Faulting at the well-known Madrid East K/Pg boundary locality in the Raton Basin of southeastern Colorado, U.S.A., recently has been interpreted to be the result of remote triggering by the Chicxulub impact in the Yucátan Peninsula, Mexico. However, a similar fault at the neighboring, correlative Madrid West locality recently has been attributed to overloading by a rapidly or catastrophically deposited crevasse splay in a sedimentological study. Therefore, this investigation seeks to provide additional insight on this problem. By focusing primarily on the timing of faulting and deposition of the crevasse splay at Madrid East and neighboring localities, this study presents evidence that faulting at Madrid East and neighboring localities post-dated the K/Pg boundary.

Fagaceae is an iconic plant family with 1000+ species distributed predominantly in the northern hemisphere. The reproductive ecology of the Fagaceae species is highly complex, and in the past three centuries, understanding the fruit masting strategies, dispersal, pathogen pressure, and storage has enjoyed steady research interest. However, the interrelationship between these factors is surprisingly less explored, undermining how acorns survive the post-dispersal period. The ability of fruits (acorns, nuts) to survive post-dispersal conditions starts during the maturation drying stage. Successful fruit production depends on the maternal environment, with a ‘good fruit production year’ occurring every few years. This phenomenon might out-maneuver predators by providing sufficient numbers to be consumed but still leaving enough for germination. Because most Fagaceae species have desiccation-sensitive (DS) fruits, survival after dispersal is challenging due to frequent dry spells. Fruit size is not phylogenetically conserved within the genus, with the average fruit mass of Castanea being 50 times higher than the desiccation-tolerant genus Fagus. Larger fruit size may be an adaptation in dry areas because prolonged drying continuously desiccates large-sized cotyledons before desiccating embryos. For species adapted to dry environments, synchronizing dispersal with the growing season could be beneficial, but exceptions exist. Many consumers, such as rodents, aid in the dispersal of fruits, but some fruits are killed in the process, especially those predated late. Upon settling on a site, the pericarp protects the embryo to a certain extent; if drying occurs, the cotyledons become the first site of water loss. However, under extreme drying, survival depends on sporadic rainfall, i.e., wet-dry cycle, which aids in maintaining the critical moisture content required by the fruits for survival. Nonetheless, these adaptive mechanisms are challenged by climate change, which affects the maturation, persistence, and seedling establishment of numerous Fagaceae species.

Physical dormancy (PY) due to a water-impermeable seed/fruit coat is one of the characteristic features of many species of Fabaceae; however, the timing and context of the evolution of this trait are poorly understood. In this investigation, fossil and molecular data are used to constrain the timing of the evolution of PY. The phylogenetic reconstruction programs GB-to-TNT and BEAUTi/BEAST are used to create chloroplast gene-based (rbcL and matK) phylogenies of taxa with well-represented fossil records. PY and non-dormancy are mapped to the terminals of the phylogeny, and ancestral states are reconstructed using parsimony. The initial evolution of PY in Fabaceae is reconstructed to have occurred sometime in the interval between divergence from Polygalaceae (late Campanian) to the diversification of crown-group Fabaceae (late Paleocene) when Fabaceae is known to have undergone multiple whole genome duplication (WGD) events across the Cretaceous/Paleogene (K/Pg) boundary. As in Nelumbo, another taxon with PY, Fabaceae may have developed PY in association with climatic change and WGD across the K/Pg boundary. The evolution of PY in association with WGD at the K/Pg boundary is an intriguing hypothesis that requires further investigation.

