No abstract

New paleomagnetic data from six different sections in the Bighorn Basin are combined with previously published results to construct a basin-wide magnetostratigraphic framework. Geomagnetic polarity reversals between chrons C26r, C26n, C25r, C25n, C24r, and C24n have been identifi ed among multiple stratigraphic sections in different parts of the basin. Using the new magnetostratigraphic framework, paleontological, paleobotanical, and isotopic information from these varied locations in the basin can now be correlated and compared to similar records from elsewhere in the world. These new data resolve previous uncertainty concerning the timing of an important episode of faunal turnover known as Biohorizon B, which is slightly below the chron C24r-C24n boundary, close to the position of the Elmo isotope excursion in marine records. Backstripping analysis using these new magnetostratigraphic data helps defi ne the time-transgressive onset of basin formation and shows the different subsidence histories of the northern and southern parts of the basin.

The three dimensional structure of vegetation is an important component of ecosystems, yet it is difficult to reconstruct from the fossil record. Forests or woodlands prevailed at mid-latitudes in North America during the early Eocene but tree spacing and canopy structure are uncertain. Here we use stable carbon isotope values (␦ 13 C ) in early Eocene mammalian faunas to infer canopy structure. We compare ␦ 13 C values in two diverse fossil assemblages from the central Bighorn Basin to values predicted for mammals in a variety of open and closed habitats, based on modern floras and faunas. We conclude that these early Eocene faunas occupied an open canopy forest. We also use carbon and oxygen isotopes to infer diet and microhabitat. Three higher level taxa have significantly different mean ␦ 13 C values, and values are negatively correlated with body mass. The pattern suggests diets high in leaves for larger mammals, and fruit or other non-foliar plant organs for small ones. A preference in the larger mammals for wetter habitats with high water availability to plants may also have contributed to the pattern.

The mammal fauna of the Willwood Formation, central Bighorn Basin, Wyoming, is ideal for paleoecological analysis because it is extensive, well studied, and continuously distributed over sediments representing the first 3 Myr of the early Eocene. The geology of the Bighorn Basin is also well known, providing a precise temporal framework and climatic context for the Willwood mammals. Previous analysis identified three “biohorizons,” based on simple counts of the first and last appearances of species. This study uses species diversity and appearance rates calculated from more extensive collections to approximate the ecological dynamic of the ancient fauna and assess whether the biohorizons were significant turnover events related to recently described climatic variation. Diversity and appearance data collected for this project are extensively corrected for uneven sampling, which varies by two orders of magnitude. Observed, standardized appearance and diversity estimates are subsequently compared with predicted background frequencies to identify significant variation. Important coincident shifts in the biotic parameters demonstrate that ecological change was concentrated in two discrete intervals ≤300 Kyr each that correspond with two of the original biohorizons. The intervals coincide with the onset and reversal of an episode of climate cooling identified directly from Bighorn Basin floras and sediments. Ecological changes inferred from the diversity and turnover patterns at and following the two biohorizons suggest short- and long-term faunal response to shifts in mean annual temperature on the order of 5–8°C.