The Cryptogamic, or spore-producing, plants of today are composed of three nonvascular, bryophyte groups (mosses, liverworts, and hornworts) and several vascular groups (ferns, club mosses, and horsetails). All of these plants produce abundant spores, which serve as propagules for dispersing and, to some extent, preserving plants through periods of ecological stress. Plant spores are typically formed as the end products of meiosis (reduction division) from a dividing sporocyte, or spore mother cell (smc). Because of this, they typically occur in groups of four, with each individual spore bearing a characteristic trilete, or Y-shaped mark on its common contact surface. Spore walls, composed of an inert, heterogeneous polymer called sporopollenin, are extremely resistant to the chemical vicissitudes of the terrestrial environment. This property of typical plant spores ultimately allows them to be quite abundant in fine grained clastic rocks. Although fossilized spores represent only a small part of the once-living plant, in many cases, they represent an important component of the plant fossil record, especially when the preservation of macroscopic tissues is lacking.

Many Paleontologists share the opinion of McGhee (1996), who wrote “Prior to the Devonian, there was no terrestrial ecosystem to speak of. Some primitive plants precariously establishing a beachhead in protected coastal areas was about it. The interiors of the continents of the planet Earth were as barren as the rocky landscapes of Mars.” Thus, it was with trepidation that I reported paleosols containing trace fossils of early land animals in the late Ordovician, Juniata Formation, of Pennsylvania (Retallack and Feakes, 1987; Retallack, 1992a, 1992b, 1993). My late colleague, Jane Gray, engendered considerable debate by reporting Ordovician and Early Silurian spores like those of liverworts (Gray and Boucot, 1977; Gray, 1985). This spore, trace fossil and paleosol evidence for life on land in the Ordovician has remained controversial (Buatois et al., 1998; Shear, 1998), but evidence for Ordovician life on land has continued to accumulate. Especially important was discovery of myriapod trackways from mid-Ordovician (Llandeilian-Caradocian) Borrowdale Volcanics of the Lake District, England (Johnson et al., 1994). Abundant arthropod burrows and tracks, and a single body fossil of an euthycarcinoid in the fluvial-eolian Tumblagooda Sandstone of Western Australia (White 1990; McNamara and Trewin, 1993; Trewin and McNamara, 1995) are now thought to be late Ordovician in age (Iaksy et al., 1998). An enigmatic assemblage of arthropods and plants from a mid-Ordovician paleokarst in Tennessee (Caster and Brooks, 1956) is now thought to have been lacustrine (Gray, 1988a). The fossil record of Ordovician land plants also has improved with the discovery of possible megafossil mosses (Snigirevskaya et al. 1992), and possible late Ordovician trilete spores (Nøhr-Hansen and Koppelhus, 1998; Richardson 1988; Strother, 1991; Strother et al., 1996). But the most abundant evidence for Ordovician life on land remains fossil soils, now exploited by increasingly thorough and sophisticated studies (Retallack, 1985, 1992a, 1992b, 1993; Feakes et al., 1989; Driese and Foreman 1991, 1992a, 1992b; Driese et al., 1992, 1997; Mora et al., 1991, 1996; Mora and Driese, 1993; Yapp and Poths, 1992, 1994, 1996; Yapp, 1993, 1996). Mounting evidence from fossils and paleosols now presents an increasingly detailed view of Ordovician ecosystems on land.

Paleosols Crop Out extensively in the Appalachian region of eastern North America, from the Canadian Maritime Provinces southward to the Tennessee-Alabama border along the western side of the Appalachian Orogen (Fig. 1). The paleosols occur primarily in terrigenous clastic redbed deposits ranging in age from Late Ordovician to Early Permian (Mora and Driese, 1999; Driese and Mora, in press), which encompass a time interval characterized by rapid evolution and diversification of terrestrial ecosystems (Table 1). These Appalachian paleosols formed under relatively constant sediment-source area and pedogenic environments (Table 1); thus they share generally uniform physical and chemical properties. As such, they are suitable particularly for investigating evolutionary advances in the gross morphology of plant roots and rhizomes, preserved as traces, and their relationship to physical and chemical features in the paleosols (Table 2). Important changes in plant abundance, rhizome and root systems, and adaptive capabilities are recorded in the paleosols, concurrent with a proxy record of changing environmental conditions. Paleosols represent a still-underutilized record of the terrestrial ecosystem (Retallack, 1990).

Ecosystem Stability and taxonomic stasis are the opposite aspect in the history of life from ecosystem perturbation and reorganization, or extinction and origination. Naturally, change is more exciting than stasis, and change also is more useful in many ways as, for instance, in the development of biostratigraphic frameworks. Paleontological preference for geologically rapid change has gone so far that a fast evolving group of organisms has been used to create an orthostratigraphy (Orthochronologie of Schindewolf, 1950), which is claimed to be the “real” biostratigraphy. By implication, all other taxonomic groups are relegated to a secondary status of merely delivering “parastratigraphies,” which may be useful (locally or regionally), but are not the “real thing.” Ammonites, conodonts, foraminifera, and a few other taxa hold the distinction of being the “chosen” taxa, and it should be self-evident that only marine organisms qualify for this role. On the other hand, organisms on land always have been known to demonstrate coarser stratigraphic resolution. By implication, they might be more prone to ecosystem and taxonomic stasis over the long run. This, in itself, is an important observation.