Premise of research. From a purely palynological perspective, lowest Danian fern-dominated assemblages of the Williston Basin (North Dakota and southwestern Saskatchewan) and the Raton Basin (northeastern New Mexico and southeastern Colorado) appear indistinguishable, although this perspective has yet to be corroborated by macrofloral studies across this same region. No ferns have been collected from the lowest Danian barren series of the Raton Formation in more than a century, and this section remains sparsely sampled. Methodology. New collections of fossil ferns from the basal portion of the barren series or informal middle member of the Raton Formation, which directly overlies the K/Pg boundary, are compared with contemporaneous assemblages in other North American depositional basins. Pivotal results. Dennastra sorimarginata McIver and Basinger (Dennstaedtiaceae Lotsy) and Woodwardia arctica (Heer) Brown (Blechnaceae Newman) that produce Cyathidites diaphana (Wilson and Webster) Nichols and JL Brown and Laevigatosporites haardtii (Potonié and Venitz) Thomson and Pflug, respectively, are recognized as characteristic ferns in the basal portion of the barren series, in addition to Osmunda macrophylla Penhallow. Conclusions. In accordance with palynological perspectives, lowest Danian fern macrofossil assemblages from the Raton Basin support the concept of a geographically widespread “postdisaster” pteridophyte flora represented predominantly by Dennstaedtiaceae and Blechnaceae, with other minor components of more northern assemblages. The enigmatic hypothesis of a landscape dominated by anemiaceous or schizaeaceous ferns is rejected on the basis of this new evidence, which suggests that Cyathidites diaphana–producing dennstaedtiaceous ferns were the K/Pg boundary “disaster taxon” and among those that flourished in the Raton Basin.

Researchers at Tanis, North Dakota, U.S.A., cited faulting associated with soft sediment deformation of the K/Pg boundary clay at Madrid East in the Raton Basin, U.S.A., as additional evidence of "more-instantaneous effects" that preceded the longer-term, climatic effects of the Chicxulub impact. However, Madrid East is among numerous localities where the initial phase of the K/Pg boundary fern spike initiated within rather than above the K/Pg boundary clay crosscut by this fault. Accordingly, this phase of the K/Pg boundary fern spike must have preceded the cessation of seismic aftershocks of the Chicxulub impact—a conclusion also reached by researchers at the deep marine depositional setting of Gorgonilla Island in Colombia, South America. The observation that the Chicxulub impact winter was shorter in duration than post-impact seismicity need not signify a major dilemma for contemporary climate models of the Chicxulub impact, a dilemma originally exacerbated by the common perception that this phase of the fern spike was dominated by tropical tree ferns. Because tree ferns have exceptionally long generation times (ca. 100 y according to contemporary estimates), they could not have had ample time to reach sporing stage if they were killed back to rhizomes or other belowground reserves by a complete shutdown of photosynthesis and subsequent impact winter. Alternatively, the dennstaedtiaceous affinity of spores at Madrid East suggests that these ferns had a generation time of only 2–4 y, which better fits contemporary estimates on how long it took for the effects of Chicxulub to wind down.

Paleobotanists debate whether the Cretaceous/Paleogene boundary (KPB) event was selective. As the hypothesis that the KPB event selected for plants with fast-return leaf economic traits (e.g. deciduousness) has lost empirical support in recent investigations, researchers have turned to alternative hypotheses to explain an abrupt decline in primary productivity across the KPB. Two contemporary hypotheses designed to explain selectivity among plants across the KPB are that (1) polyploids exhibited greater survivorship than their diploid progenitors or counterparts (i.e. the KPB-whole genome duplication or WGD hypothesis) and that (2) plants with desiccation-tolerant (DT), i.e. orthodox, seeds exhibited greater survivorship than plants with desiccationsensitive (DS), also known as recalcitrant, seeds. Late embryogenesis abundant (LEA) protein gene families are perceived to confer DT and seed longevity among vascular plants. Non-parametric Wilcoxon signed-rank test for matched pairs and a Mann-Whitney U test reveal that plant lineages perceived to have undergone WGD across the KPB exhibit significantly greater numbers of LEA genes than those that did not. On the basis of these data, this investigation elicits a merger between the KPB-WGD and KPB-seed traits concepts. However, emphasis is shifted from the concept of WGD as an immediate adaptation to climatic stress at the KPB (the KPB-WGD hypothesis) to the concept that WGD was an exaptation, which, by definition, fortuitously enhanced the survival of vascular plants across the KPB but that probably evolved initially in other climatic contexts.

