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Fox Hills I, a new Upper Maastrichtian megafloral zone within the Williston Basin of North Dakota /
Abstract
Thesis (B.S. Geology with Honors)--St. Lawrence University, 2003. Includes bibliographical references (leaves 107-115).
... The presence of Taxodium-related vegetation is further corroborated by e.g. macrofossils of Taxodium present in upper Maastrichtian deposits of the Fox Hill Formation, North Dakota (Peppe, 2003) and in Canadian deposits of the Edmonton Fm./Horseshoe Canyon Fm., pollen referable to those of Glyptostrobus, Sequoia and Taxodium have been reported (Srivastava, 1970) together with fossil taxodiaceous wood (Ramanujam and Stewart, 1969). ...
... Although bennettites have been encountered in the Maastrichtian of western North America (Stockey and Rothwell, 2003), they are generally sparse by the close of the Cretaceous and cycads are the most likely source of such pollen (Nichols and Johnson, 2008). This is further supported by results by Peppe (2003) who states that macrofossils of the cycad Nilssonia are strikingly common in the upper Maastrichtian Fox Hill Formation. Uncommon gymnosperm pollen taxa in the assemblage include Classopollis spp. ...
... Just below, or perhaps partially lateral to, that indurated unit are siltstone facies containing a diverse flora similar to that described as the zonal flora Hell Creek III by Johnson (2002) in Bowman County, North Dakota. This flora (Peppe, 2003) served as additional evidence of the diachronous relationship between deposition on the Cedar Creek Anticline and that in the Missouri River Valley. Furthermore, a study of pollen from the plant-rich bed indicated early-middle Late Maastrichtian palynomorphs of the Wodehousia spinata pollen zone, including W. spinata and Tsudypollis spp. . ...
ABSTRACT
As part of a study of the Vertebrata found in the Late Cretaceous (Early Maastrichtian to Middle Late Maastrichtian) Fox Hills Formation, 48 sites in western and central North Dakota were collected to interpret the chondrichthyan and osteichthyan paleofaunas. Based mostly on teeth, 19 shark species, 16 skate and ray species, and one ratfish species were recognized. Of those, three taxa are new, including Cretalamna feldmanni n. sp., “Myliobatis” foxhillsensis n. sp., and Dasyatis northdakotaensis n. sp. New chondrichthyan species occurrences for the Fox Hills Formation include: Squalus ballingsloevensis, Plicatoscyllium derameei, Cretorectolobus olsoni, Carcharias cf. C. tenuiplicatus, Cretalamna feldmanni n. sp., Paranomotodon toddi, Squalicorax pristodontus, Palaeogaleus navarroensis, Archaeotriakis rochelleae, Paraorthacodus andersoni, Synechodus turneri, Walteraja exigua, Dasyatis northdakotaensis n. sp., Rhombodus levis, “Myliobatis” foxhillsensis n. sp., and morphotypes of placoid scales and dermal denticles.
Twenty species of bony fishes were identified from teeth and other skeletal parts, two were vertebral morphospecies, two were based on scales, and four were recognized from otoliths. New osteichthyan occurrences in the Fox Hills Formation include: a lepisosteid, Melvius sp., Cyclurus fragosus, Protosphyraena sp., Belonostomus longirostris, Xiphactinus vetus, Paratarpon? sp., Pollerspoeckia siegsdorfensis, cf. Bathylagus sp., Enchodus cf. E. ferox, and “Apogonidarum” maastrichtiensis.
The Fox Hills Formation is Early Maastrichtian in Bowman County, southwestern North Dakota. The Bowman County sites yielded the oldest fossils of this study. Sites in the Fox Hills type area in north-central South Dakota and south-central North Dakota are Middle Late Maastrichtian based on the presence of Hoploscaphites nicolletii and Hoploscaphites nebrascensis Ammonite Zones and the Wodehousia spinata Pollen Zone. Age relationships of these fossil sites suggest temporal range extensions for several of the Fox Hills fish taxa.
