Coarse-grained volcaniclastic sediments are a common part of the non-marine geologic record along most continental margins. Many of these volcanic sediment units contain well-preserved assemblages of fossil plants. Because much of the work on depositional environments, dating, correlations and paleoenvironments has been derived from studies of plants, it is important to understand the processes that incorporate the plants into the volcaniclastic sediments. In this paper I plan to concentrate on processes that bury and preserve trees, logs and stumps.

Plant fossil assemblages are biased samples of the capacity of the once living source vegetation to produce litter (leaves, flowers, pollen, spores, fruits, seeds, twigs, branches, trunks, roots etc.). The “image” that the depositional system “sees” of the vegetation is in terms of isolated plant organs (or parts of organs) that are produced in greater or lesser quantities depending on the organ type. For instance throughout the life of a tree hundreds of thousands of leaves, many millions of pollen grains but only one trunk are produced. Only very rarely do plant fossil assemblages consist of entire or nearly entire plants. Instead assemblages consist of a mixture of organs in various states of completeness derived from a number of different taxa (each of which produces different organs in different relative amounts) growing at various distances from the depositional site.

Ancient coastal physiographic provinces have been sites of detrital plant accumulations since the advent of terrestrial vegetation in the Silurian. These lowlands contain remnants of vegetation from a diversity of ecological habitats in a variety of preservational modes and depositional environments. Deltaic regimes within these coastal systems have acted as principal locales for the preservation of in situ (autochthonous) and transported (hypoautochthonous/allochthonous) community elements. A vast majority of our understanding concerning plant diversity and community structure has been centered in the refugia of lowland deltas because of the propensity of these terrigenous sites to become part of the stratigraphic record either by autocyclic or allocyclic mechanisms. Deltaic systems have played a major role in the accumulation of hydrocarbons either as subaqueous (topogenous) and subaerial/aerial (ombrogenous) peats, to be subsequently metamorphosed to coal (McCabe, 1985), or precurssors to natural gas and petroleum (Durand et al., 1986). In certain instances, these peats have been subjected to permineralization and provide the scientific community with an insight into the original standing community biomass (Phillips et al. 1985). Where peats have been coalified, palynomorphs recovered from coal macerations provide information on trends in regional vegetation. Additionally, depending on the relative rates of clastic vs. organic sedimentation in these environments, deltas have acted as repositories for coalified compression and authigenically cemented megafloral and microfloral assemblages (taphocoenoses).

Vascular plants are the most conspicuous organisms on Earth, accounting for some 97 % of our planet's standing biomass. The nearly 300,000 extant vascular plant species exhibit tremendous morphological and ecological diversity. Along with the 20,000 or more species of bryophytes, algae, lichens, and cyanobacteria that also live on land, they fuel a complex terrestrial ecosystem containing animals, fungi, protozoans, and bacteria. The richness of terrestrial life has evolved during the last 10 % of Earth history; there is no evidence for non-microbial land plants or animals in rocks older than the mid-Ordovician.

While fossil evidence supports a mid-Silurian or earlier establishment of land plants, it is clear that major diversification occurred in the Devonian. This has been summarized in several ways in recent years - the number of plant taxa (perhaps best expressed at the generic level) rapidly increases from the Early to Middle Devonian (Knoll et al., 1984; Niklas et al., 1980; Knoll, 1985); the first appearance of new structures increases through the Early and Middle Devonian and then tends to level off (Chaloner and Sheerin, 1979; Knoll et al., 1984); structural and reproductive complexity of plants increases (Chaloner and Sheerin, 1979; Edwards, 1980; Banks, 1980; Gensel and Andrews, 1984); and floral composition changes such that generalizations about plant diversity or communities can be made and seven biostratigraphic zones can be recognized (Chaloner and Sheerin, 1979; Banks, 1980). I will discuss specific aspects of these trends in the Early and Middle Devonian in this section, including consideration of biogeography and paleoecology. Vascular plants will be emphasized, but one should keep in mind that a number of non-vascular, ones also existed, some of which will be mentioned briefly. Banks' (1980) biostratigraphic zones, of which the first five are most applicable, provide a useful framework in which to discuss major events in the diversification of land plants.

Taken as a whole, Devonian and Carboniferous floras represent vastly different evolutionary stages in the colonization of the Earth's surface by plants. When only Late Devonian and Mississippian floras are considered, however, all of the floral innovations that led to and characterize the increasingly sophisticated plant communities of the Carboniferous began in the Famennian. When examined carefully enough, the stratigraphical succession of floras at the Devonian/Mississippian boundary reveals a gradual decline of typical Late Devonian elements and an equally gradual increase of plant types that eventually characterize the Early Mississippian. Furthermore, significant floral changes occur throughout the Late Devonian as archaic genera, that originated in Middle Devonian or earlier, disappear and new types appear that will become important in the Mississippian. Within the Mississippian too, the earliest floras contain elements left over from the Devonian, but show the origins or expansions of groups that ultimately were of great importance in the Pennsylvanian.

