Go to the key  ---INDEX to SECTION on FOSSILS and TIME--- Major Phyla
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The purpose of this document is to familiarize you with some of the major groups of fossils which have proved important in determining relative time (in allowing us to establish a sequence of events for the history of the Earth).
WHAT IS A FOSSIL?
A fossil is any evidence of past life. The branch of geology that studies fossils is called paleontology.
Commonly, paleontology is divided into three areas: Invertebrate paleontology is, by far, the most useful of the three. Not only do invertebrates have a longer geological record and occur in much greater number than vertebrates, but they are most common in marine sediments which themselves are much more widespread than sediments that formed on land (terrestrial sediments).

Vertebrate paleontology is significant because it provides information on the evolution, distribution and habits of the group of organisms to which we belong; and because vertebrates have been the most successful large* animals to adapt to land.

Paleobotany. Despite occasionally spectacular finds, the fossil record of plants is not as well known as that of animals, and in general, plant fossils have not yet turned out to be as useful geological tools as animal fossils. Palynology, the study of plant pollen has been extremely useful in the study of past ecosystems.
(* The Phylum Arthropoda with its millions of species is by far the most successful group to adapt to land)
 
 

FOSSILIZATION

There are many different ways in which an organism can leave evidence of its presence.
Direct evidence means that we have some part of the animal or plant itself.  Rarely, we find preserved the remains of the entire animal. In one famous case, a baby mammoth (an extinct relative of elephants) was found frozen in the permafrost (permanently frozen ground) of Siberia, complete with the mouthful of buttercups he was chewing on when he fell to his death.  Usually, however, we are not that lucky.  Most often, but not always, the soft tissues decay; the picture is that of actual mammoth hair. Usually we find hard parts such as shells, bones or teeth. Often, these hard parts are embedded in solid rock and have themselves turned to rock, or petrified. Petrification can occur in several ways. If the original material is still there, but all the voids have been filled by some mineral, we speak of permineralization.  If the original material has been replaced by another, we call it replacement. Sometimes, only a thin film of organic matter remains, as we often see in the case of leaves, graptolites or the remains of some fishes. This is carbonization.
 
Two examples of 
carbonization. 
Both are from the 
Green River shale.

 
 

Not all evidence includes the actual remains. Indirect evidence includes molds, casts, imprints, coprolites and tracks. Molds are the hollows left in the surrounding material (the matrix) after the remains of the organism have been dissolved or removed. Some of the most famous historical molds were discovered in Pompei where the remains of fleeing Romans were entombed in the vulcanic ash of the 79AD eruption of Mt. Vesuvius. Archeologists were able to fill these molds with plaster and these casts provide us with a glimpse of the people as they fell and died from suffocation. This process occurs when a different mineral fills in molds and preserves a replica of the original. Leaves and other fossils often leave their imprints on rocks and, in a way, tracks are nothing but a special kind of imprint. In some cases we only know that an organism existed because we have found its tracks.  Finally coprolites, (fossilized feces) have given us much information about the digestive systems and dietary habits of many an extinct animal, as well as for early man.

Factors of Preservation

Some species are represented by the remains of millions of individuals, while the extistence of another species may have been recorded by only a few, or even a single individual. Still others have only left their tracks behind, and some vanished without a trace. What is it that makes some species such good candidates for preservation while there is no record of others?  Preservation of fossils depends on a number of factors.  Some are obvious. It stands to reason that a common species with lots of individuals and that is geographically widespread over lots of environments, and exists for long geologic times will be more easily preserved than a rare species that lives in a very restricted environment.

If an organism has hard parts, it will stand a better chance for preservation. Bones and teeth, shells and wood preserve more easily than the body of a jellyfish. It also helps to bury an organism as rapidly as possible, preferably in fine-grained sediment. This keeps carrion feeders, bacteria and other decomposers from destroying the remains and also protects them from the ravages of weather. This can happen in floods, vulcanic ash falls, dust storms, in burial in permafrost, sinkholes, bogs or tarpits, etc. One of the richest vertebrate deposits ever to be found was excavated from the tar pits of Rancho La Brea in Los Angeles, CA.  Fossilized human brain remains have been found in springs in Florida. Of all the environments that have preserved remains however, marine sediments remain the richest source of fossils.

