A Basic Introduction To Fossils
How They Are Formed
Fossils are found in rocks that were
usually laid down under water (sedimentary rocks), such
as shale and limestone. Some fossils, such as woolly mammoths from Siberia, are very well preserved (they
still have meat that is just about edible), but in most cases
only certain hard parts such as bone or shell remain, and
these have been partially or totally replaced by minerals
that have seeped in from the surrounding sediments.
Fossilization is thus quite a rare event since only a
small proportion of the creatures that die (especially those
that live on land) are likely to be enclosed in sediments
before they are eaten or decay completely. Other organisms, such as worms and flowers, have few or no resistant
hard parts and so are less likely to be found as fossils.
The diagram below gives an example of how a fossil forms.
1. An Ichthyosaur dies 150 million years ago and
its skull falls to the sea-floor.
2. Layers of sediment are deposited and the skull is
crushed by the weight of the overlying material.
The sediment hardens into rock. Components of
the bone dissolve away to be replaced by minerals
seeping from the surrounding sediment.
3. Earth movements raise the rock above sea-level.
4. Erosion exposes the fossil on a beach, where it
is found.
FOSSILS AND TIME
The earliest fossils are those of tiny microbes
which are about 3400 million years old but fossils only
start to become relatively common in rocks that are
younger than 600 million years. The age of a rock, and
hence of the fossils it contains, is deduced by measuring
the rate of radioactive decay of certain chemicals in different rock beds. The most recent fossils are several thousand
years old; remains that are younger than this have usually
not been replaced by any minerals and generally belong to
types of organisms that are still alive today.
The Geological Timescale on Page 1 shows the major divisions into
which geologists have divided the history of the Earth and
the main events that have taken place. Basically, geological time is divided into four main sections called Eras.
These are further divided into the Periods shown in the table. Although the
names seem daunting they are just a convenient way of
referring to different parts of the Earth's history.
Most of the Earth's history, from its formation
4600 million years ago to 570 million years ago, belongs
to the Pre-Cambrian Era. It was towards the end of this
time that the first simple life appeared on this previously
barren planet.
The succeeding Palaeozoic Era (570 - 232 million
years ago) saw an abundance of life (and hence fossils).
The seas were inhabited by creatures such as trilobites, and
later by a variety of fishes. The first land life appeared
roughly halfway through the Era and later there were huge
coal forests inhabited by insects, amphibians, and
finally reptiles.
The Mesozoic Era (232 - 65 million years ago) is
appropriately called the 'Age of Reptiles'. This was the
time when dinosaurs dominated the land, pterodactyls
ruled the air, and a variety of reptiles inhabited the seas.
The first mammals appeared near the start of the Era; birds
and flowers roughly halfway through. In the seas trilobites
were extinct and ammonites became dominant instead.
The end of the Mesozoic Era saw the extinction of
the dinosaurs, pterodactyls and ammonites. Instead,
mammals and birds became more important. Throughout
the Caenozoic Era life on Earth grew increasingly similar
to that of today.
Man finally appeared near the end of the Caenozoic,
in the Quaternary Period.
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Fossil Facts - What They Are - How They Are Formed - Where To Find Them
Dinosaurs - Cetiosaurus - Compsognathus - Diplodocus -
Edmontosaurus
Euoplocephalus - Gallimimus - Giganotosaurus -
Heterodontosaurus - Hypsilophodon
Iguanodon - Parasaurolophus -
Proceratosaurus - Protoceratops - Rebbachisaurus
Stegoceras - Stegosaurus - Titanosaurus - Triceratops - Tyrannosaurus Rex (T Rex) - Velociraptor
Fossil Finds - Cyprus - Folkestone, UK - Herne Bay, UK - Lake District, UK
Norfolk, UK - Thanet, UK - Walton, UK
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