Since their first appearance in the Middle-Late Silurian, land plants have played an increasingly important role in shaping terrestrial ecosystems and landscapes. It is difficult to overestimate their role because they form the framework for terrestrial ecosystems, provide habitats for terrestrial animals, form an important part of the food chain, affect weathering processes and have a direct impact on soil formation, and, last but not least, play a primary role in the oxygen/carbon cycles.

The past decade has seen many advances in research on vertebrate faunas of the Triassic period. The end of the Triassic now is cited widely as the dawn of modern terrestrial ecosystems, and currently the earliest mammals, turtles, lissamphibians (frogs, toads and salamanders), lizards, and crocodiles are all documented from this period. Admittedly many of these early members of present day higher order taxa were very different from their modern counterparts. For instance, the earliest crocodiles were highly active cursorial forms (e.g., Crush, 1984), and the mammals were very different to the living placentals and marsupials. Nevertheless, they possessed many of the key morphological characteristics that diagnose the group and that may well have contributed to their ultimate success. However, the Triassic was also a time of bizarre and enigmatic tetrapods, some of whose relationships are the subject of considerable debate. Indeed, in the last year this debate has reached new heights with suggestions that certain rather unusual Triassic non-dinosaurian tetrapods may have more bearing on bird origins than theropod dinosaurs. This debate has been fueled by the discoveries of feathered dinosaurs from China which, on the face of it, one might expect to dampen the search for alternative hypotheses regarding bird origins.

Mesozoic soil ecosystems are intriguing because they are the products of many unique factors and events in geologic history. One of the most notable events that likely shaped Mesozoic soil ecosystems was the Permo-Triassic extinction. At that time, estimates of 20% of plant species, 50% of tetrapod genera, roughly 60% of insect families, and approximately 91 to 97% of shallow marine life became extinct (e.g., Padian and Clemens, 1985; Raup, 1986; Niklas et al., 1980; Wing and Sues, 1992; Labandeira and Sepkoski, 1993; and references therein). Over the span of the next 180 million years the supercontinent Pangea disassembled and the continents moved toward the configuration we see today (Scotese and Golonka, 1992); the Pangean mega-monsoonal climate pattern deteriorated into more zonal climates through to the Cretaceous (e.g., Parrish et al., 1982; Dubiel et al., 1991; Parrish, 1993); sea-level rose and flooded continental interiors, reaching its second greatest maximum since the Ordovician (e.g., Haq et al., 1987, 1988); and several major evolutionary episodes occurred, including the evolution and diversification of the angiosperms, mammals, birds, and many of the neopteran insects (Lillegraven et al., 1979; Carpenter and Burham, 1985; Friis et al., 1987; Wing and Sues, 1992; Labandeira and Sepkoski, 1993).

When one hears the term “ground cover,” one immediately thinks of “grasses.” This perception is so deep-seated that paleobotanists even have been overheard to proclaim that “there was no ground cover before grasses.” Today grasses are so predominant in many environments that this perception is perpetuated easily. On the other hand, it is difficult to imagine the absence or lack of ground cover prior to the mid-Tertiary. We tested the hypothesis that different forms of ground cover existed in the past against examples from the Recent and the fossil record (Table 1). The Recent data were obtained from a large number of sources including those in the ecological, horticultural, and microbiological literature. Other data were derived from our knowledge of Precambrian life, sedimentology and paleosols, and the plant fossil record, especially in situ floras and fossil “monocultures.” Some of the data are original observations, but many others are from the literature. A detailed account of these results will be presented elsewhere (Pfefferkorn and LePage, in preparation).

The middle jurassic through Cretaceous was the heyday of gymnosperms. Gymnosperms—a paraphyletic group of seed-bearing, non-flowering vascular plants including conifers, ginkgos, seed ferns, cycads, and cycadeoids—comprised as much as 80% of global floras throughout this time interval. Even the much-heralded rise of angiosperms in the mid- to Late Cretaceous did little to shake the Mesozoic dominance among terrestrial floras of gymnosperms (in particular, conifers; see Tiffney, 1997). By the end of the Cretaceous, angiosperms comprised—depending upon whose estimate is being used—somewhere between 40 and 60% of the world's floras (Lidgard and Crane, 1988; Tiffney, 1997), leaving plenty of ecospace available for gymnosperms. The lower part of Figure 1, redrawn from Tiffney (1997), documents the flux of the major groups of plants throughout the Late Triassic-through-latest Cretaceous interval. The figure reaffirms that in the Mesozoic, gymnosperms were the floral force to be reckoned with.

