What are fossils?

Fossils (from Latin fossus, literally "having been dug up") are the preserved remains or traces of animals (zoolites), plants, and other organisms from the past.

A fossil record is found in sedimentary rocks and is relevant to the history of life on Earth.

Fossils range in age from the youngest at the start of the Holocene Epoch, 10,000 years ago, to the oldest from the Archaean Eon, up to 3.4 billion years ago.

Fossils are a record of past life forms.

What does this fossil look like?

How are fossils formed?

After the animals or plants died, they were quickly buried (sinking in mud, buried in a sandstorm, etc). Over time, layers of sediment covered the remains.  The parts of the animals that didn't rot (usually the harder parts like bones and teeth) were encased in the newly formed sediment.  After a long time, the chemicals in the buried animals bodies or plants changed. As the bone or plant material slowly decayed, water infused with minerals seeped into the bone or plant material and replaced the chemicals with rock-like minerals. 

In the end we get a heavy, rock-like copy of the original object - a fossil. The fossil has the same shape as the original object, but is chemically more like a rock!

Petrified wood is the name given to a special type of fossilized remains of a tree or tree-like plants having completely transitioned to stone by the process of permineralization.

All the organic materials have been replaced with minerals (mostly a silicate, such as quartz), while retaining the original structure of the stem tissue.

 The petrifaction takes less than 100 years. It  occurs underground when wood becomes buried under sediment it is initially preserved due to a lack of oxygen. Mineral-laden water flowing through the sediment deposits minerals in the plant's cells and as the plant  decays, a stone mold forms in its place.

Paleontology is the branch of biology that studies the forms of life that existed in former geologic periods, primarily by studying fossils. 

Paleontologists study fossils and learn about the history of animal and plant life during different periods of time.