In northeastern Montana, fossil localities in the Garbani Channel Complex and other early Paleocene (Puercan 3 Interval Zone) localities are preserved in the Tullock Member of the Fort Union Formation. They document an early phase in the recovery of the terrestrial fauna of the North American Western Interior after the mass extinction marking the Cretaceous-Paleogene (K-Pg) boundary. The cimolestids Puercolestes simpsoni and Betonnia tsosia were typified on fragmentary jaws and isolated teeth found in Puercan 2 and 3 Interval Zones (Pu2, Pu3) in the Nacimiento Formation, San Juan Basin, New Mexico. The available samples of these genera from both New Mexico and Montana are small and dominantly consist of isolated teeth. Characters of upper cheek teeth, P4 and M1–M3, justify provisional recognition of Puercolestes sp. cf. Pu. simpsoni and the somewhat smaller Betonnia sp. cf. Be. tsosia in Pu3 local faunas in the Tullock Member. In contrast, discovery of characters distinguishing the isolated lower cheek teeth, p4s and m1–m3s, of these species must await recovery of dentulous dentaries documenting the patterns of morphological variation of their lower dentitions. Fossils from the Tullock Member add support to the current interpretation that cimolestids were taxonomically diverse and geographically widespread but relatively rare members of the faunas of the North American Western Interior during approximately the first million years of the Paleocene.

Three species of the cimolestid Procerberus are currently recognized in the northern North American Western Interior in latest Cretaceous and earliest Paleocene (Puercan North American Land Mammal Age) faunas: P. formicarum, P. andesiticus, and P. grandis. Analysis of a new topotypic sample of P. formicarum from the Bug Creek Anthills locality provides an estimate of the range of variation of its postcanine dentition. The three currently recognized species occur in Puercan 1 (Pu1) interval zone faunas, but two other occurrences indicate that the genus originated and initially diversified prior to the Cretaceous/Paleogene boundary. Rare occurrences at several localities and entries in faunal lists suggest even greater taxonomic diversity. Limited evidence suggests continued diversification in the later Puercan and possible survival of the genus into the Torrejonian. Procerberus grandis or a closely related species may be a sister group of primitive taeniodonts.

Since Michael J. Novacek (1977) established Prodiacodon crustulum, the hypodigm of this species has been greatly increased. Currently, over 100 isolated teeth, but only a single dentulous fragment of a dentary, are known. Reconstruction of its dentition has been based on comparisons with the more completely represented dentition of P. puercensis Matthew, 1929, from younger Torrejonian 1-3 North American Land Mammal Age (NALMA) local faunas of the San Juan Basin, New Mexico. Prodiacodon crustulum is known from the Puercan 3 (NALMA) Garbani Channel and, possibly, Purgatory Hill local faunas in Montana. It differs from P. puercensis in the smaller size of its postcanine dentition and lesser development of minor cusps on the upper and lower molars. Given the uncertainties in reconstruction of its dentition, the new sample adds little toward illustrating the phylogenetic relationships of the species to more recent Paleogene and Neogene leptictids. Recent studies support recognition of the clade Leptictida McKenna, 1975, including the leptictids and the Cretaceous species of Gypsonictops Simpson, 1927. More data are needed to test the hypothesis that P. crustulum is the earliest known representative of the Afrotheria Stanhope et al., 1998.

The mass extinction at the Cretaceous- Paleogene boundary marks one of the most important biotic turnover events in Earth history. Yet, despite decades of study, the causes of the Cretaceous-Paleogene boundary crises remain under debate. An important tool that has the capacity to greatly improve our understanding of the events around the Cretaceous-Paleogene boundary is the geomagnetic polarity time scale (GPTS). The GPTS is used for age control in numerous Cretaceous-Paleogene boundary studies, including the timing of Deccan Traps volcanism, a majority of studies in marine sections, and studies on climate and ecological change across the Cretaceous-Paleogene boundary. The current calibration of the GPTS for circum- Cretaceous-Paleogene boundary polarity chrons (C30n-C28n) from the Geologic Time Scale draws heavily on astronomical tuning and uses a 40Ar/39Ar age for the Cretaceous- Paleogene boundary as a tie point that has since been shown to be 200 ka too old. Furthermore, complex sedimentation has been recorded in marine sections immediately following the Cretaceous-Paleogene boundary, which can possibly obscure orbital signals and complicate cyclostratigraphic interpretation. An independent test of the cyclostratigraphy for this time period is imperative for confidence in the astronomical time scale. Further, polarity reversal ages given in the GPTS do not include uncertainty estimates, making them unsuitable for quantitative chronometry. Recent calibrations have been attempted using U/Pb geochronology on zircons; however, U/Pb zircon dates may be biased by pre-eruptive zircon residence times of tens to hundreds of thousands of years. In this study, we provide constraints on the timing and duration of the most important circum-Cretaceous-Paleogene boundary chron, chron C29r, using high-precision 40Ar/39Ar geochronology and magnetostratigraphy on fluvial sediments from the Hell Creek region, Montana. Here, we show results for 14 new magnetostratigraphic sections, and 18 new high-precision 40Ar/39Ar dates, which together provide six independent constraints on the age of the C29r/C29n reversal and two constraints on the C30n/ C29r reversal. Together, these results show that the duration of C29r was 587 ± 53 ka, consistent with the most recent Geologic Time Scale calibration and previous U-Pb age models. We further present new geochronologic data for the Cretaceous-Paleogene boundary that provide the most precise date yet, of 66.052 ± 0.008/0.043 Ma. Integration of our results into the extensive paleontological framework for this region further provides important constraints on rates of terrestrial faunal change across the Cretaceous-Paleogene boundary.

