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.
environment to support fossilisation, the dead organism should be exposed to
the following conditions:
lack of oxygen (O2),
- submergence under water, ice or permafrost,
- extremes of temperature,
- extremes of pH (potenz hydrogenous), or
- extreme aridity over a long period.
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:
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
Alethopteris seed fern fossil leaf from Oklahoma, USA. Photo by
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.
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
A piece of opalised, petrified wood. Photo by
A Jurassic-era flower, embedded in amber. Photo by