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.

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.

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.

Climate change has been suggested to be a factor in the distribution of reptiles in the Cretaceous and Paleogene, but comparatively few studies have examined the possible role of climate within lithostratigraphically or biostratigraphically constrained local areas. Here, we reexamine the role of climatic change on the distribution of turtles within the Late Cretaceous–earliest Paleogene record of the Hell Creek area of northeastern Montana, where long-term field studies have produced a relatively dense record of faunal change during a period of climatic fluctuation. An improved stratigraphic record allows us to document previously unavailable range data for the Hell Creek–Tullock turtle taxa and assess changes in richness and abundance through approximately the last 1.9 m.y. of the Cretaceous and the first 750 k.y. of the Paleocene and provide revised estimates for survivorship and extinction across the Cretaceous-Paleogene boundary.

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.

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This chapter documents survivorship across the Cretaceous/Tertiary (K/T) (and Lancian/Puercan) boundary for all 111 species of nonmarine vertebrates from Garfield and McCone counties, Montana. Species-level survivorship appears to be between about 53 and 64 percent after the artifacts caused by paleobiogeography, rarity of some species, and differing evolutionary rates are taken into account. Without correcting for these artifacts, survival is an artificially low 32 percent. If specimens from the Bug Creek interval are early Paleocene (Puercan) in age, survivorship is an almost certainly incorrectly high value of 82 percent. This is because an undetermined number of latest Cretaceous (Lancian) species in the Bug Creek interval probably were reworked from Cretaceous sediments and were extinct by Bug Creek time. Comparison of the Lancian/Puercan transition with the older Judithian/Lancian and younger Puercan/Torrejonian transitions suggests percentage survival is similar among the three (55, 48, and 58 percent, respectively) after the Lazarus effect is considered. These results are not easily explained by a catastrophic mass-extinction scenario for the K/T transition, at least for nonmarine vertebrates. Rather, a geologically rapid but noncatastrophic change, such as the loss of range and habitat diversity during the Late Cretaceous marine regression, is commensurate with the analysis.