Fox Hills fishes were derived from deep and shallow marine, brackish, and freshwater habitats. Five groupings were identified based on qualitative assessment of these habitat preferences. These groupings are: “offshore marine,” “nearshore marine,” “brackish water/estuarine–strong tidal influence,” “brackish water/estuarine–weak tidal influence,” and “riverine/lagoonal–strong freshwater influence.” Tooth morphology and comparison to modern analogs indicate presence of the following feeding types: omnivore, general invertebrate, molluscivore, pelagic piscivore, benthic piscivore, and scavenger. Species representing all feeding types occur in each of the five habitat groupings. Feeding competition was thus partitioned by habitat preference. When coupled with paleogeographic distribution information, the Fox Hills fish fauna indicates that some taxa represent a recurring assemblage of species that have a “large-river delta” habitat preference, as found today on major deltas of most continents.
Paleogeographic conditions in the Western Interior Seaway (WIS) were dominated by the physiographic conditions of the Hell Creek Delta and Dakota Isthmus complex, which is composed of lagoons, estuaries, and barrier island shorelines. The Fox Hills fish paleofauna includes taxa restricted to the WIS and those that also occurred in the Texas Gulf Coast, Mississippi Embayment, Atlantic Coastal Plain, Greenland, and Sweden. Pelagic, deep marine lamniform species were cosmopolitan and ranged to Europe
and North Africa. The Fox Hills fish fauna is most similar to the fish faunas of the Maastrichtian Kemp Formation, Texas, Severn Formation, Maryland, and Navesink and New Egypt formations, New Jersey.
The Fox Hills paleofauna documents fish extinction at the close of the Cretaceous. None of the 35 chondrichthyan species and none of the 20 osteichthyan species recovered from the Fox Hills Formation are found in the Paleocene worldwide. 58% of Fox Hills chondrichthyan and 77% of osteichthyan genera, and 20% of chondrichthyan and 33% of osteichthyan families, did
not survive after the Cretaceous. Support for this interpretation is provided by comparison of the Fox Hills paleofauna to the Paleocene Cannonball Formation paleofauna in North Dakota. None of the 13 Cannonball chondrichthyan species, nor any of
the four Cannonball osteichthyan species, occur in the Fox Hills Formation. Thirteen chondrichthyan genera (Squatina, Squalus, Ginglymostoma, Carcharias, Odontaspis, Cretalamna, Palaeogaleus, Galeorhinus, Paraothacodus, Synechodus, Myliobatis, Dasyatis, and Ischyodus) range across the K-Pg boundary.
... The K/Pg boundary, and the latest Cretaceous and earliest Paleocene floras have been studied extensively in the Williston Basin (e.g., Berry, 1934;Brown, 1939Brown, ,1962Shoemaker, 1966;Williams, 1988;Johnson, 1989;Johnson et al., 1989;Johnson and Hickey, 1990;Johnson, 1992Johnson, ,1996Johnson, , 2002Peppe, 2003;Wilf et al., 2003;Wilf and Johnson, 2004;Smrecak, 2006;Wilf et al., 2006;Peppe et al., 2007). These studies have shown a marked difference between Cretaceous and Paleocene floras. ...
... The Fox Hills flora is from the Linton Member of the Fox Hills Formation and is late Maastrichtian in age (c. 66 Ma; Peppe, 2003;Peppe et al., 2007; Table 1). We grouped these taxa by floral zone following Peppe (2009Peppe ( , 2010. ...
