The word fossil means literally "dug up." More practically, they are remains or evidence of once living organisms. Fossils are of great value for geologists. They are used for stratigraphic correlation and for studying the evolution of life on Earth through time.
Organisms are made up of organic matter ("soft parts") and inorganic skeletal materials ("hard parts"). There are various modes of fossil preservation:
preservation of hard parts is the most common. Shell, bones, and other skeletal materials may be preserved mostly unaltered or in varying degrees of alteration. Over time shell and bone material may be recrystallized, obscuring the finer structure.
preservation of soft parts is rare because organic matter is subject to decay (oxidation). Soft parts may be preserved for hundreds of thousands or millions of years if rapidly buried in an oxygen poor environment, such as in an organic rich swamp, or if frozen in ice, such as wooly mammoths and the recently found "ice man" preserved in ice.
permineralization is the molecule by molecule replacement of an organisms original material for by stable inorganic materials. Petrified wood is a good example. Wood is organic matter that is subject to decay. But sometimes buried logs will have the woody organic matter replaced by silica (quartz) slowly as the groundwater seeps through the wood. Some of the woods structure is retained, such as the wood grain.
carbonization is another mode of preservation of organic matter. Sometimes as organic matter, say a leaf, slowly oxidizes, their may be a thin film of carbon left. There may also be a leaf impression.
molds and casts may be left if shell material dissolves before the mud that the shells were encased in. Molds are hollows in the shape of the shell left after the shell has dissolved. Casts are the mud left from the inside of the shell after the shell has dissolved.
trace fossils such as tracks and trails, burrows, and coprolites (feces) may tell us much about a fossil organisms habits and diet.
The Six Kingdoms of Life (viruses not included)
Living things have the ability to maintain themselves and reproduce. Anything from the lowliest, single-cell bacterium to the most complex vertebrates and towering redwood trees have that ability. Viruses, on the other hand are only quasi living. They are not much more than bundles of DNA. To survive, they force host cells to carry out these functions for them.
There are millions of species of organisms living on the Earth today. Many millions of others that lived in the past are now extinct. Scientists and natural philosophers (such as Aristotle in ancient Greece) have recognized that organisms can be organized according to similarities and differences.
The most basic difference between organism is the distinction between prokaryotes and eukaryotes. Prokaryotes are the simplest, most primitive kinds of single-celled organisms. They are the bacteria. They lack a nucleus to hold their genetic material. The DNA (genetic material) in a prokaryotic cell lies in a bundle within the cell. Bacterial cells are quite small (microscopic) and very simple. Eukaryotes are more advanced organisms. They may be single celled, like amoebas and paramecia that you may have seen under a microscope in elementary school, or they may be complex, multicellular organisms like humans. Essentially, all living things on Earth besides bacteria are eukaryotes. Eukaryotic cells have a nucleus that contains the DNA on structures called chromosomes. Eukaryotic cells also contain many bodies called organelles that perform specialized functions to aid in energy processing, etc. Eukarytotic cells are larger than prokaryotic cells, though normally also microscopic.
Many kinds of organisms must obtain their energy by breaking down (digesting/metabolizing) organic matter. Animals, fungi, and some prokaryotic and eukaryotic single-celled organisms do this. Many other organisms capture energy by the process of photosynthesis in which sunlight is used to produce carbohydrate (energy store) from carbon dioxide and water. Plants, algae, and some kinds of bacteria do this. Some organisms obtain their energy from dissolved ions by a process known as chemosynthesis. Some of the most primitive bacteria do this.
The tree of life is organized from the most general (kingdoms) down to the most specific level (species). Kingdoms can be subdivided into phyla (phylum is the singular); phyla into classes; classes into orders; orders into families; families into genuses; genuses include many species.
For example, some of the phyla of Kindom Animalia are Phylum Chordata (including the vertebrates), Phylum Arthropoda (including insects, spiders, and crustaceans), and Phylum Porifera (the sponges). humans belong to Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Primates, Family Hominidae, Genus Homo, Species Homo sapiens.
For now we will just introduce the kingdoms. Later, we will discuss the most important fossil groups from each kingdom.
Archeobacteria are characterized by living in extreme conditions. Some kinds of archeobacteria live in extremely hot water, such as at hot water vents at the midocean ridges called black smokers or in hot springs. Others live in very acidic waters, or very salty waters, or in environments devoid of oxygen. They are considered primitive bacteria. Some of these conditions are similar to condition under which life may have first arisen on Earth. Perhaps the archeobacteria include the oldest kinds of organisms on Earth.
The oldest known living things on Earth are a type of eubacteria. The oldest fossils yet found are 3.5 billion year old cyanobacteria. These are photosynthetic bacteria formerly known as "blue-green algae." They are not true algae. True algae are eukaryotes. By about 3.2 billion years ago the fossil record contains the first large fossils called stromatolites. These formed from filamentous cyanobacteria that trapped sediments in mats that often grew into mounds.
(Under some schemes all bacteria are included in one kingdom)
Protists are mostly single-celled (mostly microscopic) eukaryotes, though some simple multicellular organisms are also lumped in with the protists. Some protists secrete a shell and are important contributors to the fossil record.
Plants are multicellular organisms that produce their own energy and via photosysnthesis. They are primary producers on the land. They include everything from mosses to ferns to conifers, like pine trees, to broad leaf trees, like oak trees, to grasses. Plants probably arose during the lower Paleozoic.
Fungi are multicellular and single-celled organisms that break down dead organic matter within their cells. The multicellular fungi are plant-like in that they lack senses and the ability to move about like animals, but they are incapable of photosynthesis. Mushrooms belong to the Fungus Kingdom. Fungi do not fossilize easily so they are poorly represented in the fossil record. The oldest fossil fungi are from the Cambrian Period.
Animals are multicellular organisms that ingest organic matter to obtain energy and materials for life. Animals run the gamut from simple sponges and corals, to complex invertebrates like insects and crustaceans, to complex vertebrates like reptiles and mammals. Animals arose during the late Proterozoic, around 600 million years ago or so.
Taxonomy is the classification of living organisms. Taxonomy groups living things into Kingdoms, which are subdivided into phyla, classes, orders, families, genuses, and species. This was first established by Linneus in the 18th century.
The traditional means of organizing the various genera is with the phylogenetic tree or the branching tree of life. The groupings of similar genera implies relationships among organisms.
cladistics is a method of classification that assumes that living things have ancestral relationships derived from their evolution. A clade is a group of organism that all share a common set of "primitive" traits (those acquired from some common ancestor).