Michael Krings

Introduction to Paleobotany, How Fossil Plants are Formed

It is natural to wonder about prehistoric life—how these organisms lived, what their patterns of behavior were, and even why they became extinct. Although the paleontologist is interested in the geologic history of animals, the paleobotanist is concerned with the plants that inhabited the Earth throughout geologic time. In a general sense, the paleobotanist is a plant historian who attempts to piece together the intricate and complicated picture of the history of the plant kingdom. Although molecular and genetic analyses of living plants have become increasingly important as tools in reconstructing the phylogeny and evolutionary history of plants, the discipline of paleobotany, in all its various forms, remains the only method by which this history can be documented and visualized. One of the aspects of paleobotany which makes it unusual and interesting is that it is inherently interdisciplinary and can be approached from either a biological or a more geological perspective—or both together. Each perspective presents a variety of questions that are unique to that discipline. Today more than ever before, the questions being asked by paleobotanists necessitate that both the botanical and geological perspectives be fully understood. Paleobotanists who have been trained primarily as biologists are interested in research directions, which include all aspects of the organisms themselves. Because the majority of fossil plants are generally preserved in rocks as disarticulated plant parts, that is leaves, stems, pollen, or reproductive structures, a major aim of paleobotany is to reconstruct the whole plant, that is to say, to put the pieces of the puzzle back together.

Origin and Evolution of the Seed Habit

The origin and subsequent evolution of the seed habit is a fascinating subject that is well documented in the fossil record beginning in the middle to late Devonian, when the first seeds were found. Before considering the earliest seeds, however, it is important to trace the steps involved in the evolution of other reproductive systems that preceded the seed habit. The seed habit is the most complex and evolutionarily successful method of sexual reproduction found in vascular plants, and development of the seed habit represents one of the most significant evolutionary events in the history of vascular plants. Today, seed plants are found in a wide variety of habitats owing to the selective advantages that this type of reproduction provides over that found in pteridophytes, including independence from liquid water for fertilization and the capacity for embryo dormancy in a changing environment. This chapter presents a discussion on homospory, heterospory, the seed habit, and carboniferous seeds.

How Old are the Fungi

Fungi are no different from other lineages of organisms in that they are not represented in the rock record as a continuous and uninterrupted sequence of fossils. As we learn more about them, however, it becomes obvious that they are an ancient group that must have evolved relatively early in geologic time, perhaps more than 1.5 billion years ago. Despite the lack of body fossil evidence for the earliest organisms that possessed a heterotrophic lifestyle, there are other approaches and techniques that have been used by evolutionary biologists, including those practicing systematics, molecular ecology, genetics, and paleobiology to date certain events in geologic time and thus infer when organisms like fungi first appeared. The use of phylogenetic systematics and the construction and use of molecular clocks have provided hypotheses on the origin of evolutionary lineages of fungi as well as patterns in their evolutionary history.

How Fungal Fossils Are Formed and Studied

Evidence of the existence of biological systems at various points in geologic time can be recorded in the fossil record in many ways. While everyone has a visual image of dinosaur bones or the remains of mammoths that once roamed the Earth, invertebrate shells, or plant leaves, what about fossil fungi and other types of microorganisms? What do fossil fungi and other microorganisms look like? Where are these found, how are they preserved, and what methods can be used to study aspects of the fossilized microbial world? In this section we present the most common modes of preservation for fossil fungi, discuss how some of these organisms are studied, and how they contribute to our understanding of past ecosystems.

Gymnosperms with Obscure Affinities

There are a number of enigmatic gymnospermous plants that deserve mention because of their interesting morphology and, in some instances, unusual internal structure. Some of these are known from very few specimens or are restricted both geographically and stratigraphically, for example, the Hermanophytales. In other instances, the plants are known in some detail, but their affinities continue to remain elusive, for example, the Vojnovskyales. Some, such as the gnetophytes, have been included in phylogenetic analyses of seed plants. As additional specimens of existing taxa are discovered, which are more completely preserved, together with the discovery of new genera and species, some of the enigmatic taxa today will perhaps be easier to classify in the future.

