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Age and Correlation of Fossiliferous Late Paleocene–Early Eocene

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Age and Correlation of Fossiliferous Late Paleocene–Early Eocene PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3474, 26 pp., 12 ®gures, 2 tables May 11, 2005 Age and Correlation of Fossiliferous Late Paleocene±Early Eocene Strata of the Erlian Basin, Inner Mongolia, China GABRIEL J. BOWEN,1 PAUL L. KOCH,2 JIN MENG,3 JIE YE,4 AND SUYIN TING5 ABSTRACT 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 diversi®cation 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 (AL- MAs). 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]). Copyright q American Museum of Natural History 2005 ISSN 0003-0082 2 AMERICAN MUSEUM NOVITATES NO. 3474 early Eocene terrestrial and marine records. Using this trend and previously documented con- straints 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 con®rmed through further sampling, this result would imply that the ®rst appearance of the modern mammal orders Primates, Artio- dactyla, and Perissodactyla in Asia at the base of the Bumbanian ALMA did not signi®cantly precede their ®rst appearances in Europe and North America at the Paleocene/Eocene bound- ary. Fossil sites in the Erlian Basin promise to be central to resolving the debate about whether these clades lived and diversi®ed in Asia before dispersing throughout the Northern Hemi- sphere at the Paleocene/Eocene boundary. INTRODUCTION pearance of these groups and lack of closely allied fossil outgroups in early Paleogene The transition between the Paleocene and faunas led workers to conclude that the ap- Eocene epochs was one of the most dynamic pearances represent immigration from an un- intervals of the Cenozoic. Long-term trends seen locus of evolution (e.g., McKenna, in climate ushered in the warmest period of 1973). Phylogenetic analysis of recently dis- the Cenozoic (Zachos et al., 2001). Through- covered Paleocene mammal fossils from Chi- out this gradual shift in climate, faunas and na, however, has led some workers to con- ¯oras evolved in a series of abrupt events clude that Asian faunas include taxa closely leading to the establishment of the ®rst truly allied to the new groups, and that the origins modern terrestrial ecosystems evidenced in of the earliest Eocene debutants may be the fossil record of the Holarctic continents. traced to a low-latitude Asian Eden (Beard, Abrupt changes in climate and faunal and 1998; Beard and Dawson, 1999). ¯oral composition coincided at the Paleo- A simple test with the potential to verify cene/Eocene boundary, when transient global Asia as a source of emigration at the Paleo- warming (e.g., Kennett and Stott, 1991), dis- cene/Eocene boundary, if not as an evolu- persal of mammal groups throughout the tionary locus, would be to evaluate the rel- northern hemisphere (e.g., McKenna, 1973), ative age of the ®rst appearances of clades and shifts in ¯oral composition (Harrington, on the three Holarctic continents. If the early 2003) led to reorganization of terrestrial eco- representatives of one or more of the modern systems on geologically short timescales mammal groups were found to have greater (;100 thousand years (ky); Rohl et al., 2000; antiquity in Asia than in Europe and North Bowen et al., 2001; Farley and Eltgroth, America, this would support Asia as a source 2003). of evolution or emigration. Conversely, ®nd- One of the most profound changes in ter- ing that the Asian ®rst appearances lagged or restrial ecosystems that occurred near the Pa- were synchronous with those on other con- leocene/Eocene boundary was the abrupt ®rst tinents would con¯ict with such a hypothesis. appearance of several modern groups of Recent work has signi®cantly clari®ed the mammals in faunas from Asia, Europe, and relative age of early Paleogene faunal turn- North America (Gingerich, 1989; Hooker, over events on the Holarctic continents. Of 1998; Ting, 1998). The P/E ®rst appearances key importance is a globally correlatable car- included the ®rst representatives of Primates, bon isotope excursion documented in marine Artiodactyla, and Perissodactyla, three extant and terrestrial sedimentary rocks (Koch et orders that went on to diversify and become al., 1992; Zachos et al., 1993) and recently important components of modern faunas, and chosen as the de®ning stratigraphic datum Hyaenodontidae, a family of carnivorous for the Paleocene/Eocene boundary (Ouda, mammals that suffered extinction in the Mio- 2003). The base of the d13C excursion has cene (e.g., Gingerich, 1989). The abrupt ap- been shown to correlate with the P/E ®rst 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 3 appearances to within ;10 ky in North lithological, biostratigraphic, paleomagnetic, America (Bowen et al., 2001), where the new and isotopic data from three fossil-bearing groups appear at the base of the Wasatchian stratigraphic sections in the Erlian Basin. North American Land Mammal Age (NAL- These data allow re®ned stratigraphic corre- MA; Gingerich, 2001). Work in southern Eu- lation of rock units within the basin and sug- rope has shown that the P/E ®rst appearances gest correlations between these units and the there are unlikely to predate those in North GPTS. We then examine the implications of America (Cojan et al., 2000). In northern Eu- these correlations for the potential antiquity rope, taxa characteristic of the P/E ®rst ap- of the Bumbanian ALMA and associated pearance event ®rst appear at the Dormaal modern mammal groups in Asia. locality of Belgium (Smith, 2000). By cor- relation to other early Paleogene localities in GEOLOGICAL SETTING Belgium and northern France for which bulk organic carbon isotope data are available, Fossil mammals of the Erlian Basin occur this fauna is thought to occur within the CIE within a widely distributed package of sili- and be time-equivalent to the earliest Was- ciclastic sedimentary rocks. These rocks are atchian faunas (Steurbaut et al., 1999; Ma- largely undeformed and outcrop primarily in gioncalda et al., 2004). widely separated low mesas and in shallow In Asia, the ®rst representative of Hyaen- washes. As a result of their poor local ex- odontidae occurs in a fauna assigned to the posure and the relative isolation of outcrops Gashatan Asian Land Mammal age (AL- in this region, stratigraphic correlation of MAs; Meng et al., 1998), which has been rock units within the basin is dif®cult and no shown to be Late Paleocene in age (Bowen clear regional stratigraphy has been devel- et al., 2002). Other important appearance oped. Several stratigraphic units have been events, however, including the ®rst Asian pri- de®ned based on a combination of litholog- mates, perissodactyls, and perhaps artiodac- ical and faunal characteristics, and these tyls (Tong and Wang, 1998) occur within the were recently reviewed by Meng et al. (1998, subsequent Bumbanian ALMA (Dashzeveg, 2004). 1988; Ting, 1998). Earlier work (Bowen et The rocks at our study sites include red al., 2002; Ting et al., 2003) has shown that and tan mudstones, calcareous mudstones, the base of the Bumbanian is no younger and siltstones, commonly preserving milli- than the P/E boundary, at ;55 million years meter-scale laminations of probable lacus- ago (Ma; Wing et al., 1999), and that Gash- trine origin, massive, brick red mudstone atan index taxa persist at least until the end beds commonly preserving features indica- of chron C25n of the geomagnetic polarity tive of paleosol development (see below), timescale (GPTS) ;55.9 Ma (Cande and and laterally extensive sandstone and con- Kent, 1995). These correlations leave an un- glomerate beds of ¯uvial origin. These rocks constrained window of time some 900 ky have been previously assigned to the Nom- long during which modern groups may have ogen, Arshanto, and Irdin Manha Forma- evolved and diversi®ed in Asia or, alterna- tions, although the details of their assignment tively, archaic groups may have persisted be- has differed among studies (see Meng et al., fore the impending restructuring of their 2004).
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