ABSTRACT The Cretaceous/Paleogene (K/Pg) boundary Classopollis pollen “spike” in Patagonia, Argentina, is viewed as a singular event with no coeval analog anywhere else in the world. Review of global palynological records, however, reveals that similar K/Pg boundary Classopollis spikes involving monotaxial C. classoides (Pflug) Pocock and Jansonius assemblages have previously been reported from the Colorado Basin, Argentina, as well as the Tarim Basin, China, suggesting that this was a global phenomenon. The presence of this morphotaxon in the Danian strata of western North America has previously been interpreted as evidence of reworking from older, pre-Turonian (i.e., Triassic through Early Cretaceous) sediments during the Laramide orogeny. This hypothesis appears supported by the results of contemporary detrital zircon studies coupled with the physical degradation of pollen. However, the interpretation that all Danian examples of this pollen, particularly those from coal deposits, have been reworked from much older sediments is questioned on the basis of this review. Within this context, assorted hypotheses regarding the enigmatic coup de grâce of Classopollis-producing Cheirolepidiaceae are also considered, particularly the hypotheses that the ecologic distribution of Cheirolepidiaceae retracted to include xeric, upland habitats (e.g., the Rocky Mountains) or brackish-water, physiologically dry habitats (e.g., the margin of the Cannonball Sea) during the Late Cretaceous and Danian.

The age of the Rugubivesiculites Zone of the lower Kayan Sandstone in western Sarawak, Borneo, is disputed. Detrital zircon (U–Pb) geochronology suggests a maximum depositional age (MDA) of ca. 72–71 Ma; however, palynology suggests a Paleocene age. The overlying Proxapertites Zone, for example, encompasses both the upper part of the Kayan Sandstone (non-marine) and the un-deformed Engkilili Formation (marine), which is known to post-date deformation of the Lubok Antu Mèlange ca. 60 Ma. Among the palynological concepts that led Muller to originally assign a Cretaceous age to the Rugubivesiculates Zone was the abundance of well-preserved, non-reworked Classopollis pollen, which was perceived at the time to have gone extinct prior to the end of the Cretaceous. Contemporary palynological perspectives recognize a "spike" or abundance anomaly in Classopollis pollen in Danian deposits in both the Northern and Southern Hemispheres. Therefore, Muller's original dataset are statistically reevaluated for evidence of a Classopollis spike. This study finds strong evidence for an increase in Classopollis pollen above background levels within the lower part of the Kayan Sandstone (p < .001–0.002), representing a highly significant, stratigraphically sharp increase in the proportion of these pollen (p < .00001), consistent with the interpretation that these deposits are Danian in age.

There is no consensus on the cause of decline in conifer diversity across the K/Pg boundary, particularly in relation to the evolution and modernization of the tropical rainforest biome and the global rise of angiosperm diversity. In order to elucidate this issue, the pattern of conifer turnover across the K/Pg boundary in the Raton and Denver basins is revisited in light of additional fieldwork and improved chronostratigraphy. Turnover between the late Maastrichtian tropical evergreen climax conifer assemblage and the early Paleocene pioneer swamp cypress assemblage was stratigraphically abrupt, occurring within +/- 1-2 m of the K/Pg boundary. As the swamp cypress Glyptostrobus is known to be intolerant of competition, its persistence through the P1 Zone (lowest Danian palynological zone, which represents the first-400 ky of the Danian) in relatively stable, peat-forming depositional settings in Colorado is interpreted as evidence of arrested succession associated with the extinction of the tropical evergreen climax community. Although this basic pattern has historically been attributed to selection for deciduousness across the K/Pg boundary, this investigation highlights the related fact that tropical evergreen araucarian conifers and other trees in tropical evergreen climax forests characteristically do not produce soil seed banks, particularly as the archaic climax conifers that not only crossed the boundary but which also thrived in tropical, drought-prone settings are known to exhibit seed dormancy (e.g., Cheirolepidiaceae). As these findings parallel those of recent paleobotanical investigations in South America (Colombia and Patagonia, Argentina), this fundamental turnover pattern is regarded to be a global phenomenon underpinned by a unifying causal factor that is, a relatively severe K/Pg asteroid impact winter with global effects. Other popular but more controversial turnover hypotheses, such as the concept that conifers and other gymnosperms exhibit lower propensity for polyploidization- driven diversification than angiosperms, are also critically reconsidered in light of new and emerging evidence.