There were two major kinds of Pennsylvanian-age tropical swamps: coal swamps, represented by most coal seams, and clastic swamps, commonly associated with coal as roof shale or sandstone floras. Such environments were part of depositionally-complex lowlands that also included non-swamp habitats. In general, the Pennsylvanian-age coal and clastic swamps were dominated by non-seed (lower vascular, spore-producing) plants, and the mesic, non-swamp vegetation was characterized by seed-plant (pollen-producing) dominance. On occasion, some drier-site kinds of vegetation are preserved, providing a glimpse of other kinds of plants in the lowland regions of the tropical belt; among these are the earliest known conifers (Scott and Chaloner, 1983) in the Middle Pennsylvanian.

Upland floras (Chaloner, 1958), clastic substrate inhabitors (Dimichele et al., 1985), extrabasinal floras (Pfefferkorn, 1980), dryland floras (Havlena, 1960), seamforming Flöznahe and more distant Flözfremde (Daber, 1959; Havlena, 1961) mesophilous and xerophilous floras (Havlena, 1971), Callipteris-Walchia assemblage (Gothan and Gimm, 1930; Gothan and Remy, 1957), Megalopteris-Cordaites assemblage (Cross, 1977; Leary, 1981; Read and Mamay, 1964), Gigantopteris flora and Callipteris flora (Read and Mamay, 1964), early Permian floras (White, 1934), roof nodule floras (Stopes and Watson, 1907), the Mesophytic flora (Frederiksen, 1972), and “Permian” aspect (Elias, 1936a,b), are some of the terms that have been devised in recognition of the observation that many Late Paleozoic plant communities did not form coal swamp peats and their vegetational composition changed in response to global tectonism. The peat swamps clearly changed floral composition through time (Phillips, et al., 1974; Phillips, 1981; Dapples and Hopkins, 1969), but can be reasonably easily characterized, because “coal” swamp plants were commonly deposited in situ, or very near their sites of growth.

The paleobotanical literature contains frequent references to paleofloristic regions (often called “floral provinces”) for the Late Paleozoic. Halle (1937) published an early map showing the distribution of four different floral associations in Asia, viz. the Glossopteris Flora, the Angara Flora, the European Permo-Carboniferous Flora and the Cathaysia Flora. Gothan and Weylan (1954) completed this map by adding information from different parts of the world. In 1962 Wagner published a general map showing a Permo-Carboniferous Equatorial Belt separating the northern hemisphere Angara Flora from the southern hemisphere Gondwana Flora. The diagrammatic representation by Meyen (1969) shows the increasing diversifications of these floras with geological time. A later version (Meyen in Chaloner and Meyen, 1973, and in Vakhrameiev, et al., 1978) shows an early differentation into two major units, i.e., the Arcto-Carboniferous Kingdom and the Gondwana Kingdom. The former would be subdivided into the Euramerican and Angara areas, to be distinguished from the Early Carboniferous onwards.

In this chapter the, early Mesozoic land flora of the northern hemisphere is discussed briefly, and some of the subtle but significant evolutionary changes that took place during the time are summarized. Comments about climatic implications of the fossils are also included as well as comments about the environments of deposition of the fossils. Representative plant megafossils of the Early Mesozoic are illustrated. The Early Mesozoic as used here includes all of the Triassic and Jurassic Periods and the Neocomian Stage of the Cretaceous.

Study of the angiosperm pollen and leaf record contained in the sediments of the Potomac Group has elucidated the history of early angiosperm diversification. Although records of angiosperm pollen now extend back to the Barremian in a number of areas (see reviews by Doyle, 1977 and 1978), the Potomac sequence is significant because it provides the longest and most complete sample of data on early angiosperm evolution in a context where the pollen and megafloral records can be examined together with the sedimentology. This has provided an unparalleled opportunity for the development of insights into the ecological and adaptive aspects of the initial radiation of the flowering plants.

Studies of Tertiary plant megafossils typically have aimed at gathering paleoclimatologic and/or biogeographic data. The apparent similarities of Tertiary fossil plants to extant plants have made tempting the use of present plant/climatic relations to infer past climates. Additionally, plant-bearing, nonmarine deposits of Tertiary age are more widespread than older deposits, which allows stratigraphic and geographic distributions of Tertiary plants to be determined with greater accuracy than for older plants.