Introduction

Ferns traditionally have been identified as megaphyllous plants that reproduce by sporangia borne on leaves (i.e., fronds; Bower 1923; Kaplan and Groff, 1995). Although not all species have the entire set of characters, ferns typically are recognized by sporophytes that display unipolar growth (Rothwell, 1995), are dominated by fronds (Kaplan and Groff, 1995), and are devoid of secondary growth. Many have highly branched fronds, mesarch xylem maturation, and a rhizome stele that is dissected by leaf gaps. Plants that display various combinations of these features occur in the fossil record from the Middle Devonian (i.e., 390 million years ago) to the Recent, but there has been considerable systematic turnover with several prominent clades replacing one another through geological time (Rothwell, 1999). In practice, botanists have traditionally recognized as “ferns” those species that are left over after all other euphyllophytes have been removed to clades with clearly identifiable synapomorphies (Rothwell, 1999).

Up to the present, attempts to define ferns within a phylogenetic framework have met with only limited success (Figure 13.1) due to a combination of (1) limited information about many extinct ferns and fern-like plants, and (2) the restricted taxon sampling available for phylogenetic analyses that include only living species. The paleontological record of ferns is incomplete, and thus far has been sampled for only a small fraction of the available fossils (Stockey and Rothwell, 2006). This leaves us with an often confusing picture of inadequately known extinct species that can be difficult to comprehend and appreciate.

Late Cretaceous vegetation

Paleobotanists divide Phanerozoic time into three eras: the Paleophytic, Mesophytic, and Cenophytic (Traverse 1988b). The Mesophytic–Cenophytic boundary lies nowhere near the Mesozoic–Cenozoic boundary. Because it is defined by the first appearance of the angiosperms, the Cenophytic begins in the earliest stages of the Cretaceous, some 60–70 million years before the K–T boundary. Angiosperm evolution was rapid in the Cretaceous and global summaries based on megafloral and palynofloral data (Crane and Lidgard 1989, Lidgard and Crane 1990, Drinnan and Crane 1990) show an equatorial origin in the Barremian followed by an explosive race to polar latitudes by the Cenomanian. Nearly all Cenomanian and post-Cenomanian megafloras are dominated by angiosperms. In terms of temperature, Wolfe and Upchurch (1987b) using leaf margin analysis documented a gentle warming from the Albian to the Santonian, followed by a gradual cooling to the early Maastrichtian, culminating with a rapid late Maastrichtian warming. Huber et al. (1995) used isotopic analysis of marine foraminifera to generate a Late Cretaceous temperature curve with similar trends but with no obvious latest Maastrichtian warming. Subsequent more detailed analysis of magnetostratigraphically calibrated marine and terrestrial records shows warming occurred within the last 500 000 years of the Cretaceous both in deep-sea cores and in terrestrial deposits (Wilf et al. 2003).

The latest pre-angiosperm Mesophytic Era (Early Cretaceous, Berriasian–Barremian) is characterized by the Wealden flora of southern England (and correlative floras in France, Belgium, Germany, Spain, and Portugal).

At the scale of western North America, the paleobotanical record appears to document a mass extinction of plant species over the time represented by a centimeter of sediment. Before 1980, temporal resolution of the rock record was coarse and no such statement could have been made with any sense of certainty. The K–T boundary provided a testable hypothesis that is still being examined. Our results seem to suggest that Traverse (1988a) was incorrect in asserting that all changes in floras throughout time have been due to gradual replacement. It now appears that it is possible for plants, like animals, to suffer abrupt mass extinction.

The radiation of the angiosperms that began at the start of the Cretaceous was well under way by the end of the period some 80 million years later and most terrestrial ecosystems either contained or were completely dominated by angiosperms. The K–T extinction, while selective at the environmental scale as evidenced by the apparent preferential survival of mire plants, was not obviously selective at a taxonomic level. That is to say, no major plant groups disappeared at the boundary, and the damage primarily occurred at the species level as local ecosystems independently suffered the after-effects of the impact. Floral recovery from the event seems to have taken the duration of the Paleocene with the awkward exception of the Castle Rock rainforest, whose presence less than two million years after the event remains enigmatic.