USE OF FOSSILS

To many, fossils are curiosities and are sometimes aesthetically pleasing. But what do fossils actually tell us?

First of all they are records of past environments. If we find sharks' teeth in a layer, we can reasonably assume that this layer formed in a marine environment. But the information is much more subtile than that.  For example, fossil pollen studies are a standard tool of climatalogy.  And cypress pollen will tell us a different story from that of oak or spruce pollen. Given enough data we can reconstruct entire ecosystems, past climates and infer geography.

Secondly fossils are the documents of evolution. The gene pool of any species changes only slowly. Because the human lifespan is short, there is little chance for watching evolution in action. Even if we could, changes are so gradual that we may not even be aware of them. Fossils give us a snapshot of what life was like when they were flourishing. Even though these pictures may be more blurred than we like, and lack much of the color and detail of the living things themselves, they nevertheless allow us to see how life has changed over time and to realize that we and all the other species alive now are but the latest in a long succession of ever changing forms.

Thirdly, fossils are time markers. Because evolution never repeats itself, if we know the succession of life forms, then we can create a time line based on this sequence. Much of the history of the earth is based on the powerful concept that every interval of time is characterized by its unique life forms.  Conversely, once we have established this sequence, we can fit any fossil we find into its proper time interval, based on its stage of evolution. Because all (nearly) fossils were entombed into the rock layer as it was forming, if we know the age of the fossil, we know the age of the rock layer.  Some fossils are of more help in this than others. We call such useful fossils index fossils. To be a good index fossil, a fossil must be common, easily recognized (even by geologists) geographically widespread, and rapidly evolving.

GEOLOGIC TIME

Based on this changing succession of life, we have subdivided the Earth's history into time segments characterized by the life forms that existed during those time intervals. Below is an abbreviated version of the Geologic Time Scale which is a fundamental calendar for all geologists.
EON ERA PERIOD Based on the great changes in the record of life, the Earth's history has been subdivided into four major intervals of time, eons, characterized principally by the life forms that existed at that time. These eons have been further subdivided into smaller units, eras, periods and epochs. This calendar for the earth is referred to as the Geologic Time Scale, and it is this time scale to which geologists usally refer when discussing its history. Remember that although such a scale (like this one) is often labeled with absolute dates, it is based on fossils. Fossils are powerful tools; and in many ways they are the only reliable ones available to a geologist when it comes to unravelling the history of the earth. Still, while we may be able to ascribe an extremely accurate relative age to a fossil, a layer or an event, we cannot know how many thousands or millions of years ago that event took place. Such determinations, called absolute dates, are done by using techniques which involve physical or chemical changes which occur at known rates. The most powerful of these techniques of absolute dating are radiometric that use the decay of radioactive atoms to measure time. 
Phanerozoic 
700 MY-PRESENT
Cenozoic 
66MY-Present
 Quaternary      1.8 MY-Now
Tertiary          66-1.8 MY
Mesozoic 
248-66 MY		 Cretaceous     144-66 MY
Jurassic          202-144 MY
Triassic          248-202 MY
Paleozoic 
700-248MY 		 Permian          286-248 MY
Pennsylvanian 320-286 MY
Mississipian    360-320 MY
Devonian        408-360 MY
Silurian          438-408 MY
Ordovician     505-438 MY
Cambrian       544-505 MY
 
Proterozoic 
2.5-0.7 BY		 Late 1.3-0.7 BY Vendian**            700-544 MY
No Periods older than the Vendian
Middle 1.6-1.3 BY
Early 2.5-1.6 BY
Archean 
3.9-2.5 BY		 Late 3.0-2.5BY
  Middle 3.4-3.0 BY
Early 3.9-3.4 BY
Hadean 
>3.9 BY		 No Hadean Eras >3.9BY
 **NOTE: Although the Vendian (=Ediacaran) is usually considered a period of the Late Proterozoic Era, some authorities consider it the oldest period of the Paleozoic.
 