Flowering plants are a classic example of a group arising late in Earth history and yet achieving very high diversity, abundance, and ecological and morphological variety in a great array of environments and climatic conditions on all continents. Thus, the success of flowering plants raises basic questions about how new lineages become inserted into existing terrestrial ecosystems. To what degree did flowering plants replace older lineages competitively, and to what extent did their expansion depend on large-scale environmental disruption or extinction of older groups? Is the higher taxonomic diversity of flowering plants a consequence of higher rates of speciation, lower rates of extinction, or both? Have flowering plants expanded the total area and range of habitats occupied by terrestrial vegetation? What were the effects of the diversification and spread of flowering plants on the structure of habitats and the types of resources available to terrestrial heterotrophs?

Perhaps the most conspicuous of associations between insects and plants is pollination. Pollinating insects are typically the first and most obvious of interactions between insects and plants when one encounters a montane meadow or a tropical woodland. The complex ecological structure of insect pollinators and their host plants is a central focus within the ever-expanding discipline of plant-insect interactions. The relationships between plants and insects have provided the empirical documentation of many case-studies that have resulted in the formulation of biological principles and construction of theoretical models, such as the role of foraging strategy on optimal plant-resource use, the advantages of specialized versus generalized host preferences as viable feeding strategies, and whether “pollination syndromes” are meaningful descriptions that relate flower type to insect mouthpart structure and behavior (Roubik, 1989; Ollerton, 1996; Waser et al., 1996; Johnson and Steiner, 2000). Much of the recent extensive discussion of plant-insect associations has centered on understanding the origin, maintenance, and evolutionary change in plant/pollinator associations at ecological time scales and increasingly at longer-term macroevolutionary time intervals (Armbruster, 1992; Pellmyr and Leebens-Mack, 1999). Such classical plant-insect association studies—cycads and cycad weevils, figs and fig wasps, and yuccas and yucca moths—were explored at modern time scales and currently are being examined through a long-term geologic component that involves colonization models based on cladogenetic events of plant and insect associates, buttressed by the fossil record (Farrell, 1998; Pellmyr and Leebens-Mack, 1999; A. Herre, pers. comm.). In addition to tracing modern pollination to the earlier Cenozoic and later Mesozoic, there is a resurgence in understanding the evolutionary history of earlier palynivore taxa (spore, prepollen and pollen consumers), which led toward pollination as a mutualism (Scott et al., 1992).

Many changes were taking place in the terrestrial landscape near the end of the Mesozoic. Angiosperms are first unequivocally recorded in Cretaceous strata; a rapid diversification and increase in abundance occurred during the Campanian-Maastrichtian. This shift from ferns-gymnosperm to angiosperm dominance was of major importance to terrestrial herbivores. Increasing complexity in the dental batteries of hadrosaurs and ceratopsians also occurred simultaneously. Stable carbon isotopes from teeth have provided detailed information on ecosystem and dietary shifts during the Miocene-Pliocene, with the advent and spread of C4 grasses. However, during the Late Cretaceous all plants probably used only the C3 pathway and carbon isotopes are less useful in distinguishing browsing and grazing habits. However, a large range of carbon isotope values is produced by C3 plants, and indicates that environmental parameters (aridity/humidity) are more important than those produced by taxonomic effects.

There are two principal mechanisms of photosynthesis amongst the plants of the world. One produces a 3-carbon compound as the primary photosynthate and is called C3 photosynthesis. The other produces a 4-carbon compound as its primary photosynthate and is called C4 photosynthesis. This latter method is of rather recent origin, and its inception had important consequences for the flora and fauna of the world. The C4 plants make up a significant portion of global net primary productivity, especially in low latitudes. C4 plants are predominantly warm-season grasses, while C3 plants include most dicotyledons and cool-season monocotyledons. In this review we answer the questions about how C4 photosynthesis differs from C3 photosynthesis, where C4 plants are found, and review the paleoecological history of C4 photosynthesis.

Most of what we know about ecology inevitably comes from Late Quaternary ecosystems, particularly those of the past few decades. 20th Century ecosystems are the only ones for which we have direct observational and experimental studies. We can obtain detailed records of ecological dynamics at decade- to century-scales over the past few centuries using historical, permanent-plot, demographic, and paleoecological techniques, but only for a select few ecosystems. Radiocarbon-dated fossil assemblages provide records of ecological changes over the past 25,000 years. Because 14C-dating allows independent dating of the assemblages with precision of 50–500 years, detailed spatiotemporal patterns of change can be outlined, but these are limited to certain kinds of organisms and to regions with a high density of fossil sites. When we go beyond the radiocarbon barrier 30–40,000 years ago, the density of datable terrestrial assemblages decreases sharply, and our views of past ecosystems become murkier.