Explore the vast range of titles available.

The Cretaceous/Paleogene (K/Pg) mass extinction was a pivotal event in Earth history, the latest among five mass extinctions that devastated marine and terrestrial life. Whereas much research has focused on the global demise of dominant vertebrate groups, less is known about changes among plant communities during the K/Pg mass extinction. This study investigates a suite of 11 floral assemblages leading up to and across the K/Pg boundary in northeastern Montana constrained within a well-resolved chronostratigraphic framework. We evaluate the impact of the mass extinction on local plant communities as well as the timing of post-K/Pg recovery. Our results indicate that taxonomic composition changed significantly from the Late Cretaceous to Paleocene; we estimate that 63% of latest-Cretaceous plant taxa disappeared across the K/Pg boundary, on par with other records from North America. Overall, taxonomic richness dropped by ~23–33% from the Late Cretaceous to the Paleocene, a moderate decline compared with other plant records from this time. However, richness returned to Late Cretaceous levels within 900 kyr after the K/Pg boundary, significantly faster than observed elsewhere. We find no evidence that these results are due to preservational bias (i.e., differences in depositional environment) and instead interpret a dramatic effect of the K/Pg mass extinction on plant diversity and ecology. Overall, plant communities experienced major restructuring, that is, changes in relative abundance and unseating of dominant groups during the K/Pg mass extinction, even though no major (e.g., family-level) plant groups went extinct and communities in Montana quickly recovered in terms of taxonomic diversity. These results have direct bearing on our understanding of vegetation change during diversity crises, the differing responses of plant groups (e.g., angiosperms vs. gymnosperms), and spatial variation in extinction and recovery timing.

Leaf adpression fossils vary in their organic content, relief, and quality of preservation. Some of the most enigmatic adpressions, known as leaf molds, retain fine morphological and anatomical details despite being found in coarse sandstones—a widespread phenomenon attributed to the presence of fine-grained minerals on the fossil surface. Previous taphonomic studies have demonstrated the importance of microbial biofilms in promoting mineralization and argued that authigenic iron oxides can serve as the preserving medium. Here, we propose that this role is played more commonly by biologically precipitated aluminosilicate phases (clays). To test this hypothesis, we conducted energy dispersive X-ray spectroscopy (EDS) analysis of thin sections through fossil leaves from five localities differing in age and depositional environment. Point spectra taken directly from the leaf-sediment interface revealed that cation-rich clays separate the leaf fossils from the matrix. Additional EDS analyses of biofilms on a fossil leaf and on modern oak leaves decaying in freshwater also revealed aluminosilicates, for which we infer a biofilm-mediated, authigenic origin. These results are the basis of a novel ‘Biofilm-Clay Template' taphonomic model, whereby microbially mediated clay authigenesis is commonly the first step in leaf adpression preservation.

Aptian paralic and Paleocene intracratonic mires that bracket the northern reaches of the Western Interior Seaway preserve Milankovitch orbital forcing cycle signals and record the dynamic interplay with craton-linked salt dissolution tectonism in the Alberta Basin foreland and the adjoined intracratonic Williston Basin to the southeast. These mires, now lignite-bearing strata, along the northern Seaway provide a proxy record for Milankovitch orbital signals that would not have been otherwise recorded because of the voluminous clastic sediments accumulated along the western Canadian portion of the Cretaceous Seaway, in contrast to pervasive orbital forcing signals recorded by organic-rich cyclic carbonate deposits accumulated along the central and southern reaches of the Seaway. Prior to opening of the northern segment of the Seaway, the base levels of the paralic mires of the lower McMurray Formation (Aptian) were hydraulically linked to the rising sea-level fluctuations resulting from southward transgression of the Boreal Sea. In contrast, inland fluvial-margin peats of the Paleocene Ravenscrag and Fort Union formations accumulated following the closure of the Cretaceous Seaway. These mires were increasingly sensitive to aquifer-eustatic controls on the water table–mire relationship. Orbital forcing signals were preserved in the central Williston Basin mires in contrast to their masking by salt dissolution collapse-subsidence structures that exerted dominant control on peat mire configurations across the northern Williston Basin. Paralic mires accumulated at the top of the lower McMurray Formation record short-term orbital forcing, possibly representing an obliquity cycle recorded by lithotype sequencing in petrographic profiles. Mires accumulated along the coastal areas during the transition from a lowstand system tract to an early transgressive system tract. The linkage of sea level with coastal water table fluctuations permitted pervasive accumulations of paralic mires at the top of the lower McMurray Formation. Internal mire cycles resulted in coal lithotype profiles responsive to Milankovitch fifth-order and higher sub-cycles. In contrast, concurrent cataclysmic sinkhole collapses and differential fault block displacements were responsive to underlying salt removal patterns. The sea level/water table linkage was punctuated by the salt tectonism and orbital forcing signals were masked, resulting in sinkholes infilled with different coal lithotype sequences. Following the closure of the Western Interior Seaway, inland Paleocene fluvial-margin mires across the northern intracratonic Williston Basin were responsive to larger scale salt removal events in the subsurface. The collapse-subsidence structures were propagated several km up-section, diffused but sufficient to control mire configurations and mask orbital forcing signals. Southward into the central and southwestern Williston Basin, these strong salt dissolution tectonism patterns were out-of-phase with the repetitive mire accumulations, permitting the recognition of the markedly weaker signals of the 100 kyr orbital eccentricity cycle.