• Paleobotanists have long used models based on leaf size and shape to reconstruct paleoclimate. However, most models incorporate a single variable or use traits that are not physiologically or functionally linked to climate, limiting their predictive power. Further, they often underestimate paleotemperature relative to other proxies. • Here we quantify leaf-climate correlations from 92 globally distributed, climatically diverse sites, and explore potential confounding factors. Multiple linear regression models for mean annual temperature (MAT) and mean annual precipitation (MAP) are developed and applied to nine well-studied fossil floras. • We find that leaves in cold climates typically have larger, more numerous teeth, and are more highly dissected. Leaf habit (deciduous vs evergreen), local water availability, and phylogenetic history all affect these relationships. Leaves in wet climates are larger and have fewer, smaller teeth. Our multivariate MAT and MAP models offer moderate improvements in precision over univariate approaches (± 4.0 vs 4.8°C for MAT) and strong improvements in accuracy. For example, our provisional MAT estimates for most North American fossil floras are considerably warmer and in better agreement with independent paleoclimate evidence. • Our study demonstrates that the inclusion of additional leaf traits that are functionally linked to climate improves paleoclimate reconstructions. This work also illustrates the need for better understanding of the impact of phylogeny and leaf habit on leaf-climate relationships.
Seven fossil leaf species are described from impression fossils collected from the Upper Cretaceous (Maastrichtian) Fox Hills Formation in south-central North Dakota, USA. They are Marmarthia johnsonii n. sp., Nilssoniocladus yukonensis n. comb., Nilssoniocladus comtula n. comb., Mesocyparis borealis, Rhamnus salicifolius, Paloreodoxites plicatus, and Zingiberopsis magnifolia. These species represent some of the elements of the Fox Hills flora that have paleogeographic ranges to the northwest (N. yukonensis, N. comtula, and M. borealis) and to the southwest (M. johnsonii, R. salicifolius, P. plicatus, and Z. magnifolia) of the Fox Hills type area. The identification and reappraisal of these species represent an effort to understand the biogeographic relationships of Late Cretaceous floras across the Northern Hemisphere.
THE rapid radiation of angiosperms during the Late Cretaceous has been thought to reflect their rise to vegetational dominance1-3. The number of species in a clade and its vegetational importance are not necessarily related, however. Quantitative studies of the recently discovered Big Cedar Ridge flora, found preserved in situ in a mid-Maastrichtian volcanic ash in central Wyoming, USA, reveal that dicotyledonous angiosperms accounted for 61% of the species but constituted just 12% of vegetational cover. Dicots, many of which appear to have been herbaceous, were abundant only in areas disturbed just before burial. By contrast, free-sporing plants were 19% of the species but 49% of cover. The only abundant and ubiquitous angiosperm was a single species of palm (about 25% of cover). A comparably low abundance of dicots was found in two other nearly contemporaneous floras buried by volcanic ash, whereas coeval floras from fluvial environments are dominated by dicots4. This shows that, even as late as the mid-Maastrichtian, in northern mid-latitudes there were areas away from streams that were not yet dominated by dicots. Despite vigorous taxonomic diversification during the previous 30 Myr3, dicots played a subordinate role in these areas of fern-dominated vegetation.
Estimates of past precipitation are of broad interest for many areas of inquiry, including reconstructions of past environments and topography, climate modeling, and ocean circulation studies. The shapes and sizes of living leaves are highly sensitive to moisture conditions, and assemblages of fossil leaves of flowering plants have great potential as paleoprecipitation indicators. Most quantitative estimates of paleoprecipitation have been based on a multivariate data set of morphological leaf characters measured from samples of living vegetation tied to climate stations. However, when tested on extant forests, this method has consistently overestimated precipitation. We present a simpler approach that uses only the mean leaf area of a vegetation sample as a predictor variable but incorporates a broad range of annual precipitation and geographic coverage into the predictor set. The significant relationship that results, in addition to having value for paleoclimatic reconstruction, refines understanding of the long-observed positive relationship between leaf area and precipitation. Seven precipitation estimates for the Eocene of the Western United States are revised as lower than previously published but remain far wetter than the same areas today. Abundant moisture may have been an important factor in maintaining warm, frost-free conditions in the Eocene because of the major role of water vapor in retaining and transporting atmospheric heat.
