A fossil can simply be described as "any evidence of past life". Fossils are relatively rare because most organisms are 'designed' to decompose once they die, and so miss out on any long-term preservation. There are however occasions when for one reason or another an organism will fossilise in an environment conducive to the process.

For an environment to support fossilisation, the dead organism should be exposed to the following conditions:It is particularly common for small single-celled algae and pollen to be preserved because pollen grains in particular have a hard decay-resistant coat called an exine. Other commonly fossilised plant parts include floral reproductive organs, stems, wood, and leaves.

You might be wondering what exactly constitutes a fossil? Many people recognise the impression-types, however these are one of several fossil types recognised by palaeobotanists:

Impression-type

These are the most commonly recognised type of fossil. They are created when plant parts are covered in a layer of mud, rock, or ash. The ash may become wet, and 'set' like concrete, preserving the shape of the plant. The plant itself still degrades, and no anatomical material remains. These fossils give an idea of the morphology of the plant. They are extremely common.

Compression-type

These are the result of the plant material being buried under pressure. The carbon (C) in the material changes its structure and becomes coal. The details of the plants often remain, including cellular details. Fossils composed of black coals are of the highest quality. Most fossils date back to the Carboniferous period of 345 million years ago.

Petrifaction

The word petrifaction means 'turned to stone', and that is literally what happens with this type of fossil!

The carbon atoms in the plant material are exposed to minerals in the soil, and are gradually exchanged with calcium (Ca), magnesium (Mg) and/or silicon (Si) atoms. Sometimes other minerals are involved. The complete cellular detail is preserved, but can be difficult to date (see the section on dating).

Amber

Amber fossils are composed of compressed and preserved sap from trees - usually conifers. The sap flows over plant parts and they are completely preserved, including their DNA (deoxyribonucleic acid). A lot of amber was produced in the Jurassic period of 135-190 million years ago. Amber fossils are of very high value in terms of what they contain. Naturally they hold much potential for reviving extinct species owing to their DNA content.

Coprolite

Coprolite is preserved animal faeces, and can be a petrifaction, impression or compression. Coprolites often contain preserved plant matter and sometimes provide 'glimpses' of former plant communities, as several plants from the area that the animal was grazing in may be preserved. Unfortunately animals don't eat all of the plants of any one area, and so the full extent of plant communities must be determined using other methods. All of the above are 'macrofossils'.

Microfossils of pollen grains, epidermal cells, cuticle, and stomata also are found frequently. They are often used to identify plants to genus - sometimes species.