Premise of research. Paleobiogeographers and pteridologists in particular debate the relative importance of vicariance, peripatry, and whole-genome duplication (WGD; i.e., sympatric speciation) in shaping biogeographic and macroevolutionary patterns. Contemporary debates focus on whether there is a general role for polyploidization in plant macroevolution and whether intercontinental disjunctions are caused primarily by vicariance or by peripatry. Continental collision associated with the Great Asiatic Floral Interchange in the Paleogene/Neogene is currently perceived to be among the most important factors responsible for the geographic radiation of stenochlaenoid ferns, although this hypothesis does not take into account molecular phylogenetic relationships and has not been rigorously tested using historical biogeographic approaches.Methodology. The paleobiogeographic radiation of stenochlaenoid ferns is investigated by constructing molecular phylogenies using chloroplast (rbcL, atpB, and rps4) genes in BEAUTi and BEAST, by subjecting this chloroplast gene-based phylogeny to model comparisons involving historical biogeographic techniques (i.e., BioGeoBEARS in RASP 4) to determine the best-fitting model, by estimating divergence times using a relaxed molecular clock with fossil constraints, and by evaluating these estimates in the context of previous studies.Pivotal results. BioGeoBEARS reveals that these data fit the DIVA-like + J (i.e., peripatry-accommodating) model significantly better than other, exclusively vicariant models. The initial diversification of the rain forest-inhabiting stenochlaenoid fern crown group is dated to the Paleocene-Eocene thermal maximum, with subsequent diversification of Stenochlaena from Telmatoblechnum, presumably accompanied by WGD, in the mid-/late Eocene. Stem group stenochlaenoid ferns first appeared near the Cretaceous/Paleogene (K/Pg) boundary.Conclusions. Contrary to conventional views in paleobiogeography, WGD and peripatry appear to be of primary importance for understanding the biogeographic radiation of stenochlaenoid ferns. WGD appears disassociated from the K/Pg event and is placed instead in the global climatic context of the expansion of the tropical rain forest biome in the mid-/late Eocene.

The Cretaceous/Paleogene (K/Pg) boundary fern-spore spike concept was first introduced by R.H. Tschudy 40 years ago and established the precept that ferns are so-called 'disaster taxa' that flourish after natural disasters because of the high colonization potential of their wind-blown spores. Among the least understood topics at the crux of contemporary K/Pg boundary fern-spike studies is whether Stenochlaena-like or other stem blechnaceous ferns were among those that flourished after the K/Pg event, contributing to the Laevigatosporites-dominated phase of the fern-spore spike. For instance, dispersed Stenochlaena-like spores - e.g. Polypodiisporites usmensis (van der Hammen) Khan & Martin first appear in the upper Eocene stratigraphic record of South America, whereas megafossils of Stenochlaena-like ferns are first known from the upper Paleocene strata of this same region. Beyond this traditional focus on taphonomic and taxonomic uncertainty regarding the identity of dispersed spore producers, however, there is further discordance between contemporary paleobotanical and molecular phylogenetic perspectives on the timing of diversification of stem lineages of blechnaceous ferns in relation to the K/Pg boundary. This investigation reconciles these two perspectives by constraining molecular clock divergence time estimates using contemporary fossil data. If this reconciliation is correct, then paleopolyploidization (whole genome duplication or WGD) associated with the origin of the genus Stenochlaena J. Sm. predated evolution of polocytic stomata, a heavily ornamented exospore, and a hemi-epiphytic to epiphytic habit observed in the crown lineage. Critically, this novel perspective elicits a merger of the K/Pg boundary fern spike and K/Pg boundary-WGD concepts, suggesting a link between polyploidy and the 'disaster taxon' concept.

