Fossils: Window to the past

What is a fossil?

Fossils are not just a bunch of bones that make up an old dinosaur model. Fossils are the remains of an ancient organism or the traces of activity of such an organism. There are two types of fossils: body fossils and trace fossils. Body fossils include preserved remains of an organism (i.e. freezing, drying, petrification, permineralization, bacteria and algea). Trace fossils are the indirect signs of life that give evidence of the organism's presence (i.e. footprints, burrows, trails & other evidence of life processes).

The beginning

Life began in the sea. The earliest evidence of life on earth is of marine animals, during the Precambrian era. But there is only sparse evidence of life before the Cambrian era. The oldest known Precambrian rocks, found in Africa and Australia, are believed to be more than three billion years old. The fossils found among them are of the oldest known organisms on earth. Usually, fossils found in these old rocks are microfossils, such as elongated bacteria, Eobacterium and other water environment fossils. Bacteria represents the first stage of recognizable organized life and scientists have found well defined remains of algae and bacteria from nearly two billion years ago.

How old is this fossil?

The most common fossils are found in sedimentary rock. Sedimentary layers act as evidence of the changing climate or movement of the continents during the passage of time. The Law of Superposition or Steno's law states that in a pile of undisturbed sedimentary rock, the oldest bed will lie at the bottom and the youngest on top. Layers of strata in different locations may have the same composition but carry fossils of a different time period, therefore a technique of zoning or an index fossil is used. Index fossils are specific animals or plants that had a broad geographical distribution but existed for relatively short periods of time. These fossils allow geologists to establish a parallel between layers of sediment. Some excellent guide fossils are ammonites; each species lived for relatively short periods of time but had such a broad geographical distribution. They can be found today in stratigraphic rock layers often separated by great distances. The appearance of the same ammonite in different layers in different localities gives evidence that those layers were deposited at the same time. Each time period is marked by an abundant radiation of many new life forms or the mass extinction of past life forms.

Types of fossils

Fossils undergo a variety of different fossilization processes, depending on the characteristics of the particular organism. There are various levels of fossil preservation, each containing its own clues pertaining to the organism. Fossilization at the cellular level varies in all organic compounds since not all cellular types are equally resistant to decay and decomposition. The same hold at the tissue level, where some tissue types are more susceptible to fossilization. The other two kinds include the organ level and organism level which provide information in the field of morphology and biology of the ancient organism. Although there are an endless number of categories, we will focus on the broader mode of classification.

• Permineralization: is the occurrence of decay of organic substances and filling of mineral material into every cavity of the organism, still retaining most information about the fossil.
• Compressions: the two-dimensional compression which retains organic matter of the organism.
• Impressions: the two-dimensional imprint most commonly found in silt or clay, without organic material present.
• Casts & Molds: caused by deposits of sediment in cavities of organism, resulting in a three dimensional model.
• Compactions: preservation of organic material with slight volume reduction.
• Molecular fossils: deals with chemical data, preserving organic material, but providing no information concerning the structure of the organism.
• Freezing: ideal fossils that are rare, everything up to internal organs are preserved in cold storage.
• Amber: biological specimen that is encased in the hardened resin of a tree, in which the entire body may be preserved.
• Drying & Desiccation: fossils that have been thoroughly dried.
• Wax & Asphalt: almost as good as freezing, but with the usage of natural paraffin.
• Coprolites & Gastroliths: these categories deal with the indigestable remnants of meals.
• Trace fossils: typically formed when an organism moves over the surface of soft sediment and leaves an impression of its movement behind.

How are fossils found?

There are certain techniques that paleontologists might exercise to find fossils, but it mostly has to do with chance and luck. However, all paleontologists need a place to start. Paleontologists must have an extensive knowledge of the stromatolites, the different eras, and which environment was most suitable for certain organisms. With this information, a collector can eliminate certain localities in his or her search. For example, if a collector was interested in finding fossils of animals of ancient rocky shores, he or she would eliminate formations and beds in which remains are likely to be rare and poor. Paleontologists can also follow leads that other paleontologists or collectors have left behind in published reports. These clues help narrow the focus of the search. For example, marsh plants are most abundant in shales and sandstones between beds of coal, and coals are found in limy shales and massive limestones, many of which are the remains of ancient reefs. Using these techniques, the process of elimination, and perhaps some luck, paleontologists have retrieved many different fossils and animal remains from a variety of geological areas.

Conditions that lead to fossilization

There are many conditions that contribute to the formation of fossils. The most common include the possession of hard parts, such as a skeleton or shell, and a rapid burial after death. Besides being tough and hard, the organism must come to rest in a place where it can be buried before it decays or disintegrates. If the organism is not buried deeply and quickly, aerobic bacteria will reduce it to rubble. Water, given enough time, can also dissolve it. For this reason, fossils of some organisms are more rare than others. The skeletons that contain a high percentage of mineral matter are most readily preserved. The soft tissue that is not intimately connected with skeletal parts is least likely to be preserved. Other conditions that lead to fossilization include resting an environment that was biologically inert, areas that are receiving a large, steady supply of sediment (deltas of major rivers), and parts of the earth below sea level compared to those above the sea level. The ideal place to become a fossil is at the bottom of a quiet sea or lake where the prospective fossil is safe from damage and can be covered rapidly with sediment. Clay offers excellent conditions where the sediment protects the tissues and helps to exclude predators and solvent water.

What do fossils tell us?

Answers to this question can be broad and depend upon particular fossil locations. Nevertheless, there are certain general ideas that can be drawn from different fossils accordingly. Different fossils, depending upon how they were preserved, tell us different things. For example, fossils that are preserved in amber give us an extraordinary amount of information about the anatomy of that organism; since the organisms that are preserved in amber, mostly insects, are usually preserved intact without any disintegration of organs, muscles, and coloring. Even bones may tell a great deal about the soft anatomy. For instance, the area where the muscle attaches to the bone leaves marks that indicate size, shape, and functions of these varied organs. Also, the cavities and the the channels in skulls give us an idea of their intelligence, behavior, and their principle features. Certain parts of certain fossils can also tell us about growth, injury, disease, form, function, activities, and instincts. Fossils also record the successive evolutionary diversification of living things, the successive colonization of habitats, and the development of increasingly complex organic communities. Fossils can tell a great deal about their surroundings and the conditions under which they lived. Finally, study of fossils contributes greatly to the study of evolution. They are the only direct record of what has occurred in sequences of reproducing populations and in the course of the time on an evolutionary scale.