Overview

Several scenarios for events at and following the K–T boundary have been proposed, some of them predating the one set forth in Alvarez et al. (1980), the publication that substantially renewed speculation on the subject. In this chapter we summarize the most prominent hypotheses, including that advanced by the Alvarez team. In the final section of this chapter, we discuss our own preferred scenario, which is based on our understanding of effects of the boundary event on plant life.

Other reviews of K–T boundary scenarios, most of which emphasize the impact hypothesis, are presented in books by Albritton (1989), who also reviewed the whole concept of catastrophic events in Earth history and the way they have been viewed since the seventeenth century; Allaby and Lovelock (1983); Alvarez (1997); Frankel (1999); Hsü (1986); and Powell (1998). Archibald (1996) and Officer and Page (1996) proposed alternative scenarios to account for the mass extinctions at the end of the Cretaceous. In general, these books feature the history of dinosaurs and tend to neglect the fossil plant record, exceptions being Hsü (1986) and Powell (1998); some of them overlook plants entirely. This short shrift or omission from prominent books on the subject served as our inspiration for writing this book.

Disregarding a few unconventional and even unscientific (in the sense of being untestable) ideas about causes of the K–T extinctions (which also focus on dinosaurs), the mainstream hypotheses center on four themes: climate change, regression of epicontinental seas, a major episode of volcanic eruptions, and impact of an extraterrestrial body.

In this book we have attempted to provide an overview of the state of knowledge of plants and the K–T boundary. The center of North America has yielded a rich floral record and the high quality and vast extent of exposures hold much promise for further refinement. Nearly all of the K–T boundary sections that contain evidence of the impact event are in North America. For this reason, it is difficult to make global generalizations other than to say that more sections are needed on other continents. In fact, it is fair to say that without the North American record, we would be hard pressed to argue for major floral change at the K–T boundary. Only the New Zealand sections document floral change relative to the iridium anomaly, and in those sections it is primarily the fern-spore spike that supports the concept of an impact as a causal mechanism. Were the New Zealand sections studied in isolation, it is questionable whether the fern-spore spike would have been recognized as significant. Only when taken in context of the North American fern-spore spike is the New Zealand occurrence interpretable. Nonetheless, its presence and close association with an iridium spike is one of the strongest arguments for the global reach of the immediate and deleterious effects of the bolide impact. The loss of forest canopy at the K–T boundary in New Zealand was followed by the nearly complete recovery of the pre-existing Cretaceous forest.

First attempts in Europe and North America

The understanding of the nature of floral change across the K–T boundary has been delayed both by the nature of the plant fossil record and by poor temporal resolution in Cretaceous nonmarine rocks. The rapid rise of angiosperms in the Early Cretaceous and the rarity of thick, continuously fossiliferous, Cretaceous sections further complicated this situation. Angiosperms dominate modern vegetation and account for more than 80% of living species, but they appeared abruptly in the fossil record over a span of 25 million years in the Early Cretaceous. Early Cretaceous angiosperm leaves are superficially similar to living ones, and this similarity gave rise to the misconception that extant angiosperm genera first appeared in the Early Cretaceous. This sudden appearance, known as Darwin's abominable mystery, set the stage for several misconceptions and stratigraphic problems. Moreover, paleobotany has traditionally been summarized at the stage level and no great stock has been placed in obtaining the precise age of fossil floras. Before the Alvarez challenge, it was considered sufficient to state that a flora was Cenomanian or Campanian, or perhaps late Cenomanian or early Campanian. These stages are 6.1 and 12.9 million years in duration, respectively, and clearly represent too long of a time bin to be relevant to resolving change over short periods of time.

We perceive three increasingly precise scales of temporal resolution of the K–T boundary. Stage-level resolution is on the order of millions of years; subchron-level resolution is on the order of tens to hundreds of thousands of years; and impactite-level resolution, while not directly measurable, is on the order of one to a few years. At the resolution of stage, the boundary between the Cretaceous and Paleogene periods is the boundary between the Maastrichtian Stage (5.1 Ma in duration) and the Danian Stage (3.8 Ma in duration). At the time of the Alvarez discovery, the resolution of most K–T boundary paleontological studies was at the stage level, at best. For most terrestrial K–T sections outside of North America, this level of temporal resolution is still prevalent today. At the subchron level of temporal resolution, the K–T boundary event occurred within the polarity subchron C29r, whose duration was between 570 and 833 thousand years, depending on which calibration is chosen. Resolution at the subchron level often brings recognition of paleontological events to the outcrop scale on the order of tens to hundreds of meters of section. Biostratigraphic zones can have similar durations to subchrons and thus may fall into this category of resolution. The third level of resolution is the impact layer, where physical and biological observations can be directly related to the K–T boundary impactite. At this level, events are resolved at the centimeter, or even millimeter, scale.