Fossilized leaf mines and other traces of phytophagous insects provide a unique window into ecological and evolutionary associations of the past. Leaf-mining flies (Diptera: Agromyzidae) are an important component of the recent leaf-mining fauna, but their fossil record is sparse compared to other mining insect lineages; many putative agromyzid body fossils and traces are dubiously assigned. Agromyzid leaf mines often can be distinguished from those of other insects by the presence of an intermittent, fluidized frass trail that may alternate between the sides of the mine. Here, we describe two new Paleogene leaf mine fossils, Phytomyzites biliapchaensis Winkler, Labandeira and Wilf n. sp. from the early Paleocene of southeastern Montana, USA, occurring in leaves of Platanus raynoldsii (Platanaceae); and Phytomyzites schaarschmidti Wappler n. sp., from the middle Eocene of Messel, Germany, occurring in leaves of Toddalia ovata (Rutaceae). These fossils both exhibit frass trails indicative of an agromyzid origin, and P. biliapchaensis also exhibits associated stereotypical marks identical to damage caused by feeding punctures of extant adult female Agromyzidae prior to oviposition. Phytomyzites biliapchaensis represents the earliest confirmed record of Agromyzidae, and one of the earliest records for the large dipteran clade Schizophora. Plant hosts of both species belong to genera that are no longer hosts of leaf-mining Agromyzidae, suggesting a complex and dynamic history of early host-plant associations and, for the early Paleocene example, an evolutionary, possibly opportunistic colonization in the midst of the ecological chaos following the end-Cretaceous event in North America.

The Corbulidae are one of a handful of a primarily marine bivalve clades that exhibit a remarkable radiation, marked by increased species richness and divergent morphologies, within a long-lived lake. For corbulids, this diversification occurred within the lower to middle Miocene Pebas Formation of western Amazonia. Only one taxon associated with this radiation (Anticorbula) remains extant. We conducted a series of phylogenetic analyses to characterize diversification of Corbulidae within the Pebas Formation and relate that diversification to geologically older freshwater corbulids from the Paleocene Fort Union Formation of the northern Great Plains (United States). We used these results, as well as a quantitative examination of morphospace occupation, to infer whether Pebasian corbulids represent a true species flock, and whether the lacustrine system represented by the Pebas Formation represents a cradle of, or reservoir for, freshwater corbulid diversity. We conducted two sets of phylogenetic analyses using shell morphology characters. A genus-level data set incorporated type species of freshwater corbulid genera, any Paleocene representatives of these genera, and selected brackish and marine corbulid genera. A species-level analysis added all described freshwater corbulid taxa to the genus-level matrix. Our results were highly resolved (few most-parsimonious trees), but not particularly robust (low branch support). For the genus-level matrix, we used a taxon jackknife procedure to explore the effects of taxon sampling on tree stability and topology. Jackknife results recover a subclade of freshwater taxa (including both Anticorbula and Pachydon species and the Paleocene Ostomya sp.) in 92.4% of trees, although placement of this subclade across the ingroup varies, as do the topologic positions of other freshwater species. Freshwater and marine corbulids also are morphologically distinct from each other, a factor that likely reduced the robustness of our phylogenetic results. By combining these results with paleoecologic, stratigraphic, and morphologic data, we infer that freshwater corbulids arose once within the family, prior to the Cenozoic, with three distinct freshwater lineages present at their first appearance in the late Paleocene of North America. Within the Miocene Pebas system of South America, we reconstruct supralimital morphologic evolution within three lineages as freshwater taxa became variously adapted to the fluid, dysoxic muds characterizing lake-bottom facies representative of the Pebas lacustrine system. In addition, corbulids apparently successfully coped with high predation pressures from co-occurring shell-crushing predators. Finally, we consider that freshwater Corbulidae were primarily fluvial taxa throughout their geologic history, with a relatively ephemeral radiation within the Pebasian lake system, thus making the Pebasian system a cradle of diversity for several corbulid lineages.

