s What is a fossil? - Hull Museums Collections

What is a fossil?

Detail of Starfish

Fossils are the remains of creatures and plants, which lived on earth many millions of years ago, which have become trapped and preserved in rock. They come in a variety of sizes from micro-organisms to large skeletons and trace fossils.

The word fossil comes from the Latin 'fossilis' meaning 'dug up', the ancient Greeks used the term to describe any distinctive materials dug up from the earth, including minerals, rocks and organic remains. It was not until the nineteenth century that the word began to be applied only to the remains and traces of pre-existing life preserved in rocks.

How fossils are formed?

FishWhen marine animals or plants died their remains accumulated on the sea floor where they were covered in mud, silt or sand. Over a very long period of time these sediments became compacted into sedimentary rocks encasing the animal or plant remains with-in the rock. Not all life became fossilised; most simply disintegrated away before it could be preserved.

When land animals or plants died the soft parts of their bodies, such as skin and internal organs, usually decomposed or were eaten by scavengers. However, if the hard parts of their bodies, such as bone, shell or wood, were quickly covered by water, sediment or volcanic ash, they might have been preserved. Teeth are commonly found because they are the hardest parts of animals and therefore more likely to be fossilised.
Plant (Ptilophyllum pectin)

Although bone, shell and wood are hard components they are full of very small air spaces. Upon burial, water would seep into these spaces and deposit minerals. Often, over millions of years, all of the original bone or shell would dissolve away leaving a complete mineral replacement set in the surrounding rock. The organism remains are then said to the petrified, literally meaning 'turned to stone'.

Trace fossils

In some cases the entire animal will decay away leaving a 'mould' that is filled with sediments or minerals that form a natural 'cast'. There also cases where footprints made in soft ground have created moulds that are later filled with sediments, making casts. These are known as trace fossils because they indicate the activity of an animal or plant and therefore the presence of former life. Other trace fossils include burrows and root tunnels.

Tridactyl dinosaur footprint

Fossils are not only found in solid rock but have been discovered trapped in tar pits, amber and even in ice, preserving flesh, hair and sometimes stomach contents. Woolly mammoths, for example, have often been discovered trapped in ice and in 1997 an entire mummified woolly mammoth was found in the Siberian permafrost.

The importance of fossils

Very few individual organisms are preserved out of the billions that have lived throughout geological time, and it is rare to find a complete specimen. Fossils are important to both palaeontologists (scientists who study ancient life) and geologists (scientists who study the history of the earth) because they provide vital clues about ancient life forms, ancient environments and links to the ancestors of modern organisms.

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