19.1 The Fossil Record

Fossils and Ancient Life

The best way of gathering information about extinct (dead) species is through fossils. However, fossils vary in size, type and quality of preservation, and only small portions of populations have been preserved.

Fossils can range from large, intact animals to bacteria to growing embryos to pollen grains. Sometimes, fossils are just small parts of organisms, or sometimes footprints or dropping, known as trace fossils. Most fossils are made in sedimentary rocks, but others are conserved by other means.

There is a common process in which fossils are made and preserved in sedimentary rock. Firstly, sediments, which can be clay, silt, mud or sand in water bodies, or compact sand in deserts, collect around the dead body of an organism. Then they begin to cover the dead organism, while the tissue of the organism often decays. If the parts of the organism that have not decayed are saturated or replaced by minerals, the parts will become fossilised. Sometimes, organisms are covered up too quickly for major decay to happen, resulting in fossils with the tissue still left. After long periods of time, the pressure exerted on the sediment and remains will form rock in the place of the remains.


Figure 19-1a, Fossilisation

There may be many things that are not part of the fossil record, and unknown, but there really is a lot of information for palaeontologists, who are people who study fossils to find information about the past, to use. For example, they can see the physical structure of these old organisms, their environment and how they lived. They can also infer how species have evolved by comparing fossils to each other and to living species. We can also find out how animals moved, the conditions of the environment and sometimes even complete ecosystems of the past.

Dating Earth’s History

There would not be much use for fossil if we had no way of telling how old they were, but luckily we have several methods of doing this today.

One of the more basic ways is relative dating, which is comparing fossils and the sediment layers around them. This allows scientists to find out if a fossil is older or younger than another, but no exact dates. For this method to work, we need index fossils, which are particular fossils that we can use to compare other fossils with. Index fossils must be species that existed for a quite short period of time, but can be found in many areas around the world. Trilobites are a common index fossil.

To find out the more specific time period the fossil is from, we can use radiometric dating. Radiometric dating uses radioactive isotopes of common elements, and the way they decay into stable isotopes. We look at something called a half-life, which is the time it takes for half of the radioactive material to decay, when we use radiometric dating. Since different elements have different half-lives, different elements are looked at depending on the age of the fossil. One common element used in radiometric dating is carbon-14, which exists in small amounts in the atmosphere and is therefore taken in by plants during photosynthesis. Animals that then eat the plants or other animals that have eaten plants get this isotope into their system, and when the organism dies it stops consuming this element. This is the reason behind why carbon-14 is so useful for most types of organism. The half-life of carbon-14 is about 5,730 years, which means that only organisms that existed during the last 60,000 can be dated using this isotope. Older organisms can be dated by relative dating, or other isotopes such as potassium-40, uranium-238 and rubium-87.

Geologic Time Scale

Using both relative and absolute dating scientists have created a timeline of the Earth’s history, known as the geologic time scale. The geologic time scale is arranged into eons, eras and periods.

Figure 19-1b, Geologic Time Scale

Using relative dating, index fossils and rock layers, scientists have found large differences in the fossils between particular rock layers. These differences were then used to mark different times, which were then given specific ages using radiometric dating. However, the geologic time scale is always changing as new information is found.

The geologic time scale is divided into four eons: the Hadean Eon, the Archean Eon, the Proterozoic Eon and the Phanerozoic Eon. The Hadean Eon is the time between the formation of Earth and about 4 billion years ago, and contains mostly the formation of rocks. The Archean Eon comes after the Hadean and lasts until about 2.5 billion years ago, and contains the formation of the first living organisms. The Proterozoic Eon lasted between about 2.5 billion years ago until about 542 million years ago. The Phanerozoic Eon began after that and continues until today. As you can see, the different ages on the geologic time scale are irregular in length. The Eons are arranged into Eras. The Phanerozoic Eon is split into three eras; the Paleozoic, the Mesozoic and the Cenozoic Eras. These are then further divided into periods, which can be between 2 and 100 million years long.

Many of the ages in the geologic time scale are named after places or characteristics of that time. Names were given to the divisions in the Phanerozoic Eon before the later ages were discovered, so the Proteozoic, Archean and Hadean Eons are called the Precambrian Time.

Life on a Changing Planet

Today, we have an idea of the environment of many regions of the Earth. However, these regions have not always had these environments. The Earth has been affected by many small and large changes throughout its existence.

For example, the climate has changed several times, such as in the ice ages and the Mesozoic heat wave. These changes were not actually such huge temperature changes, but they still had major impacts on the planet. The geological forces have also had a great impact on the Earth, by forming mountains and volcanic activity. Changing winds and ocean currents have affected both the environment and certain physical features of the planet, such as the mountains. Another major factor of the biological aspect of the planet is the slow movement of continents. A long time ago, all the continents were stuck together, but over billions of years they moved to the point where they are today, and they are still moving. This has affected the ocean currents and the distribution of organisms. One accepted explanation for this is the theory of plate tectonics, which is the idea that parts of the Earth’s surface move over time. Another thing that has affected both the physical and biological aspects of the Earth is comets and meteors that have hit the Earth.

Sometimes though, it is actually the organisms that change the physical features of the Earth, rather than the other way around. For example, when the first photosynthetic organisms came into existence, they stored carbon in their bodies which lowered the carbon levels in the atmosphere, causing the temperature of the Earth to decrease. There are more examples of this, in which organisms have altered the land, water and atmosphere. Organisms are affecting the environment today as well, by for example the nutrient cycles.




19-1a Fossilisation. Digital image. WWW.TRILOBITA.DE. Web. 12 Dec. 2012. <http://www.trilobita.de/english/fossilize.htm>.

19-1b Geologic Time Scale. Digital image. Geologic Time Scale Notes. Web. 12 Dec. 2012. <http://home.comcast.net/~mjmayhew42/Biology%20notes/geologic_time_scale_notes.htm>.