Disjunction between East Asian and eastern North American plants has been recognized since the time of Darwin. Although there is considerable evidence for congruent vicariance associated with late Neogene/Quaternary cooling among angiosperms, similar studies among specific fern families (e.g., Osmundaceae Martinov) have appeared incongruous with this pattern. Pteridologists continue to debate whether long-distance dispersal of wind-blown spores could have produced intercontinental disjunctions among ferns. To date, however, state-of-the-art historical biogeographical approaches have not been applied to this problem. In this investigation, multiple chloroplast gene (rbcL, atpA, atpB, and matK) sequences for ferns in the family Onocleaceae Pic. Serm. are drawn from GenBank, including those from the recently sequenced chloroplast genome of the East Asian fern O. sensibilis L. var. interrupta Maxim., and used to create molecular phylogenies using Bayesian (BEAUTi and BEAST) techniques. Using contemporary approaches for relaxed molecular clock divergence time estimation with fossil calibration, divergence time estimates for East Asian and eastern North American populations in the Onoclea sensibilis species complex are consistently reconstructed as the Pliocene (ca. 5 – 3.4 Ma), and the best-fitting historical biogeographic model is a DIVA-like (exclusively vicariant) model using BIOGEOBEARS in RASP4, with a low probability of peripatry. Accordingly, these molecular and fossil data appear congruent with the pattern observed among angiosperms, despite the propensity for long-distance dispersal of wind-blown spores in pteridophytes. The lack of evidence for peripatry in this lineage may be related to the presence of short-lived green spores in onocleoid ferns; however, this hypothesis requires further investigation.

Premise of research. A two-phase fern spike occurred immediately after the Cretaceous-Paleogene (K/Pg) mass extinction event. Solely on the basis of palynological evidence, researchers have traditionally attributed the first phase of this spike to the proliferation of a single species of a Cyathea-like fern in the earliest Danian. This traditional perspective is challenged by recent investigations linking Anemia-like fern foliage with Cyathidites spores at K/Pg boundary localities in the Raton Basin, where the fern spore spike was first discovered by R. H. Tschudy. Although evidence emerging from neighboring basins appears to corroborate this new perspective, it remains to be seen whether this generalization applies to the northern Great Plains. Methodology. A comprehensive list of the most common fern megafossils collected from earliest Danian plant localities across western North America was compiled to determine the number and potential identity of Cyathidites-producing ferns. Pivotal results. Three K/Pg survivors commonly collected from basal Paleocene strata in western North America conceivably produced psilate, trilete (Cyathidites) fern spores essentially identical to those observed at the K/Pg boundary fern spore spike: Anemia elongata (Newberry) Knowlton, Dennastra sorimarginata McIver et Basinger, and the Coniopteris-like fern "Dennstaedtia" americana Knowlton. Conclusions. Arborescent ferns were not among the first plants in western North America to thrive in the immediate wake of ecological collapse at the K/Pg boundary. Basal polypod ferns can be linked to Cyathidites spores in the northern Great Plains, perhaps because of a latitudinal climatic gradient. These results could explain taxonomic inversion in the dual-phase fern spore spike between western North America and New Zealand.

The Pierre Shale is a marine deposit that accumulated in the Cretaceous Western Interior Seaway during the onset of Laramide tectonism in the southern Rocky Mountains region. In the eastern part of the Raton Basin near Trinidad, Colorado, ammonite biostratigraphy suggests that the base of the Pierre Shale lies within or slightly above the Lower Campanian Scaphites hippocrepis III ammonite range zone (81.8–80.5 Ma), and the top of the Pierre Shale corresponds with the Lower Maastrichtian Baculites clinolobatus Zone (69.59 + 0.36 Ma). These data are consistent with previous estimates for the age of the base of the Pierre Shale in the Raton Basin, and indicate that the top of the Pierre Shale (base of the overlying Trinidad Sandstone) in the eastern part of the basin lies near the Lower-Upper Maastrichtian substage boundary. A Late Maastrichtian age for the Trinidad Sandstone near Trinidad has a bearing on the timing of geological events associated with the eastward retreat of the Western Interior Seaway from the region during the Late Cretaceous. Introduction The Pierre Shale was deposited within the Western Interior Seaway during an early phase of Laramide tectonism, which resulted in areas of regional subsidence and uplift associated with subduction of the Farallon tectonic plate beneath western North America (Baltz, 1965; Cather, 2004; Slattery et al., 2015; Heller and Liu, 2016). In the Raton Basin of northeastern New Mexico and south-central Colorado (Fig.1), the Pierre Shale is underlain by the Niobrara Formation and overlain by the Trinidad Sandstone (Lee, 1917). The age of the base of the Pierre Shale is constrained by ammonite biostratigraphy of the uppermost part of the Niobrara Formation in the Raton Basin and surrounding regions (Scott et al., 1986; Molenaar et al., 2002; Merewether et al., 2011). The age of the top of the Pierre Shale is similarly constrained by ammonite biostratigraphy of the upper part of the formation.