Hornworts and Bryophytes

Hornworts and liverworts have been interpreted as occupying a position intermediate between the green algae and vascular plants. Others suggest that the bryophytes in general represent examples of evolutionary failures; perhaps they originated from early vascular plants, such as the rhyniophytes, and then, as the group continued to evolve, vascular tissue was lost. Today, however, these views have changed on the basis of a variety of ultrastructural, biochemical, and molecular data suggesting that the principal bryophyte groups had separate origins, and that hornworts, liverworts, and mosses represent the earliest divergent lineages of extant land plants, although the specific order of their divergence still remains unresolved. Prior to the development of the first efficient microscopes in the late eighteenth century, most people, including scientists, regarded hornworts, liverworts, and mosses as tiny flowering plants. The distinctness of these organisms first became widely acknowledged in the decades following the documentation of the bryophyte life cycle by Wilhelm Hofmeister in 1851. Beginning in the twentieth century, scientists became increasingly interested in the origin and evolution of the hornworts and bryophytes, and their relationships to other groups of fossil and modern plants. Bryophytes and hornworts are unique among extant embryophytes in that they have a gametophyte-dominant life cycle; the sporophyte usually is relatively short-lived and permanently dependent on the gametophyte. Physiologically, they are poikilohydric, meaning that they cannot control water loss; when the environment dries out, bryophytes also desiccate.

Ferns and Early Fernlike Plants

Ferns first appeared in the Devonian and today more than 10,000 species can be found in a wide variety of habitats. A fern is sometimes described as a vascular cryptogam with foliar-borne sporangia. That definition generally works for extant ferns, but defining a fossil fern is more difficult. All extant ferns are perennial and herbaceous and, although some are treelike, none produce secondary xylem. There is a wide variety of stem anatomy in the ferns, ranging from simple protosteles in most of the Paleozoic forms to complex dictyosteles. The features that are used to characterize most living ferns are less applicable for many fossils that represent various degrees of evolution within the ferns, especially those from the Devonian. The classification that is provided in this chapter does not include examples of all fossil ferns, but is rather intended to provide a framework with which to discuss some of the plants that are believed to have a phylogenetic relationship with modern ferns. Detailed accounts of ancient- and modern-appearing fossil ferns that focus on biodiversity and ecology during the Mesozoic and Cenozoic, and evolution of leptosporangiate ferns are important references that help to frame discussions about the evolution of these highly adaptive vascular plants and their role in ecosystems through time.

Fungal intruders of enigmatic propagule clusters occurring in microbial mats from the Lower Devonian Rhynie chert

Microbial mats in the Lower Devonian Rhynie chert represent diverse communities of organisms, which probably not only co-occurred in these structures, but also variously interacted with one another. However, little is known about these interactions. Three different types of fungi interact with clusters of small propagules that frequently occur within the Rhynie microbial mats. One of the fungi occurs in the form of small mycelia and single reproductive units within individual propagules, while the second is characterized by apophysate, epibiotic sporangia and multibranched rhizoidal systems that extend through the clusters and penetrate individual propagules. The third fungus consists of what is interpreted as a distal sporangium or spore from which a long, tubular stalk reaches into the propagule cluster. One specimen of the latter fungus occurs inside a specimen of the second fungus and, moreover, shows evidence suggestive of hyperparasitism in the form of conical callosities. This discovery supports the suggestion that microbial mats in the Rhynie paleoecosystem were complex structures based on the presence of numerous interactions between different organisms within the mats.

Looking for Arbuscular Mycorrhizal Fungi in the Fossil Record

Abstract The evolution and diversification of plants on land were profoundly influenced by mutually beneficial symbioses between the plants and certain fungi. The vast majority of fungi involved in these fossil associations are strikingly similar to present-day arbuscular mycorrhizal fungi (AMF), and their symbioses with plants closely resemble present-day arbuscular mycorrhizas (AM). Although fossil evidence of AM has variously been documented and illustrated throughout the late Palaeozoic to Cenozoic, the record in general remains exceedingly scant. However, we believe that more compelling evidence of AM and AMF in fossil plants can be gathered if paleobotanists are equipped with an accurate search image for mycorrhizal fungi and the core structural components of their associations with plants. This chapter presents an illustrated guide that provides researchers with a synopsis of important (i.e., recognizable in transmitted light) structural features of modern AM that facilitates the accurate identification of fossil members of this group of fungi and their discrimination from other, nonmycorrhizal fungi while examining structurally preserved plant fossils. Where available, fossil mycorrhizal fungi displaying the features included in this guide are also documented.

Bacteria and Fungus-Like Organisms

Numerous microorganisms bear some resemblance to fungi (with regard to both morphology and biology) but do not belong to that kingdom, and many of these microbes have a fossil record. Some of them can be directly observed while others are interpreted based on various types of indirect evidence. Fungus-like organisms covered in this chapter include bacteria, actinomycetes, peronosporomycetes, and various fossils included in the Mycetozoa.