All of life, animals and plants and everything else, is related. Here is a tree showing roughly how the animals and plants mentioned in this fossils website are related to each other and when they lived. I have also added amphibians (such as frogs), birds and mammals (which includes human beings) so you can see where they fit in. There is a tremendous amount of life not mentioned here, fungi, worms, bacteria and much more. However, they don't fossilise well, and they do get very complicated! Click on a name in black to see the fossils. The names in grey are not links.
You can see that some of the lines are in black, and some in grey. The black lines show roughly when this particular animal or plant has a fossil record. The grey lines show how the animals or plants are connected.
Science consists of theories supported by evidence. Fossils are invaluable evidence for creatures that lived in the past, what lived when, and how they changed over time. However, you always get a fossil of a particular animal, not one animal turning into another! If you are lucky, you do get fossils of animals which have characteristics both of more ancient animals, and more recent animals. But the fossil record is necessarily incomplete, so the grey lines show some of this uncertainty. This does NOT mean that evolution may not have happened, just that we do not know everything about it. We have masses of evidence to support it as a scientific theory, no evidence to disprove it, and little evidence to support any other theory.
Some of the grey lines show animals that do have a fossil record, but are different to those that I have named. For example, the ancestors of mammals go back into the Palaezoic, while true mammals started in the Mesozoic. I have tried to indicate this by a change in line colour, and a slight kink in the line. Please don't think that at one point in time, all proto-mammals suddenly changed into mammals! This chart is a simplification of what actually happened.
Time on Earth is divided into Precambrian, Paleozoic, Mesozoic and Cenozoic.
Precambrian covers 88% of the Earth's lifetime. This is a very important time since life started then, and major divisions happened such as between plants and animals. However, little is known about it, as there are few fossils from this time. Many Precambrian rocks have been changed or metamorphosed, and others have been destroyed by erosion. Most life was small and without structures such as bone, so would tend not to fossilise. If the diagram above was to scale, everything interesting would have been squashed into the far right-hand side of the diagram. So the Precambrain is not to scale; everything else is.
The Paleozoic era had mostly marine life, although amphibians, reptiles and insects, together with land plant life, had evolved by the end. The Paleozoic era is divided into several periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Cambrian is known for the Cambrian explosion, when many new forms of life came into being. This does not necessarily show up well in the diagram above, since the fossils that you can easily buy or find tend to be later (except trilobites). But you can see that there are a lot of divisions around this point. The Carboniferous period is so-called because European coal-fields date from this time (and coal is carbon). The Permian is known for the Permian extinction, or more properly, the Permian-Triassic extinction event. Over 90% of marine species became extinct, as well as 70% of all land organisms. Scientists are still not quite sure what caused it. Perhaps gigantic volcanic events altered earth temperatures enough to have caused the catastophe. Trilobites became extinct then, although the other common fossil classes managed to survive, even if many individual species were destroyed.
The Mesozoic era is known as the Age of the Dinosaurs. It is divided into Triassic, Jurassic and Cretaceous periods. The Jurassic period is well-known because of a certain film! However, many of the best-known dinosaurs such as Tyrannosaurus rex lived in the Cretaceous period. The Mesozoic era ended with the Cretaceous-Tertiary extinction event (or K-T extinction event). This may have been caused by an enormous meteor that smashed into the Yucatán Peninsula. Approximately 50% of species became extinct, including all of the non-avian dinosaurs. You may have thought that all dinosaurs became extinct then, but it is now considered that birds are avian dinosaurs.
The Cenozoic era is the modern age. It is divided into periods as well, but these are short compared to the others, so I have left them out. The important eras for fossils are the Paleozoic and Mesozoic.
There is more than one way of classifying animals. Originally many scientists recognised that some type of animals are so similar to others that perhaps they were connected. It's easy to see that a zebra looks like a horse, for example. Other animals are different but still had similarities. For example, a horse doesn't look much like a wolf, but both feed their young on milk, and both have four legs. They are both mammals. Scientists gradually built up a classification system, and this is given in the fossil description in this website. However, this classification system was based on observation - what looked like what. Sometimes, they changed their minds! Gradually, scientists are building up a new system based on clades, or groups of animals which are descended from a common ancestor. This largely agrees with the existing classification system. The above diagram is a loose description of the clades of the fossils. For a more complete description of clades, click here.
© Jo Edkins 2007 - Return to Fossils index