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Species discrimination in the multituberculate Mesodma Jepsen, 1940 (Mammalia, Allotheria): considerations of size, shape, and form
Size-driven preservational and macroecological biases in the latest Maastrichtian terrestrial vertebrate assemblages of North America
REVISED STRATIGRAPHIC RELATIONSHIPS WITHIN THE LOWER FORT UNION FORMATION (TULLOCK MEMBER, GARFIELD COUNTY, MONTANA, U.S.A.) PROVIDE A NEW FRAMEWORK FOR EXAMINING POST K–PG MAMMALIAN RECOVERY DYNAMICS
A systematic reappraisal and quantitative study of the nonmarine teleost fishes from the late Maastrichtian of the Western Interior of North America: evidence from vertebrate microfossil localities 1
Paleoenvironments, taphonomy, and stable isotopic content of the terrestrial, fossil-vertebrate–bearing sequence of the El Disecado Member, El Gallo Formation, Upper Cretaceous, Baja California, México
Early mammalian recovery after the end-Cretaceous mass extinction: A high-resolution view from McGuire Creek area, Montana, USA
Calibration of chron C29r: New high-precision geochronologic and paleomagnetic constraints from the Hell Creek region, Montana
High-resolution chronostratigraphy of the terrestrial Cretaceous-Paleogene transition and recovery interval in the Hell Creek region, Montana
A multivariate approach to infer locomotor modes in Mesozoic mammals
Environmental change across a terrestrial Cretaceous-Paleogene boundary section in eastern Montana, USA, constrained by carbonate clumped isotope paleothermometry
Foreword
From Tyrannosaurus rex to asteroid impact: Early studies (1901–1980) of the Hell Creek Formation in its type area
Over a century has passed since 1901 when W.T. Hornaday showed a fragment of a horn of Triceratops found in the valley of Hell Creek to H.F. Osborn at the American Museum of Natural History. The following year Osborn's assistant, Barnum Brown, was dispatched to eastern Montana and began investigations of its geology and paleontology. By 1929, Brown had published a geological analysis of the rocks exposed in the southern tributaries of the Missouri River, named the Hell Creek Formation, and published studies of some of the dinosaurs discovered there. Parts of his collections of fossil mollusks, plants, and vertebrates contributed to research by others, particularly members of the U.S. Geological Survey. From 1930 to 1959, fieldwork was slowed by the Great Depression and World War II, but both the continuing search for coal, oil, and gas as well as collections of fossils made during construction of Fort Peck Dam set the stage for later research. Field parties from several museums collected dinosaurian skeletons in the area between 1960 and 1971. In 1962, concentrations of microvertebrates were rediscovered in McCone County by field parties from the University of Minnesota. Ten years later, field parties from the University of California Museum of Paleontology began collecting microvertebrates from exposures in the valley of Hell Creek and its tributaries. The research based on this field research provided detailed geological and paleontological analyses of the Hell Creek Formation and its biota. In turn, these contributed to studies of evolutionary patterns and the processes that produced the changes in the terrestrial biota across the Cretaceous-Paleogene boundary.
In 1907, Barnum Brown named the Hell Creek beds (Formation) for the strata exposed in the Hell Creek Valley and other downstream tributaries of the Missouri River. In the absence of a stratotype section, a lectostratotype is herein proposed for the Hell Creek Formation based on 84.2-m-thick exposures at Flag Butte (local name) in Ried Coulee (archaic use; East Fork of Hell Creek) and East Ried Coulee, tributaries of Hell Creek, Garfield County, Montana (sec. 29, T. 21 N., R. 38 W., NAD27 CONUS; base 47.55931°N, 106.88111°W; top 47.55533°N, 106.86810°W). The formation is underlain with general conformity by sandstone beds of the Fox Hills Formation (as characteristically known, the Colgate Member is absent) and is for the most part conformably overlain locally by the Tullock Member of the Fort Union Formation. The upper contact at Flag Butte is demarcated at the base of the IrZ lignite bed (above an iridium anomaly). The boundary has been demonstrated to be somewhat unconformable in areas to the west. The IrZ bed is also missing at Bug Creek in McCone County. In its type section, the Hell Creek Formation is subdivided (simply and informally) into Ried Coulee (lower Hell Creek), East Ried Coulee (middle Hell Creek), and Flag Butte (upper Hell Creek) units, each containing a sandstone and a mudstone lithofacies. Formational thickness varies with local depositional and erosional history of various coastal-deltaic environments across the Williston Basin and a trend of overall thinning to the east and northeast.