The warmest global temperatures of the Cenozoic Era occurred in early Eocene time, following a warming trend that started in late Paleocene time. The greater Green River Basin of southwestern Wyoming is one of the best areas in the Rocky Mountains for paleobotanical investigation of the Paleocene-Eocene climatic transition. Intensive sampling has resulted in the recovery of an estimated 189 species of plant macrofossils from the Tiffanian, Clarkforkian, Wasatchian, and Bridgerian land mammal "ages." The leaf morphologies and taxonomic affinities of these fossils were used in combination with other indicators to evaluate Paleocene-Eocene climates. Following cool humid conditions in the Tiffanian, the Clarkforkian was humid and subtropical, and several plant families with modern tropical affinities appeared. However, as in the Tiffanian, Clarkforkian floras had low diversity and were dominated by a single species in the birch family. Mean annual temperature (MAT) rose from an estimated 12 °C in the Tiffanian to 19 °C in the Clarkforkian, while mean annual precipitation (MAP) for the Tiffanian and Clarkforkian is estimated to have been 130-150 cm. Little fossil plant material is preserved from the latest Clarkforkian or the earliest Wasatchian, which is thought to have contained an interval of cooling and drying followed by renewed warming. By the middle Wasatchian, the time of the Cenozoic thermal maximum, the inferred MAT was about 21 °C, and the MAP was near 140 cm. A second influx of plant families with tropical affinities appeared in the area, and diversity increased significantly, but most plant families known from the Clarkforkian persisted. Species turnover from the Clarkforkian to the Wasatchian was greater than 80%. A second turnover of more than 80% of species (but not families) from the Wasatchian to the early Bridgerian accompanied drying and increased seasonality of precipitation. The early Bridgerian MAT is inferred to have been near 20 °C and the MAP to have been about 80 cm. Except for the Tiffanian and possibly portions of the early Wasatchian, paleoclimates during the study interval were predominantly frost free. Although the moderating influence of the Green River lake system has been suggested as a possible explanation for mild Eocene winters in Wyoming, this study shows that virtually frost-free climates existed in the area prior to and independent of significant lake development.
Abstract--The latest Maestrichtian marine deposition in the Western Interior is represented by the Fox Hills Formation in North and South Dakota, which contains a relatively large, nearshore molluscan fauna. Restudy of the Gastropoda from the Fox Hills Formation in North Dakota indicates a fauna of at least thirty-seven species in twenty-four genera and has introduced eleven new generic-level taxa to the record from the North Dakota Maestrichtian. These genera include Neritina, Syncera, Cancellaria, Remera?, Amuletum, Semitriton, Hecorhyncus, Rhombopsis, Graphidula, Perissitys, and Goniocylichna, and are based both on reassignment of known faunal elements and discovery of previously unknown taxa. Paleozoogeographic distributions based upon the published literature indicate that the Fox Hills gastropod fauna had a strong contribution from Gulf Coast and Mississippi Embayment stock with a lesser contribution coming from faunal sources to the northwest. Species from Gulf Coast lineages show a marked reduction of shell ornament, a cold-water feature, indicating that by Fox Hills time in North Dakota the Maestrichtian regression was well in progress with seaway connections to the Gulf Coast already severed to produce a northward regression and to subject the area solely to the influence of cold-water marine conditions from the northwest. These cold-water conditions may reflect a latitudinal climatic gradient but it is equally likely that they were a response to marine current flow and thus may have been the cause of, rather than the result of, climatic effects as suggested by some workers and as postulated herein. The last southern connections of the seaway were to the southeast where two species, Euspira rectilabrum (Conrad), and Goniocylichna bisculpturata Wade, of the Ripley Formation occur in the Fox Hills Formation in North Dakota. Since migration directions are reversible, and because Maestrichtian faunas to the northwest are still poorly understood, it is recognized that G. bisculpturata may have had origins from either source.