Fosssils have thrilled, excited fascinated and challenged countless people. Their aesthetic beauty alone has assured them a place in our homes and hearts. Everyone who has ever seen the skeleton of one of the large dinosaurs has stopped in awe. The mere existence of fossils has sparked controversy; and understanding how they formed, lived  and their relationship to other fossils has given us a clearer glimpse of the past. Without them we would have but a fraction of the knowledge we posess about the earth.
 
 

CLASSIFICATION OF FOSSILS

Living things and the world they inhabit are the product of four billion years of evolution. Life is an organic whole whose components are all related to a greater or lesser degree. In order to express this inter-relationship, we use the same classification for fossils as for living organisms. This classification system (=taxonomy) was devised in the latter part of the XVIIIth Cent. by Carl von Linne, a Swede whose great invention was twofold: first, binomial nomenclature (the naming of each species with two unique names, often called the scientific name). Secondly, the Linnean classification scheme indicates the relationships between all species that were and are alive, and allows us to sort organisms into appropriate categories based on the degree of relationship rather than by differences. Linne had no concept of genetics in the modern sense, but his classification method foreshadowed this century's discoveries that leave no doubt that all organisms are genetically related.
Why bother with complicated foreign scientific names?
We use binomial nomenclature, (or scientific names) rather than common (every day) names because common names lead to confusion. Suppose we want to talk to someone about a salamander. Most people know what that is, or at least they think so. Unfortunately that is often not the case. For example, in most of the english-speaking world a salamander is an amphibian, related to newts and frogs. In the rural SE US, a salamander is a rodent (related to rats) that burrows and makes mounds in sandy soil. (In fact salamander is a corruption of "sandy mounder"). In other parts of the US this rodent is known as a pocket gopher. In the South and West, though, a gopher is a tortoise (land turtle). Such confusing examples are all too common.

Also, when people interact commonly with another species they tend to use lots of names for that same species; think of the variety of names applied to horses or dogs. If they do not, they tend to lump species that have a similar appearance under the same name. " What kind of tree is this?   That's an oak...".  So all of these common names interfere with our ability to communicate.  What makes binomial nomenclature so useful is that each species has a unique pair of names and that removes any ambiguity. Also, there are no common names for fossil species or for those that disappeared before man began to record his descriptions of the world. Thus taxonomy allows clarity in communication.

Taxonomy also clarifies relationships. It is generally accepted that life originated but once on this planet. Because all organisms arose from this original life, they are all related and that means we can put them into taxa (=groupings) which express their degree of relationship. These groupings or categories are hierarchical, meaning that they are arranged in a series of ever larger, broader groupings. The only real grouping is the species,commonly defined as a group of organisms that interbreed naturally and produce fertile offspring. All other categories (=taxa, the plural of taxon) express only our knowledge as to the degree of relationship between species, and therefore, as our knowledge changes, so does taxonomy.

This functional definition of a species creates a special problem for the paleontologist, because we can't tell how fossil organisms reproduced. We assume that if a group of organisms is genetically different enough to be a species, these differences will be large enough to affect the physical appearance of the organisms, and these differences will show up in the preserved parts. Although this makes identification and classification of fossil species occasionally difficult, it works well enough in practice to sort out the lines of evolutionary relationships and to put together a classification scheme valid for both modern and fossil species.

Linnean classification
In the Linnean classification, the fundamental taxon is the species. Closely related species make up a genus. Related genera in turn make up a family. Families make up orders and orders, a class; classes make up a phylum and phyla in turn, a kingdom the highest level of classification commonly used in taxonomy.
                                                                                            Kingdom
                                                                              Phylum
                                                                Class
                                                  Order
                                    Family
                      Genus
              Species.
Each of these taxa (levels) can be further refined by using such prefixes as sub- or super-, and detailed classification schemes may use other specialized terms such as tribe and cohort. In the main the basic system outlined above will serve our purposes well enough.

A diagrammatic representation of the classification of two species: A man and his dog.
 
 
 
 
 
 
 
 
 

As stated before, the fundamental unit of classification is the species. Every species is identified by a unique combination of two names (binomial nomenclature): the generic (genus) and specific (species) name. The generic names comes first and is always capitalized, while the specific name is always lower case. Both names must always be underlined or italicized. for example, the scientific name for a dog is Canis familiaris or Canis familiaris.
 
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