Only a small percentage of the plants that ever lived left a record of their existence, surviving as fossils: mineralized wood, flowers in amber, leaf imprints in coal, or other indicators of life in an earlier era. Paleobotanists document this fossil record and use it to interpret the past evolution of plants.
Importance of Plant Fossils
Paleontology (or paleobiology) is the science concerned with fossils, the physical evidence of prehistoric life—including plants, animals, and microorganisms—on the earth. Paleobotany focuses on plant fossils, including algae, fungi, and related organisms, as well as mosses, ferns, and seed plants.
As most organisms decompose rapidly after death, their preservation in nature is a rare event. Most individuals are not represented in the fossil record, and even many species that must have existed have vanished without a trace.
As a branch of botany, paleobotany is of importance primarily because the record of fossil plants helps scientists understand the long process of plant evolution. Especially since the 1940’s, fossil evidence has helped to explain the origin of major classes of organisms, such as algae and fungi.
Researchers now also have evidence for the origin of the earliest vascular plants and the formation of reproductive structures, such as cones of gymnosperms (evergreen trees and relatives) and flowers of angiosperms (flowering plants). The location of fossils, including both their temporal (age) and their spatial (geographical) arrangement, is used to determine past climates.
The climates of the world have changed continuously as continents have shifted over the earth’s surface. For example, the location of coal deposits (which are the remains of giant tree ferns) in what is now Pennsylvania indicates the warmer climate that must have existed then.
Although perhaps most of the contributions to paleobotany have been made by professionally trained scientists with a solid background in geology, botany, and related sciences, amateurs have also made significant discoveries. Many valuable specimens of university and museum collections were made by people interested in paleobotany as a hobby.
How Fossils Form
|How Fossils Form|
The large-scale process by which plant parts become impregnated with minerals produces what has traditionally been called petrified wood. The modern term for this process is permineralization. Soluble carbonates, silicates, and other compounds infiltrate plant cells and the spaces between them.
Eventually, the mineral deposits may completely replace the naturally occurring organic matter, preserving the details of the plant’s microscopic architecture. Well known are the petrified forests of western United States, many of which are protected within national parks, such as Petrified Forest National Park in Arizona.
Being trapped in a sedimentary rock does not automatically guarantee that the organism will be preserved. The environment must be an anaerobic one—that is, one in which oxygen is excluded—thus preventing the decay that would otherwise result.
The process may be interrupted by the action of waves or other erosive forces which re-expose the developing fossil before the process of fossilization is completed.
Even after the process is completed, the well-preserved specimen may become distorted or altered in appearance because of the combined effects of time, pressure, and high temperatures that convert sedimentary rocks into metamorphic rocks.
As one would expect, the harder cells and tissues of plants are more likely to be preserved as fossils than are softer ones. For example, the thick-walled cells of wood and bark (called xylem) are more often preserved than are those of the pith (center of a stem) or cortex (found in stems and roots beneath the bark or outer covering). Other cells that are often fossilized are pollen grains and spores, both of which have outer shells that are highly resistant to decay.
Limestone and dolomite are among the most common types of rocks that form in such away that they trap plants and form fossils. Coal, a combustible sedimentary rock, is formed in much the same way as other rocks but is distinctive because the sediment involved is of plant, rather than mineral, origin. Within this matrix of plant-derived material is often embedded a variety of plant parts.
Special Types of Fossils
Two special kinds of rock that may contain plant and animal fossils are diatomite and amber. Diatomite is a rock that forms from the silica cell walls of a group of unicellular algae known as diatoms. Because silica is the same material that sand and quartz are composed of, it is unusually permanent.
Diatoms are found in both fresh water and salt water in great numbers and diversity. When they die, their cell walls accumulate underneath the water and become compacted over time into diatomite. The rock, itself formed by fossilization, may have fossil remains of various kinds of plants and animals preserved within it.
Amber, considered a semiprecious stone by gemologists and valued because of its beauty and distinctive appearance, is also of interest to paleobotanists. Amber is basically the fossilized resin produced by ancient cone-bearing evergreen trees. Sticky resins ooze from trees in response to injuries.
Before such resins harden, various small animals, floral parts, pollen grains, fungal spores, and other plant parts may become trapped and be preserved intact. Deposits of amber valued for their use as jewelry and as fossils are recovered mainly from two world areas: the Baltic region of northern Europe and the Dominican Republic in the Caribbean Sea.
Paleobotanists are sometimes challenged by puzzling specimens. Outright fakes are sometimes presented by pranksters, but more common are various mineral structures that bear a superficial resemblance to a plant. Such specimens are called pseudofossils. Mineral deposits called dendrites found in rock crevices bear a resemblance to fern leaves.
Acoprolite (fossilized feces) from the upper Cretaceous period in Alabama was initially mistaken for the cone of a conifer (cone-bearing evergreen tree); these specimens may be referred to as pseudo-plant fossils, as they are true fossils of animals.
During the formation of flint, bands are sometimes formed that suggest fossil mollusks or coral. Suspicious specimens require careful analysis by a specialist. In general, plants and animals, and therefore their remains, possess details and a characteristic regularity of form absent in pseudofossils.
Naming and Classifying Fossils
In order to prevent confusion, fossils, like living species, need to be named and classified in a consistent, systematic fashion recognized by paleobiologists throughout the world. The branch of biology devoted to the naming and classification of organisms is called taxonomy.
The same systemis applied to both living and fossilized plants. According to the system of binomial nomenclature, each species is given a scientific name consisting of two parts: the genus name followed by the species name.
The former is capitalized, whereas the latter is written in lower case; both are italicized. Often a name follows that belongs to the person who assigned that name. As example, the scientific name of an extinct redwood tree is Sequoia dakotensis Brown, while that of a stemless palm is Nipa burtinii Brongniart.
The ginkgo tree, Ginkgo biloba L., is considered a "living fossil". Known from the fossil record, it persists as a commonly planted shade tree. The initial following its scientific name is that of Carolus Linnaeus, the Swedish botanist who established this binomial system of nomenclature in 1753.
Species, living or dead, are classified using a hierarchical system(also by Linnaeus), which reflects degrees of similarity or dissimilarity to other species. All three trees already mentioned, because they are (or were) vascular plants, are assigned to the division Tracheophyta.
Within that division the redwood and ginkgo, because they produce uncovered seeds, are placed into the class Gymnospermopsida (naked seeded plants), whereas the palm, a flowering plant, is assigned to the class Angiospermophytina.
The plant fossil record is often used to establish natural relationships among various extant plant species and other taxa at higher levels. This is especially true of the vascular plants. In fact, the division Tracheophyta was established by A. J. Eames in 1936 to show the natural relationship between seed plants and ferns.
The basis for this new category was the discovery, earlier in the twentieth century, of Devonian fossils of a group of primitive vascular plants known as psilotophytes. They were recognized as ancestral to both ferns and seed plants. Previously, ferns and seed plants had been assigned to a separate division of the plant kingdom.
As the plant fossil record becomes more complete, further revision of the classification system becomes necessary to allow the system to more nearly reflect the true or natural relationships among the various categories. This is, at least, the goal of both paleobotanists and those who study modern plants.