Precise estimates of past temperatures are critical for understanding the evolution of organisms and the physical biosphere, and data from continental areas are an indispensable complement to the marine record of stable isotopes. Climate is considered to be a primary selective force on leaf morphology, and two widely used methods exist for estimating past mean annual temperatures from assemblages of fossil leaves. The first approach, Leaf Margin Analysis, is univariate, based on the positive correlation in modern forests between mean annual temperature and the proportion of species in a flora with untoothed leaf margins. The second approach, known as the Climate-Leaf Analysis Multivariate Program, is based on a modern data set that is multivariate. I argue here that the simpler, univariate approach will give paleotemperature estimates at least as precise as the multivariate method because (1) the temperature signal in the multivariate data set is dominated by the leaf-margin character; (2) the additional characters add minimal statistical precision and in practical use do not appear to improve the quality of the estimate; (3) the predictor samples in the univariate data set contain at least twice as many species as those in the multivariate data set; and (4) the presence of numerous sites in the multivariate data set that are both dry and extremely cold depresses temperature estimates for moist and nonfrigid paleofloras by about 2°C, unless the dry and cold sites are excluded from the predictor set. New data from Western Hemisphere forests are used to test the univariate and multivariate methods and to compare observed vs. predicted error distributions for temperature estimates as a function of species richness. Leaf Margin Analysis provides excellent estimates of mean annual temperature for nine floral samples. Estimated temperatures given by 16 floral subsamples are very close both to actual temperatures and to the estimates from the samples. Temperature estimates based on the multivariate data set for four of the subsamples were generally less accurate than the estimates from Leaf Margin Analysis. Leaf-margin data from 45 transect collections demonstrate that sampling of low-diversity floras at extremely local scales can result in biased leaf-margin percentages because species abundance patterns are uneven. For climate analysis, both modern and fossil floras should be sampled over an area sufficient to minimize this bias and to maximize recovered species richness within a given climate.
A classification of the architectural features of dicot leaves—i.e., the placement and form of those elements constituting the outward expression of leaf structure, including shape, marginal configuration, venation, and gland position—has been developed as the result of an extensive survey of both living and fossil leaves. This system partially incorporates modifications of two earlier classifications: that of Turrill for leaf shape and that of Von Ettingshausen for venation pattern. After categorization of such features as shape of the whole leaf and of the apex and base, leaves are separated into a number of classes depending on the course of their principal venation. Identification of order of venation, which is fundamental to the application of the classification, is determined by size of a vein at its point of origin and to a lesser extent by its behavior in relation to that of other orders. The classification concludes by describing features of the areoles, i.e., the smallest areas of leaf tissue surrounded by veins which form a contiguous field over most of the leaf. Because most taxa of dicots possess consistent patterns of leaf architecture, this rigorous method of describing the features of leaves is of immediate usefulness in both modern and fossil taxonomic studies. In addition, as a result of this method, it is anticipated that leaves will play an increasingly important part in phylogenetic and ecological studies.
Floral evolution in north eastern Russia during the Cretaceous period is analyzed and the regularities of florogenesis are related to coeval climates. The Neocomian to Aptian floras consist of ferns, cycadophytes, ginkgoaleans, czekanowskialeans and archaic conifers. Floral evolution is manifested in changes in the ratios of these group in relation to climatic changes and speciation. The first flowering plants appeared in the Buorkemussian Flora of the Albian. This flora is characterized also by the highest taxonomic diversity and in the culmination of all the Mesozoic plant groups. The early angiosperms were xeromorphic. They were possibly a minor component of the non-arboreal shrubby or herbaceous vegetation. A sharp cooling of the climate in the Late Albian resulted in extinction or considerable reduction of many fern and gymnosperm groups followed by a radiation of the modern conifers. The earliest angiosperm-dominated plant communities, some of them polydominant forest types, appeared during the Cenomanian time. Angiosperms came to dominance after the Late Cenomanian climatic optimum (The Grebenkian Flora). With the appearance of angiosperms the florogenetic processes intensified considerably. New ecosystem types appeared and both the taxonomic and biogeographic diversity increased. During the Late Cretaceous floral evolution the species content of successive floras was constantly replenished by 60-90% from age to age, whereas the composition of gymnosperm groups underwent minor changes at most. The most pronounced floral changes are related to a marked deterioration of climatic conditions that occurred the Middle/Late Albian, Cenomanian/Turonian and the Middle/Late Maastrichtian boundaries. These changes were accompanied by mass extinctions (mainly of the highly specialized dominant taxa of climax plant communities) and by a spread of low diversity community types dominated by ubiquitious species with broad geographic ranges. The high rate taxonomic diversification was confined to the period of optimal climatic conditions, especially when the latter followed mass extinctions.