This study documents the terrestrial palynological record at the John's Nose section, a new Cretaceous-Paleogene (K-Pg) boundary site in North Dakota, USA. In addition to Mud Buttes and Pyramid Butte, John's Nose represents the third K-Pg section in southwestern North Dakota that preserves direct evidence of the Chicxulub asteroid impact, allowing for direct comparison over the timing and trends of the palynological record in respect to this event. The palynological analysis of John's Nose section reveals the presence of 68 pollen and spore taxa. Immediately above the boundary clay, a high abundance of fern spores of the genera Cyathidites and Laevigatosporites is recorded (with 59% of the assemblage being represented by Cyathidites). This very distinctive K-Pg 'fern spike' event is correlated with the devastation of land plants immediately following the asteroid impact and matches the composition generally reported from other sites in southwestern North Dakota. Palynostratigraphy demonstrates that the placement of the K-Pg boundary based upon the identification of the Last Appearance Datum (LAD) of typical Maastrichtian taxa (K-taxa) may be misleading. The presence of occasional K-taxa up to a few meters above the boundary clay at John's Nose represents an important difference when compared to previous reports. In light of this observation, LADs should be used cautiously as the primary criteria to identify the boundary; some K-taxa may have a short-term presence in the earliest Paleogene, or be reworked. In the John's Nose section, major changes and extinction in the palynological record occur at the geochemical K-Pg boundary, indicating that a catastrophic turnover took place over a short time.

The Paleogene Order Taeniodonta Cope, 1876—peculiar heavy-bodied mammals, some with ever-growing cheek teeth—are grouped with the Late Cretaceous eutherian CimolestesMarsh, 1889, along with a host of other taxa in a superordinal group, the Cimolesta. Taeniodonts were thought to have arisen from Cimolestes indirectly, through Paleocene ProcerberusSloan and Van Valen, 1965. The recently described Paleocene AlveugenaEberle, 1999, until now known only from the upper dentition, has been put forth as a transitional form between cimolestids and taeniodonts on phylogenetic and biostratigraphic grounds. An older taeniodont, the Late Cretaceous SchowalteriaFox and Naylor, 2003, has since been described, complicating taeniodont origins. We describe here a lower jaw that we refer to Alveugena from the lower part of the Ludlow Member of the Fort Union Formation in North Dakota. The lower jaw comes from strata of early Early Paleocene age (Puercan 1 North American Land Mammal Age) ~8.5 m above a Cretaceous-Paleogene boundary, identified using palynological criteria. A cladistic analysis is here presented using new data on Schowalteria and Alveugena, added to that of Cimolestes, Procerberus formicarumSloan and Van Valen, 1965, P. grandisMiddleton and Dewar, 2004, and Onychodectes. This analysis revealed Alveugena as the sister taxon of the taeniodonts but with a closer relationship to Cimolestes than Procerberus, suggesting that taeniodonts evolved from a Cimolestes-like ancestor. We discuss the age relations of early taeniodonts and related taxa and propose a scenario of ancestor-descendent relations that minimizes, but does not eliminate, implied stratigraphic gaps.

