted close to the Paleocene/Eocene boundary. If confirmed through further sampling, this result would imply that the first appearance of the modern mammal orders Primates, Artiodactyla, and Perissodactyla in Asia at the base of the Bumbanian ALMA did not significantly precede their first appearances in Europe and North America at the Paleocene/Eocene boundary. Fossil sites in the Erlian Basin promise to be central to resolving the debate about whether these clades lived and diversified in Asia before dispersing throughout the Northern Hemisphere at the Paleocene/Eocene boundary.