We both have been fascinated by the Cretaceous–Paleogene (K–T) boundary since the late 1970s. The Alvarez discovery galvanized our individual research efforts and we have worked together on this problem since we met in 1983. Our research has focused on western North America, so our data and interpretations are largely based on field work and laboratory analyses in this region. We have also studied terrestrial K–T boundary sections in Russia, China, and New Zealand and searched, unsuccessfully, for them in Mongolia, Patagonia, and India.

In preparation for writing this book, we made a comprehensive survey of the world's scientific literature through 2006 pertaining to plants and the K–T boundary. Our bibliographic database includes more than 500 references, but we have chosen to cite only those most relevant to understanding the effects of the terminal Cretaceous event on plants. We sought to interpret objectively the data available in those publications rather than simply to repeat the conclusions of the original authors. In many instances we agree with the original authors, but in some we do not. In the latter instances, we trust we have fairly presented their views and that we have given no reason for offense in our reinterpretation.

To present a major conclusion at the outset, we deduce that the changes in plant communities that took place at the K–T boundary are inextricably and causally linked to the impact of an extraterrestrial body on the Earth in the Caribbean region – the Chicxulub impact.

Overview

From the palynomorph- and leaf-bearing intervals of latest Cretaceous and early Paleocene age in North America and Eurasia, we now direct our quest for records of plants and the K–T boundary to lands formerly or currently in the equatorial region or the Southern Hemisphere. These lands are the remnants of Gondwana. The continents formed by the break-up of Gondwana were well separated by K–T boundary time. Africa, India, and South America had moved into tropical latitudes while Australia, New Zealand, Madagascar, and Antarctica remained at mid to high latitudes.

Herngreen et al. (1996) stated that, in Late Cretaceous time, South America and most of Africa lay within the Palmae palynofloral province, which is characterized by assemblages with 10–50% pollen of the type produced by the Arecaceae (palms) and related species. That palms characterized the floras of the equatorial regions of these continents even in the Late Cretaceous is not surprising. Northern Africa was well north of the equator at 66 Ma and close to Europe and the Normapolles Province. As a result, northern Africa was in a transition zone between the Palmae and Normapolles provinces (Herngreen et al. 1996); see Figure 5.4. There was an increase in Normapolles pollen in northern Africa in the early Paleocene. Contemporaneous palynofloral assemblages from India included elements of the Palmae Province admixed with some from both the Normapolles and Aquilapollenites provinces; there is no obvious paleogeographic explanation for this.

Overview

The uppermost Maastrichtian and lowermost Paleocene rocks in the Williston Basin of western North Dakota, northwestern South Dakota, and eastern Montana contain the best exposed and most studied nonmarine record of the terminal Cretaceous event in the world. This region contains 41 (39%) of the known terrestrial K–T boundary sections (see Table 2.1 and Appendix). The Williston Basin is a large structural depression occupying much of North Dakota and parts of South Dakota, Montana (Figure 6.1), and it extends into southern Saskatchewan. The stratigraphic units in which the K–T boundary is preserved are the Hell Creek Formation, which for the most part is Maastrichtian in age, and the Fort Union Formation, which for the most part is Paleocene in age (Figure 6.2). The Hell Creek Formation is composed of fluvial sandstone, mudstone, and claystone. It includes rare, thin lignite beds and minor marine units. It was deposited at the edge of the Western Interior seaway (Roberts and Kirschbaum 1995, Johnson et al. 2002). The Fort Union Formation is composed of extensive lignite and carbonaceous shale beds, variegated mudstone of lacustrine origin, and sandstone largely of crevasse-splay origin. The facies change that marks the formational contact was caused by rising sea level during a temporary re-advance of the Western Interior seaway in early Paleocene time known as the Cannonball Sea, a Paleocene remnant of the Cretaceous seaway represented by the Cannonball Member of the Fort Union Formation (Figure 6.2).