During the first 10–20 Kyr of the Eocene temperatures warmed by 4–8°C in middle and high latitudes, then cooled again over the succeeding ∼200 Kyr. Major changes in the composition of marine and terrestrial faunas, including one of the largest mammalian turnover events of the Cenozoic, occurred during this temperature excursion. To better understand the effects of rapid climatic change on continental biotas, we studied 60 fossil pollen samples collected from 900 m of section spanning approximately three million years of the late Paleocene and early Eocene; the samples come from the Fort Union Formation and Willwood Formation in the Bighorn Basin of northwestern Wyoming, paleolatitude approximately 47°N. There are 40 samples from the 500 m of rock deposited during the one million year interval centered on the Paleocene/Eocene boundary, although pollen was not preserved well in rocks representing the short warm interval at the base of the Eocene. Overall, the palynoflora shows moderate change in composition and diversity. Two pollen taxa clearly expanded their ranges to include North America in the first 400 Kyr of the Eocene, Platycarya (Juglandaceae), and Intratriporopollenites instructus (cf. Tilia), but they account for less than 5% of pollen grains in the early Eocene. There are no last appearances of common taxa associated with the Paleocene/Eocene boundary. The most noticeable palynological changes are the decrease in abundance of Caryapollenites spp. and Polyatriopollenites vermontensis (Juglandaceae), and the increase in abundance of Taxodiaceae (bald cypress family), Ulmaceae (elm family), and Betulaceae (birch family), particularly Alnipollenites spp. (alder). There are 22% more species in the Eocene samples than in the Paleocene samples; mean richness of Eocene samples is 17% higher than the mean of Paleocene samples. The mean evenness of Eocene samples is higher than that of Paleocene samples, but the difference is not significant. The modest level of floral change during the late Paleocene and early Eocene contrasts with the major taxonomic turnover and ecological rearrangement of North American mammalian faunas observed at the same time. Faunal change probably resulted from intercontinental range expansion across Arctic land bridges that became habitable as a result of high-latitude warming, so it is surprising that climatically sensitive plants did not also experience a major episode of interchange. The absence of fossil plants from the temperature excursion interval itself could prevent us from recognizing a transient shift in floral composition, but it is clear that the flora did not undergo a major and permanent restructuring like that seen in the mammals. The contrast between the moderate floral response to warming and the strong faunal response is consistent with the idea that interactions between immigrant and native taxa, rather than climate directly, were the primary cause of terrestrial biotic change across the Paleocene/Eocene boundary.

A substantial complication to using the oxygen isotope composition (δ18O) of vertebrate bioapatite in paleoclimate studies is the need to distinguish variation due to temporal changes in the δ18O of surface waters from that due to temperature-dependent fractionation during biomineralization. One solution is multiple-taxon comparisons using data from coexisting homeothermic and heterothermic animals. Fossil emydid turtles have been suggested to be potentially useful as functional homeotherms because (1) modern emydids employ behaviors, such as basking, to restrict skeletal growth to a narrow temperature range; (2) their aquatic habitat constrains the isotopic variability of dietary inputs; and (3) emydids have a dense fossil record. But because turtles lack teeth and therefore tooth enamel, sampling must focus on bone, which is potentially more susceptible to diagenetic alteration. This study examines the δ18O of carbonate (δ18Oc) and phosphate (δ18Op) in hydroxylapatite from co-occurring emydids and heterotherms (crocodilians and gars) from the Paleocene–Eocene of the Clarks Fork Basin, Wyoming. Previous isotopic studies of this area provide an extensive data set for comparison with the results of this study. Bone and enamel δ18Oc values measured here exhibit a greater range (16‰–32‰ Vienna Standard Mean Ocean Water) than previously observed, suggesting alteration, while the range of δ18Op values (9‰–15‰) is within that predicted by presumably unaltered mammalian tooth enamel δ18Oc. While high crystallinity indices (0.28–0.55) and a lack of covariation between δ18Oc and δ18Op suggest alteration of one or both of these constituents, a strong correlation between crocodilian enamel and bone δ18Op suggests bone phosphate may be reliable.1

A pollen and spore assemblage of 50 species was recovered from the late Paleocene (pollen zone P5) Almont locality in the Williston Basin, central North Dakota, USA. This palynoflora was extracted from the same layer containing a diverse megaflora preserved in a silicified shale with compressed leaves, and anatomically preserved fruits and seeds. More than 44 megafossil genera assignable to 26 extant plant families thus far have been recognized. The palynomorphs, which are of exceptional preservation, were examined using the same-grain technique with both light microscopy (LM) and scanning electron microscopy (SEM). Additional LM and SEM studies augmented the same grain studies to provide an understanding of sculptural features, and additional, rare taxa. Of particular note are the in situ pollen types known from catkins and pollen cones, allowing for confirmation of the dispersed pollen's systematic position by tying it to its parent plant. Taxa for which in situ pollen is known from Almont include taxodiaceous conifers, Betulaceae, Hamamelidaceae, Juglandaceae, and Platanaceae, and several catkins of uncertain affinities, some with monosulcate grains. This study emphasizes the role of palynology in providing an expanded view of the flora from palynomorphs for comparison with a rich megafossil assemblage.