In past investigations the pattern of differential survival of plants across the K/Pg boundary has been viewed as incompatible with severe asteroid impact winter scenarios (i.e., an impact winter lasting more than a few months), particularly the enigmatic survival of coryphoid palms and Pandanus (screw pine). Stateof- the-art climate models based on soot, sulfate and nano-sized dust aerosols predict a global impact winter that drastically reduced precipitation and resulted in a transient period of total darkness and permafrost conditions. This suggests that the plants most likely to have been affected by the global mass-extinction event were tropical phanerophytes that produce recalcitrant seeds, which by definition are desiccation-intolerant, survive less than a year, and cannot survive freezing. However, this hypothesis has never been tested. In this study I sampled over 100 plant species from the global fossil record that have a high probability of having produced either recalcitrant seeds/disseminules (n1 = 58) or orthodox seeds (n2 = 59), based on their phylogenetic relationships with extant taxa that either are monomorphic for these traits or specifically exhibit a genetic marker for abscisic acid inhibition associated with seed dormancy and recalcitrance. A one-tailed z-test for the difference between two proportions revealed that plant taxa with a high probability of having produced recalcitrant seeds had significantly lower survivorship than plant taxa with a high probability of having produced orthodox seeds (p < 0.0001). Based on these data, it can be concluded that plants which formed a frost-tolerant seed bank during the latest Maastrichtian were significantly more likely to survive the K/Pg impact winter than plants which did not (including palms). These data clearly indicate that the K/Pg impact winter probably lasted longer than a year and that it selected for seed-based traits that effectively sorted correlated functional traits of mature plants (i.e., leaf physiognomic features). This novel hypothesis stands as an alternative to J.A. Wolfe’s classic hypothesis that a mild K/Pg impact winter selected for fast-growing angiosperms with deciduous leaves and did not affect the plant communities of the Southern Hemisphere. Potential mechanisms for the rare survival of tropical, recalcitrant-seeded plants are discussed.

Based on the results of leaf physiognomic studies, it is generally acknowledged that the Raton Basin in south-central Colorado and northeastern New Mexico hosted a tropical rainforest during the early Paleocene. However, this interpretation is problematic because the canopy-forming foliage of this forest was predominately lauraceous, and lauraceous foliage is almost invariably entire-margined. Thus, it may be necessary to look to other, taxonomy-based paleotemperature proxies to test megathermal paleotemperature estimates based solely on leaf physiognomic analyses of predominately lauraceous datasets. In this study, the pantropical fern Cyclosorus sensu lato (s.l.) is reported from the upper coal zone of the Raton Formation. The presence of this fern in the understory of a predominately lauraceous canopy suggests megathermal conditions in the early Paleocene.

The presence of the amino acid α-aminoisobutyric acid (Aib) within Cretaceous/Paleogene (K/Pg) boundary clay in the Raton and Powder River basins in Colorado and Wyoming, respectively, has been described as compelling evidence that extraterrestrial Aib survived the high-energy Chicxulub impact. Based on contemporary experiments and simulations, however, it is highly unlikely that extraterrestrial Aib survived the impact, which had peak impact pressures and temperatures in excess of 600 GPa and 10,000 K, respectively. In other words, the amino acid signature of the carbonaceous chondritic asteroid that impacted Chicxulub was undoubtedly destroyed upon impact during formation of the vapor plume or so-called “fireball.” The only organisms known to produce Aib are the suite (more than 30 genera) of cosmopolitan saprotrophic filamentous fungi that include Trichoderma Pers., which has recently been hypothesized to have thrived during the K/Pg mass-extinction event. Therefore it is proposed that the Aib horizon in the K/Pg boundary clay in the Raton and Powder River basins correlates with the K/Pg boundary fungal spike, which thus far has only been observed in New Zealand (Southern Hemisphere). This proposition is based upon superimposing the Aib horizon on the well-known iridium and fern-spore spikes, as its stratigraphic position precisely matches that predicted by the fungal spike. If correct, this hypothesis alters the conventional perspective on the tempo and mode of terrestrial ecosystem recovery in western North America, as the heavily sampled K/Pg boundary section in the Raton Basin was instrumental in shaping the traditional narrative of the rapid recolonization of a denuded landscape by ferns via wind-blown spores in the immediate wake of regional deforestation caused by the K/Pg impact event. Perhaps more importantly, it could present an alternative to traditional palynological approaches for locating the fungal spike in other terrestrial K/Pg boundary sections and could provide additional support for the generalization that global mass-extinction events are frequently accompanied by fungal spikes.