The Breien Member of the Hell Creek Formation represents the last welldocumented advance of the Fox Hills Sea into the upper midcontinent during the Late Cretaceous (Maastrichtian). This member is lithologically variable and consists of burrowed, greenish-gray, glauconitic sandstone and mudstone or burrowed, interbedded sandstone and mudstone. Iron-oxide staining is usually present. Ichnofossils, most of which are Ophiomorpha, occur throughout the member and are often associated with Crassostrea subtrigonalis. The Breien Member is exposed in isolated ravine, road cut, and butte outcrops in south-central North Dakota. It ranges in thickness from 1.5 m in Emmons County to 8.5 m in Morton County. The Breien interfingers with nonmarine, dinosaur fossil-bearing Hell Creek deposits. Both upper and lower contacts of the Breien Member with nonmarine Hell Creek strata are lithologically variable and conformable. The base of the member is 1.5-9.0 m above the Fox Hills Formation and the top of the member is 46-61 m below the Hell Creek-Fort Union formational contact. Because of lithologic variability at the Breien Member's type locality and elsewhere in its outcrop area, three reference sections are designated to characterize the Breien. Fossils are scarce in the Breien Member but indicate deposition within the Upper Cretaceous Jeletzkytes nebrascensis Zone and a Lancian age. The Breien vertebrate fossil assemblage consists of terrestrial and marine taxa, including fish, crocodilians, dinosaurs, and mammals. Marine chondrichthyan teeth are the most abundant vertebrate fossils. Most of these taxa corroborate a Late Cretaceous age for the member and indicate that the Breien was deposited in relatively shallow water, nearshore, marine environments. This marine chondrichthyan fauna is similar to, but less diverse than, the Maastrichtian Fox Hills Formation fauna and exhibits no similarity to the early Paleocene Cannonball Member fauna. This finding lends support for a major end-Cretaceous extinction of marine chondrichthyan species.
Molluscan species associations reflecting varying salinities in the final North American interior seaway occur in Cretaceous to Paleocene strata. Deposits of the last few million years of the Cretaceous sea record an open marine environment characterized by species-rich faunal associations documenting lowering levels of the Western Interior Sea. Relatively diverse molluscan associations at the beginning of the deposition of the Fox Hills Formation are dramatically reduced in both bivalves and gastropods in the younger brackish-water lithofacies of the formation, indicating continued loss of habitat. Marginal marine to marine conditions returned to the area of central North Dakota and South Dakota, as recorded in the transition deposits of the Fox Hills into the lower Hell Creek Formation. More fully marine (mesohaline to brachyhaline) conditions are documented by a relatively diverse molluscan fauna in south-central North Dakota, as evidenced in the Breien Member of the Hell Creek Formation. The short-lived species richness recorded in the Breien strata is overlain by evidence of isolated rare molluscan and arthropod fossils in the upper, but not uppermost, part of the Hell Creek. These molluscan faunules contain individual ammonite, scaphopod, and bivalve taxa that indicate a resurgence of westward shoreline advance or fluctuation in shoreline configuration and continued influence of sublittoral marine conditions. Thus, as reflected in the deltaic depositional history of the Hell Creek Formation through much of its thickness, the molluscan record of the last 1-2 m.y. of the Cretaceous indicates environmental settings characteristic of the presence of an adjacent interior sea.