Detailed understanding of ecosystem decline and recovery attending the Cretaceous-Paleogene boundary (KPB) mass extinctions is hindered by limited constraints on the pace and tempo of environmental events near the boundary. To mitigate this shortcoming, high-resolution 40Ar/39Ar geochronology was performed on tephras intercalated between fossiliferous terrestrial sediments of the upper Hell Creek and lower Fort Union Formations in the western Williston Basin of northeastern Montana (USA). Tephra samples were collected from 10 stratigraphic sections spanning an area of ~5000 km2. Several distinctive tephras can be correlated between sections separated spatially by as much as ~60 km. The tephras are thin distal deposits generally preserved only in lignite beds, which are interbedded with clastic deposits yielding vertebrate faunas of Lancian (late Maastrichtian) to Torrejonian (early Danian) North American Land Mammal Ages. Sanidine from 15 tephra samples was analyzed in 1649 total fusion experiments (1597 on single crystals) and 12 incremental heating analyses of multigrain aliquots. Ages were determined for 13 distinct tephras, ranging from 66.289 ± 0.051 to 64.866 ± 0.023 Ma, including only analytical uncertainties. This level of precision is sufficient to resolve the ages of all of the coal beds that have served as a basis for a regional stratigraphic framework. The data confirm that the Hell Creek-Fort Union formational contact is diachronous, and further support the age of the KPB impact layer at 66.043 ± 0.010 Ma (or ± 0.043 Ma considering systematic uncertainties). Application of the new results to previous magnetostratigraphic data indicates an appreciably compressed time interval between the base of chron C29r and the top of chron C28r, with a maximum duration estimate of 1.421 ± 0.066 Ma. Most notable is the implied brevity of chron C29r, with a maximum estimate of 457 ± 54 ka, and possibly as brief as 345 ± 38 ka, compared to the 710 ka estimate from the Geologic Time Scale 2012 (GTS2012). Further, application of new results to terrestrial biostratigraphy adds higher precision to the timing and tempo of biotic change before and after the KPB. Our results indicate that the timing of pre-KPB ecological decline is constrained to the last ~200 ka of the Cretaceous, adding further support to the press-pulse extinction hypothesis. Additionally, the duration of the depauperate basal Paleogene Puercan 1 disas ter fauna is confined to a 70 ka interval. Faunal recovery in this region, indicated by the appearance of primitive members of the placental mammal radiation and the restoration of taxonomic richness and evenness, occurred within ~900 ka after the KPB. These results show that biotic recovery after the mass extinction in the terrestrial realm was more rapid than in the marine.

Changes in mammalian faunal composition and structure following the Cretaceous- Paleogene mass extinction are central to understanding not only how terrestrial communities recovered from this ecological perturbation but also the evolution of archaic groups leading to extant mammalian clades. Here, we analyzed changes in mammalian local faunas during the earliest Paleogene biotic recovery on a small spatiotemporal scale. We compiled samples of mammals from four localities in the Hell Creek Formation and Tullock Member of the Fort Union Formation, in the McGuire Creek area, McCone County, Montana, USA, and placed these localities into a high-precision chronostratigraphic framework using 40Ar/39Ar tephra ages and magnetostratigraphy. Within this framework, we quantitatively compared faunal composition, heterogeneity, and richness among McGuire Creek local faunas and made broader comparisons to other earliest Paleogene faunas from throughout the Western Interior of North America. In the first ~320 k.y. of the recovery, mammalian local faunas at McGuire Creek, all of which can be placed in the Puercan 1 North American Land Mammal Age (NALMA) interval zone, underwent modest increases in taxonomic richness and heterogeneity, indicating the beginning of biotic recovery; however, no Mc- Guire Creek fauna reached fully recovered levels of taxonomic richness. Further, appearance of immigrant taxa such as Purgatorius in younger McGuire Creek faunas demonstrates important compositional changes within the Pu1 of McGuire Creek. These results highlight the difficulties with describing the nuanced mammalian recovery process using the NALMA system and emphasize the increasing importance of high-precision dating, especially when comparing faunas across large geographic distances.

Plesiadapiform mammals, as stem primates, are key to understanding the evolutionary and ecological origins of Pan-Primates and Euarchonta. The Purgatoriidae, as the geologically oldest and most primitive known plesiadapiforms and one of the oldest known placental groups, are also central to the evolutionary radiation of placentals and the Cretaceous-Palaeogene biotic recovery on land. Here, we report new dental fossils of Purgatorius from early Palaeocene (early Puercan) age deposits in northeastern Montana that represent the earliest dated occurrences of plesiadapiforms. We constrain the age of these earliest purgatoriids to magnetochron C29R and most likely to within 105-139 thousand years post-K/Pg boundary. Given the occurrence of at least two species, Purgatorius janisae and a new species, at the locality, we provide the strongest support to date that purgatoriids and, by extension, Pan-Primates, Euarchonta and Placentalia probably originated by the Late Cretaceous. Within 1 million years of their arrival in northeastern Montana, plesiadapiforms outstripped archaic ungulates in numerical abundance and dominated the arboreal omnivore-frugivore niche in mammalian local faunas.