Abstract The Cretaceous-Paleogene (K-Pg) boundary Chicxulub impact is supposed to have produced a nearly decade-long impact winter which resulted in a mass-extinction event among dicot angiosperms but which left pteridophytes comparatively unaffected. Dicot angiosperms subsequently recovered from the soil seed bank following an episode of global deforestation, although this recovery took centuries. Pteridophytes, on the other hand, are supposed to have recovered within months of the impact event, due to the characteristic, short-term viability of fern spores in the soil bank – an interpretation consistent with the assumption that the dominant fern spore at the K-Pg boundary fern spore spike, Cyathidites Couper, was produced by cyatheaceous foliage. At the K-Pg boundary section near Sugarite, New Mexico, however, Cyathidites spores are more likely to have been produced by schizaeaceous foliage, which produces spores capable of germinating after spending about a decade or more in the soil and which already commanded similar depositional settings in western North America during the Maastrichtian. Therefore, the protracted – millennial – timescale for fern dominance in the earliest Danian could be related to the unique ecology of schizaeaceous ferns that recovered from a persistent spore bank in a habitat that they already dominated, presumably by suppressing the colonization of angiosperms.

At Berwind Canyon, northeast of Trinidad, Colorado, the upper 100 m of the Pierre Shale yield the most extensive upper Campanian-lower Maastrichtian record of fossil mollusks (primarily bivalves and ammonites) in the Colorado portion of the Raton Basin. This record includes the zonal baculitid ammonites (from oldest to youngest) Baculites compressus (Say, 1820), B. cuneatus Cobban, 1962, B. reesidei Elias, 1933, B. jenseni Cobban, 1962, B. eliasi Cobban, 1958, B. baculus Meek and Hayden, 1861, B. grandis Hall and Meek, 1855 and B. clinolobatus Elias, 1933, as well as other index mollusks (e.g., scaphitid ammonites, gastropods, and inoceramids). Thus, we identify eight ammonite zones in the condensed section of the Pierre Shale in Berwind Canyon, which record an important phase in the tectonic evolution of the basin. 81 New Mexico Geological Society Guidebook, 70th Field Conference, Geology of the Raton-Clayton Area, 2019, p. 81-88.

New research reveals that ecologic modernization of the thelypteridaceous ferns probably occurred by the Paleocene, although it is not yet clear what role the K-Pg boundary event had in shaping this contemporary pattern. Goniopteroid venation where adjacent proximal (basal) lateral veinlets are either connivent at the sinus or anastomose below the sinus to form an excurrent veinlet that runs to the sinus margin is diagnostic of the thelypteridaceous genus Cyclosorus LINK sensu lato (s.l.) (i.e., cyclosoroid ferns). Today, members of the cyclosoroid ferns (Cyclosorus and related genera) are pantropical to pan-subtropical in distribution, whereas less derived thelypteridaceous ferns are generally found in higher-latitude settings. Globally, the oldest fossils of Cyclosorus s.l. are presently known only from the late Paleocene and Eocene (i.e., near the Paleocene-Eocene thermal maximum or PETM). Taxonomic revision coupled with the discovery of new fossils clearly reveal that the fossil record of this form genus extends into the late Maastrichtian-early Paleocene (K-Pg) strata in central Colorado, making these the oldest known representatives of this group in the world. These cyclosoroid fern fossils are only distantly related to Speirseopteris STOCKEY, LANTZ, et ROTHWELL from upper Paleocene strata in the northern Western Interior (Canada). Accordingly, the distribution of thelypteridaceous ferns during the Paleocene appears to parallel their modern distribution. The hypotheses that cyclosoroid ferns proliferated in central Colorado across the K-Pg boundary because of their characteristic ability to form a persistent spore bank or because of fundamental changes in canopy cover are reviewed.