The interval spanning the uppermost Hell Creek Formation to the overlying lowermost Fort Union Formation in north-central Montana encompasses a marked paleoenvironmental change (associated with the formational contact), the Chicxulub impact event, and the Cretaceous-Paleogene boundary. We have examined the record of this transition at the Hell Creek Formation lectostratotype to determine the placement of these events using a series of lithological, geochemical, palynological, and 40 Ar/ 39 Ar geochronological analyses. The claystone derived from the Chicxulub impact is identified based on lithological criteria, enrichment of iridium and osmium, and osmium isotope ratios. The impact claystone also contains a Cyathidites fern spike. The first continuous lignite horizon in the section immediately overlies this claystone and represents the Hell Creek–Fort Union formational contact. A tuff ~3 m above the impact layer is dated to 66.024 ± 0.059 Ma. Given this evidence, at the lectostratotype the Cretaceous-Paleogene boundary is coincident with the impact claystone and therefore with the formational contact. Due to poor preservation and apparent reworking of palynomorphs surrounding the formational contact, the Cretaceous-Paleogene boundary is difficult to identify based on biostratigraphically significant taxa. The presence of marine dinoflagellates is suggestive of reworking of older marine sediments during the deposition of the Cretaceous-Paleogene boundary interval.
Magnetostratigraphy of the Hell Creek and lower Fort Union Formations in northeastern Montana
Magnetostratigraphic evaluation of a well-exposed stratigraphic section in northeastern Montana has been undertaken to expand upon and better understand the timing of the Hell Creek and Fort Union Formations and the biological processes recorded within them. Characteristic remanent magnetizations show clear magnetostratigraphic patterning of chrons C28n, C28r, C29n, C29r, C30n, and possibly C30r. Differentially corrected global positioning system coordinates, including elevation, were recorded at each sample site, allowing the magnetostratigraphic framework to be precisely relocated in the field and traced laterally across the landscape. In this way, important chronologic boundaries such as the Cretaceous-Paleogene boundary can be projected into or above topography of the surrounding region. Localities in Montana that have been sampled for fossil studies were mapped and correlated to the same stratigraphic sections as the magnetostratigraphy, and so they can be compared directly to the geomagnetic polarity time scale. The new magnetostratigraphy can also be used to relate to other basins of Cretaceous and Paleogene age using information independent from biostratigraphic zonation, making it possible to directly compare the composition of coeval faunas from significantly different latitudes.
Questions of biotic and environmental change during deposition of the Upper Maastrichtian Hell Creek Formation require a robust and replicable system for intra-formational correlation of fossil localities. In this paper, we present a carbon isotope chemostratigraphic curve based on terrestrial organic carbon. Data were taken from a complete measured section spanning the full 93 m of the Hell Creek Formation at our study site. Sedimentary beds were described at the centimeter scale, and samples for carbon isotope analysis were taken at ~10 cm intervals. Each sedimentary bed was analyzed in thin section, and grain-size data were assembled based on petrographic point counts. The well-documented Cretaceous-Paleogene boundary negative carbon isotope excursion, six negative carbon isotope excursions, and four tentative positive carbon isotope excursions provide chronostratigraphic tie points within the Hell Creek Formation. We used this curve to precisely correlate 12 additional fossil-bearing localities from throughout the Hell Creek Formation across its type area. These correlations revealed significant local variation in sediment accumulation rates, confirming that simple stratigraphic position relative to the diachronous base and top of the Hell Creek Formation introduces significant error in correlation.