The Hell Creek Formation of North Dakota preserves a rich and varied palynoflora including pollen, spores, and algal cysts; this palynoflora provides insight into the nature of plant life in the region in late Maastrichtian time. This palynological record supplements extensive data from the same ancient vegetation preserved in the megafloral record, and it provides background biostratigraphic data necessary to determine the nature of the Cretaceous-Tertiary (K-T) boundary event. This study of the Hell Creek palynoflora is based upon occurrences of palynomorphs in 71 samples from 20 measured sections in Bowman and Slope Counties, southwestern North Dakota. The palynoflora documented includes 115 or more palynomorph species, 98 of which are described. Three new genera are proposed, three new species are described, and five new nomenclatural combinations are made. The new taxa are Palaeoisoetes gen. nov., Styxpollenites gen. nov., Tschudypollis gen. nov., Aquilapollenites marmarthensis sp. nov., Osmundacidites stanleyi sp. nov., Styxpollenites calamitas sp. nov., Polyatriopollenites levis (Stanley 1965) comb. nov., Palaeoisoetes subengelmannii (Elsik 1968) comb. nov., Triporopollenites triplicatus (Anderson 1960) comb. nov., Tschudypollis retusus (Anderson 1960) comb. nov., and Tschudypollis thalmannii (Anderson 1960) comb. nov. The palynofloral taxa include 67 angiosperms, eight gymnosperms, 19 pteridophytes, and three bryophytes. Of these, 33 are regarded as characteristically uppermost Cretaceous taxa (K taxa). A few of the K taxa do not range all the way to the K-T boundary. Extinction of the Hell Creek palynoflora at the K-T boundary is ∼30%. The Hell Creek Formation is within theWodehouseia spinata Assemblage Zone.
Following introductory comments on the nature of plant fossils (eg. how age is determined, systematic relationships, reconstruction and nomenclature), chapters trace the evolution of different forms from the Precambrian through to the 'living fossil', Ginkgo. The last 4 chapters provide overviews: discussion of the first coniferophytes; the diversification of conifers and taxads; the origin and early evolution of angiosperms; and major evolutionary events and trends - a summary. Throughout, description (including many line drawings) is linked with functional and evolutionary development. -P.J.Jarvis
A survey of palynofloras of terrestrial origin from the Upper Cretaceous in a north-south transect of the Western Interior Basin from the Yukon Territory to New Mexico reveals that contemporaneous palynofloras are of similar morphogenetic grade and taxonomic composition, but are not uniform. Latitudinal variation in palynofloras involves endemism that becomes more pronounced and involves successively lower-order taxa in successively younger assemblages. Diachroneity in ranges of some taxa is evident. Latitudinal variation and diachroneity in palynofloras necessitates establishment of compound first-occurrence datums for regional correlation. The datums and the chronostratigraphic boundaries they mark are: 1) tricolporate-obligate tetrad (Albian-Cenomanian), 2) triporate-oculate (Turonian-Coniacian), 3) triprojectate (Santonian-Campanian), 4) Wodehouseia-Kurtzipites (Campanian-Maastrichtian), and 5) Wodehouseia spinata-Mancicorpus vancampoae (base of upper Maastrichtian). -from Authors
Nine genera of fossil palm and palm-like leaves are recognized in this study. Amesoneuron H. R. Goeppert, Bactrites E. W. Berry, Palmacites A. Brongniart, Phoenicites A. Brongniart, and Sabalites G. Saporta are considered to be true palms. Some specimens, because of their uniquely recognizable characteristics, have been identified with the modern genus Phoenix Linnaeus. Sanmiguelia R. W. Brown, Paloreodoxites F. Knowlton and Propalmophyllum O. Lignier are also included in this study because of their superficial resemblance to palms. However, they lack one or more characters essential for their recognition as true palms (i.e. hastula, primary costa, and two orders of venation paralleling a prominent but uniform mid-vein in the individual segments). A diagnostic key and a synopsis of accepted form and organ genera, with appropriate synonymies and new combinations complete the paper. A list of genera to be excluded from the Palmae is appended.
