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.

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.

Modern amphibians (lissamphibians) are highly sensitive indicators of environmental disturbance. As such, fossil lissamphibians are an excellent model for testing causal hypotheses of the Cretaceous-Paleogene mass extinction and secondary effects of Deccan volcanism and a bolide impact (e.g., acid rain). We quantitatively analyzed high-resolution temporal changes in diversity and community structure of a succession of salamander and salamander-like lissamphibian assemblages from the Hell Creek Formation and Tullock Member of the Fort Union Formation of Garfield County, northeastern Montana (ca. 67.5–65.3 Ma). Richness, evenness, and taxonomic composition remained stable through the lower Hell Creek Formation. Peak richness (11 species) occurred in the middle of the formation coincident with a short-term drop in evenness. Following a return to preexisting levels of evenness, diversity progressively declined in the upper third of the formation. This pattern reflects plummeting relative abundances of Scapherpeton tectum and a stepwise disappearance of five species, of which three represent extirpation (33%) and two represent extinction (22%). These results suggest that ecological instability increased in the local fauna during the last ~400 k.y. of the Cretaceous. Temporal correlation with local, regional, and global changes in other aspects of the terrestrial (mammals, plants) and marine (planktonic foraminifera, mollusks) biota and environment (volcanism, paleotemperature) implies a global phenomenon (late Maastrichtian event). The post–Cretaceous-Paleogene “survival” fauna from the lowermost Tullock Member was taxonomically depauperate and predominated by the “bloom taxon” Opisthotriton kayi . Together, our results lend growing support in favor of a complex multiple-cause scenario for the Cretaceous-Paleogene mass extinction event.

Here we provide a survey of Triceratops localities and accompanying stratigraphic data from the Hell Creek Formation of northeastern Montana. The majority of the sites discussed here were relocated or discovered during the last 5 yr of the Hell Creek Project (1999–2010), a multi-institutional effort to record a large volume of faunal, floral, and geologic data on the Hell Creek Formation in order to test evolutionary, paleoecological, and geological hypotheses. Triceratops is the most abundant dinosaur in the Hell Creek Formation and one of the most common nonavian dinosaurs of the Upper Cretaceous. It is known from hundreds of specimens, which have been collected since it was first described in 1889. Although these specimens provide a wealth of morphological data on Triceratops , many lack detailed stratigraphic information and context. Detailed stratigraphic and contextual data for more than 70 specimens of Triceratops collected during the Hell Creek Project make this data set among the most comprehensive for any nonavian dinosaur.

Ceratopsid dinosaurs are notable for their common occurrences in bonebeds; however, until recently, these have not been encountered for the chasmosaurine Triceratops . The aim of this investigation is to describe the taphonomy of Quittin' Time (Museum of the Rockies locality HC-430), a Triceratops bonebed in the Hell Creek Formation, Garfield County, Montana. Using site taphonomic descriptions with an evaluation of ontogeny, inferences regarding the paleobiology of this extinct taxon are possible. The locality is associated with abundant organic material, including woody debris, large seeds, and other fragments in isolated silty lenses, all incorporated within a siltstone matrix, indicating preservation within a floodplain environment. Based on the repetition and ontogenetic stages of cranial elements, the minimum number of individuals (MNI) is three. Evidence from the location and taphonomic condition of the bones preserved in close proximity within the same siltstone unit suggests that the individuals—one young adult, one subadult, and a juvenile—likely accumulated during distinct flooding events within a narrow region of the floodplain as a result of “bloat-and-float” transport. The relatively small scale of the bonebed, both in terms of total area and number of individuals, implies that future work on Triceratops sites requires careful scrutiny of cranial elements examined within an ontogenetic framework because they are potentially critical to establishing MNI. Preservation of multiple individuals within the same unit does not necessarily provide evidence of gregarious behavior in Triceratops but rather may be a reflection of site taphonomic history and accumulation processes.

The Cretaceous-Paleogene boundary marks a critical event in mammalian evolution. Using a database of 4769 mammalian specimens from the Hell Creek and lower Fort Union formations of Garfield County, northeastern Montana, I quantified temporal patterns of diversity and community structure to evaluate faunal dynamics during the last ~1.9 m.y. of the Cretaceous, the Cretaceous-Paleogene extinction and survival, and the first ~1.2 m.y. of the early Paleocene recovery and placental radiation. Mammalian taxonomic composition and species richness remained relatively stable for most of the last ~1.9 m.y. of the Cretaceous, but the relative abundance of metatherians and evenness of paleocommunities began declining ~500–600 k.y. before the Cretaceous-Paleogene boundary. The ecological instability implied by falling evenness may be linked to the local extinction of 75% of species at or near the Cretaceous-Paleogene boundary. The early Puercan (Pu1) survival fauna from the first ~100–200 k.y. of the Paleocene is species poor and consists of a few local “bloom taxa” and an influx of immigrants. The species-rich mid/late Puercan (Pu2/3) fauna implies mammalian recovery occurred within ~600–700 k.y. of the Cretaceous-Paleogene event. The Pu2/3 and early Torrejonian (To1) faunas signal the beginning of the placental radiation—“bloom taxa” and multituberculates waned, richness and abundance of “archaic ungulates” and plesiadapiform primates increased, and the first appearance of taeniodonts, pantodonts, and possibly creodonts expanded the ecological diversity of local faunas. Together, the decoupled patterns of richness and relative abundances reveal the complexity of faunal dynamics during this seminal episode in mammalian history.