Depositional Environments Of Fossil Plants

Fossil plants are found in almost all regions of the Earth, the most notable exceptions being recent volcanic islands or in rocks that have been extensively metamorphosed (FIG. 1.9). Marine plants, such as various forms of algae (Chapter 4), are generally found in rocks deposited in marine environments (e.g., nearshore deposits, carbonate shelves, etc.). Although land plants are occasionally found in marine rocks, generally, wherever terrestrial sedimentary rocks occur, there is a good chance that fossil plants will be found in them. Sedimentary rocks are those formed by the accumulation of rock particles derived from the weathering and mechanical abrasion of existing rocks. The great majority of sedimentary rocks are formed by deposition of particles in water, but wind deposits (e.g., eolian sands, loess) can also form, and rock breakdown can occur by chemical weathering, with rock components being released into solution, later to solidify at some other place. Plant parts are typically fossilized, then, in areas where sediment is accumulating. The delta of a river is just such a depositional environment. As the course of the river constantly shifts, channels are abandoned and new ones initiated; natural levees are destroyed during flooding, and new ones built up later. A meandering river cuts into the bank on the outside of each meander, and deposits sediment on the inside of meanders, often covering plants growing along the water. When the river breaks

FIGURE 1.9 Exposed rocks in Antarctica. Zone of dark red rocks are volcanic and lack fossil plants, including microfossils.

through a levee, a rush of water and sediment, called a splay deposit, can rapidly cover the adjacent floodplain, inundating the plants growing there. Associated with the deltaic system are abandoned stream channels, often referred to as oxbow lakes, and vegetation growing along the banks of these abandoned channels may be undisturbed for some time. If a subsequent flood destroys the levee, knocking down trees and other plants growing on it, these plant parts can be carried to abandoned channels and other places where a high concentration of sediment would bury the plant fragments and fill in the oxbow. As might be expected, plant parts carried for great distances would tend to be fragmented and shredded, and those deposited close to the place of growth would be less distorted and better preserved. Plants that become preserved at the same locality where they were growing are termed autochthonous (e.g., many Pennsylvanian coal ball deposits), whereas those assemblages that have been transported are termed allochthonous. Preservation of whole plants or plant parts (usually stems and roots) in growth position is termed in situ.

The plants that once made up a community, together with the other organisms in the ecosystem, are preserved in the Earth's crust in a variety of ways, and different kinds of physical and chemical processes were involved during the process of preservation (FIG. 1.6 ) . Moreover, various environmental settings and depositional processes also result in fossils that occur in a variety of forms. Taphonomy is the study of all the processes occurring between the time the organism died and its discovery today as a fossil. These include burial by sediments of some type (e.g., sand, fine mud, ash, etc.), and diagenesis, which is defined as all the chemical and physical changes to the sediment (and the fossils within

FIGURE 1.10 Cuticle preparation of the seed fern Blanzyopteris praedentata showing numerous trichomes extending from the surface. Bar = 1mm.

it) as it is converted into rock (Gastaldo, 1989; Gee and Gastaldo, 2005).

Because of the countless ways in which plants are preserved, the paleobotanist must employ different techniques to extract the maximum amount of information from fossils. For example, when a paleobotanist finds a fossil leaf, it would first be compared to modern leaves, based simply on the overall size and shape, that is, the morphology of the leaf, to identify it. This can include describing the shape and distribution of teeth on the margin of the leaf, if present, and the shape of the base of the leaf compared to the tip, as well as the length and shape of the petiole. Next, the discoverer might examine the pattern of veins in the leaf—the venation, followed by a microscopic examination of the types and distribution of stomata (pores for gas exchange) and other structures on the surface, such as hairs (trichomes) (FIGS. 1.10, 1.11 ),ortrichomebasesifthehairsthemselves are no longer attached to the leaf. Still later, the paleobota-nist might study the ultrastructure of the cuticle (the waxy

Uniseriate Trichomes
FIGURE 1.11 Multicellular, uniseriate trichomes on Blanzyopteris praedentata cuticle. Bar = 400 ^m.

covering on leaves), or the molecules that remain part of the leaf after diagenesis (geochemistry). It is possible to determine the proportion of carbon isotopes (13C versus 12C) in many fossil plants, and to use these to reconstruct paleoenvironment or the type of photosynthetic pathway (C3 versus C4 photosynthesis) employed by the plant. And in the future? Perhaps paleobotanists will be able to extract information that reveals details about biochemical pathways, developmental mechanisms, and families of genes involved in response to parasites or herbivores that attacked the leaf surface, all from a fossil plant leaf! Although there are numerous variations on the ways in which plant fossils are preserved, there are a few basic types which we discuss later. It is important to keep in mind, however, that all preservation types can intergrade, or a fossil plant may be preserved in more than one way, for example, a compressed plant with a stem that is partially petrified. Finally, there may be certain structures that appear to be an organism, but are not of organic origin. One of the most common of these are dendrites (FIG. 1.12).

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  • Hildibrand
    What is plant depositional plant?
    2 years ago

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