G|far from being evolutionary stagnate, gymnosperms show high rate evolution during the Cretaceous. In particular, first fossil remains of Cycas are found in the Late Cretaceous of Sakhalin Island, Far East
The most important data for the reconstruction of Mesozoic cliseres are the life-form statistics, the phytosociological criteria and the dynamic criteria (i.e., the displacements of isoflores, the altitudinal migrations, etc.). The succession of fossil-plant assemblages in South Primorye as well as in the Bureja basin indicates a rise of temperature during the Late Jurassic Epoch up to Berriasian time, the cooling from Valanginian to Barremian and the amelioration of climate in the Aptian followed by the fall of temperatures in Albian time.
THEORIES that explain the extinctions characterizing the
Cretaceous/Tertiary (K/T) boundary1-3 need to be tested by
analyses of thoroughly sampled biotas. Palynological studies are the
primary means for stratigraphic placement of the terrestrial boundary
and for estimates of plant extinction4-12, but have not been
combined with quantitative analyses of fossil leaves (megaflora).
Megafloral studies complement palynology by representing local floras
with assemblages capable of high taxonomic resolution13, but
have previously lacked the sample size and stratigraphic spacing needed
to resolve latest Cretaceous floral history5,14-18. We have
now combined megafloral data from a 100-m-thick composite K/T boundary
section in North Dakota with detailed palynological analysis. Here the
boundary is marked by a 30% palynofloral extinction coincident with
iridium and shocked-mineral anomalies and lies ~2 m above the highest
dinosaur remains. The megaflora undergoes a 79% turnover across the
boundary, and smaller changes 17- and 25-m below it. This pattern is
consistent with latest Cretaceous climatic warming preceding a bolide
impact.
Profiles illustrating the abundance of iridium and the changes in the relative abundances of major groups within the palynoflora are given for 3 localities from W Canada. The peak abundance of iridium ranges from 1.35-5.60ppb and corresponds to the base of a coal and to a floral-extinction event at the 3 localities. Immediately above the iridium-peak anomaly, angiosperm pollen is conspicuous in the assemblage, in contrast to the fern-spore spike described for sections in the mid-continental United States and at the Morgan Creek locality in S-central Saskatchewan.-Authors
Intensive collecting of Maastrichtian and early Paleocene plant megafossils (primarily leaves) in the northern Rocky Mountains and Great Plains has resulted in a database of nearly 25,000 specimens from more than 200 localities in eight areas. The most completely sampled section is at Marmarth, in southwestern North Dakota, where 57 latest Cretaceous and 30 early Paleocene localities (11,503 specimens) span the Cretaceous/Tertiary (K/T) boundary. Zonation of the Marmarth megaflora based on floral composition and relative abundance results in four zones: HC I, HCII, HC in, and FU I. This megafloral zonation appears to be applicable to terrestrial sediments across the northern Great Plains and Rocky Mountains and to correlate with the Western Interior ammonite zonation. The zones reflect major megafloral changes before and at the K/T boundary. Megafloral change at the boundary is large (79 percent) and is characterized by the disappearance of most of the Upper Cretaceous dicotyledonous angiosperm taxa. It coincides with a peak in palynofloral extinctions and iridium content and with the occurrence of shocked mineral grains, all of which have become accepted as the characteristic signature of the K/T boundary and as indications of an abrupt causal mechanism. In contrast, the megafloral changes before the boundary appear to have been caused by a regional climate warming. The fact that the megafloral zones are not reflected by palynostratigraphy argues for using an integrated approach to biostratigraphy that combines the high stratigraphic resolution of palynomorphs with the high taxonomic resolution of megafossils. Results of this analysis of the terrestrial plant record are compatible with the hypothesis of a biotic crisis caused by extraterrestrial impact at the end of the Cretaceous.