The Hell Creek Formation in eastern Montana has yielded well-preserved leaf megafossil localities that provide insight into the vegetation and climate of the latest Cretaceous. Among the most basal, the PDM locality (UCMP [University of California Museum of Paleontology] PB99057 = MOR [Museum of the Rockies] HC-278) occurs in channel sandstones ~10 m above the underlying Fox Hills Formation. The locality represents a fluvial/estuarine environment. Leaf megafossil impressions were preserved on clay drapes within the channel. Angiosperms dominated the flora (13 of 17 morphotypes). Dryophyllum subfalcatum and “ Vitis ” stantoni , two common morphospecies in the Hell Creek Formation, are well represented. Gymnosperms including Metasequoia , Glyptostrobus , Cupressinocladus , and Ginkgo are rare; ferns and cycadophytes are absent. Univariate leaf-margin analysis produced mean annual temperature (MAT) estimates of ~7–11 °C (5–14 °C including overlapping estimation errors). The Climate Leaf Analysis Multivariate Program (CLAMP) produced a MAT value of 11–12 °C ± 2 °C. Leaf area analysis produced mean annual precipitation (MAP) estimates of 197 cm (+152/−86 cm) and 191 cm (+161/−87 cm), while CLAMP produced a growing season precipitation estimate of 82–90 ± 48 cm. The wetter MAP values are consistent with paleosols near the base of the formation, which lack paleosol carbonate. CLAMP results further suggest seasonality in both temperature and precipitation. Some PDM morphotypes are familiar from the Hell Creek I floral zone of North Dakota, and several are not, suggesting greater spatial and/or temporal heterogeneity in the Hell Creek Formation flora than has been previously appreciated.
Many workers consider the Cretaceous-Paleogene extinction the archetypal catastrophic pulse event caused solely by the Chicxulub bolide impact. However, based on a global scale analysis of marine animals, the Cretaceous-Paleogene boundary is a candidate for an extinction enhanced by the coincidence of press and pulse disturbances. We make a preliminary test of key predictions of the press-pulse hypothesis using palynological data. We document a local palynological extinction of 21% at the Cretaceous-Paleogene boundary, which is consistent with extinction rates of 15% to 30% at other localities in the Hell Creek type area and throughout North America. We also find a decline in the number of dicot angiosperm pollen taxa between −3.5 m and −2.5 m below the boundary. We document a low-palynospecies-richness interval between −1.4 m and −1.0 m that includes extirpation, but not extinction, of some palynospecies. These changes in species richness are not correlated with changes in depositional style or pollen preservation, indicating that they may represent a biological rather than entirely taphonomic signal. Review and reanalysis of previously published data from other localities in the western interior of North America suggest similar declines in species richness within approximately the same stratigraphic interval. However, many of the species absent during the low-species-richness interval reappeared before the Cretaceous-Paleogene boundary, suggesting changes in community structure and composition before the terminal Cretaceous event—a key prediction of the press-pulse hypothesis—rather than gradual extinction in the latest Cretaceous.
An examination of freshwater euselachian fossils from the Maastrichtian lower and upper Hell Creek Formation, and the Bug Creek Anthills (Cretaceous-Paleogene boundary), and the early Paleocene (Puercan) Tullock Member of the Fort Union Formation of Montana, USA, revealed seven taxa: Lonchidion selachos , Protoginglymostoma estesi , Chiloscyllium , Restesia americana , Ischyrhiza avonicola , Myledaphus pustulosus n. sp., and Dasyatis (Dasyatidae). Squatirhina americana is redescribed to the new genus Restesia based on unique crown morphology. Dasyatis is only reported from the Paleocene. This is in contrast to the other collected taxa, which are only known from the Cretaceous. Ischyrhiza is not reported in our samples from the Hell Creek Formation despite earlier erroneous claims; however, the taxon is present in a sample from the Bug Creek Anthills. We suggest that this taxon infrequently moved into the freshwater rivers to forage. Lonchidion selachos occurs only in the upper Hell Creek Formation, and we hypothesize that the upper Hell Creek localities were deposited during a warm interval, as the paleodistribution of L. selachos shifted significantly to the north during warmer periods in the Santonian and Campanian. Of the taxa found in the Hell Creek Formation and the Bug Creek Anthills, only Myledaphus is found in Paleogene deposits; however, specimens are extremely rare. The occurrence of Myledaphus in the Paleogene is suggested to be due to the occurrence of reworked material.