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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 identified 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 define the time-transgressive onset of basin formation and shows the different subsidence histories of the northern and southern parts of the basin. Keywords: magnetostratigraphy, Paleocene, Eocene, Bighorn Basin, mammals, plants.

Initial continental collision between India and Asia is thought to have caused significant changes to global climate and biota, yet its timing and biogeographic consequences are uncertain. Structural and geophysical evidence indicates initial collision during the early Paleogene, but sedimentary evidence of this has been controversial owing to the intense deformation and metamorphism along the suture zone. Modern orders of mammals that appeared abruptly on northern continents coincident with the global warming event marking the Paleocene-Eocene boundary are hypothesized to have originated on the Indian subcontinent, but no relevant paleontological information has been available to test this idea. Here we present new paleomagnetic, sedimentologic, and paleontologic evidence to show that the lower Eocene Ghazij Formation of western Pakistan records continental sedimentation and mammalian dispersal associated with initial India-Asia collision. Our results are consistent with the initial collision occurring near the Paleocene-Eocene boundary, but modern orders of mammals appeared later in Indo-Pakistan and thus did not likely originate on the Indian subcontinent. Keywords: India, Asia, collision, paleomagnetism, mammals.

New stratigraphic and paleontological information from the McCullough Peaks, northern Bighorn Basin, Wyoming, is incorporated into an isotaphonomic faunal database and used to investigate the impact of the latest Paleocene thermal maximum and coincident earliest Wasatchian immigration event on local mammalian community structure. Surface collections from Willwood Formation overbank deposits provide taphonomically consistent and stratigraphically resolved samples of the medium- to large-sized components of underlying mammalian communities. Rarefaction shows that the immigration event caused an abrupt and dramatic increase in species richness and evenness. After this initial increase, diversity tapered off to more typical Wasatchian levels that were still higher than those in the preceding Clarkforkian. Wasatchian immigrants were rapidly incorporated into the new community organization, representing ~20% of the taxa and ~50% of the individuals. Immigrant taxa generally had larger body sizes and more herbivorous and frugivorous dietary habits compared to endemic taxa, causing significant turnover in body-size structure and trophic structure. There was a significant short-term body-size decrease in many lineages that may have been prompted by the elevated temperatures and/or decreased latitudinal thermal gradients during the latest Paleocene thermal maximum. Rapid short-term climatic change (transient climates) and associated biotic dispersal can have abrupt and long-lasting effects on mammalian community evolution.

Paleogene hyperthermals, including the Paleocene-Eocene Thermal Maximum (PETM) and several other smaller events, represent global perturbations to Earth's climate system and are characterized by warmer temperatures, shifts in floral and faunal communities, and hydrologic changes. These events are identified in the geologic record globally by negative carbon isotope excursions (CIEs), resulting from the input of isotopically light carbon into Earth's atmosphere. Much about the causes and effects of hyperthermals remains uncertain, including whether all hyperthermals are caused by the same underlying processes, how biotic effects scale with the magnitude of hyperthermals, and why CIEs are larger in paleosol carbonates relative to marine records. Resolving these questions is crucial for their full interpretation and application to future climate scenarios. The Fifteenmile Creek area of the central Bighorn Basin, Wyoming U.S.A., exposes an early Eocene floodplain sedimentary sequence that preserves paleosol carbonates and an extensive fossil mammal collection. Previous analysis of faunal assemblages revealed two pulses of mammal turnover and changes in diversity interpreted to correlate with the ETM2 and H2 hyperthermals that immediately follow the PETM. This was, however, based on long distance correlation of chemostratigraphic records. We present new carbon isotope stratigraphy using micrite δ13C values from paleosol carbonate nodules preserved in and between richly fossiliferous localities at Fifteenmile Creek to identify the stratigraphic positions of ETM2 and H2. Additionally, we used differential GPS elevations to establish a new stratigraphic framework that assists in correlation and is independent from the biostratigraphy and previous composite lithostratigraphic sections from the area. Carbon isotope results show that the ETM2 and H2 hyperthermals, and possibly the subsequent I1 hyperthermal, are recorded at Fifteenmile Creek. ETM2 and H2 overlap with the two previously recognized pulses of mammal turnover. Comparisons between the new chemostratigraphy and fossil record suggest that the recorded amplitude of these faunal changes may be muted as a result of some stratigraphic averaging of fossils. The CIEs for these hyperthermals are also smaller in magnitude than in more northerly Bighorn Basin records. We suggest that basin-wide differences in soil moisture and/or vegetation could contribute to variable CIE amplitudes in this and other terrestrial records. [ABSTRACT FROM AUTHOR]

The Bighorn basin, Wyoming preserves complete records of middle Paleocene to lower Ecocence continental data and its geochronology is studied by geometric polarity time scale (GPTS). The biostratigraphic revision reveal that Mason Pocket, the primary fauna upon which the Tiffanian land-mammal age was originally based, is correlative to Chron 26and not Chron 25 and the age calibration of the biozone provides a robust framework for testing the synchrony of taxonomic first and last appearances.

Markov analysis was performed in the Bighorn Basin (Wyoming) to test a current depositional model. This analysis indicated that the observed sequence of facies transitions is non-random and that the transitions that deviate most from random are consistent with the hypothesized relationship between paleosol maturity and avulsion.