What are fossils? How are they formed?

Explanation

### Step-by-Step Solution:

1. **Definition of Fossils**:
Fossils are the preserved remains or traces of ancient organisms, including plants and animals, that lived in the past. They provide important information about the history of life on Earth.

2. **Types of Fossils**:
- **Vertebrate Fossils**: These include the remains of animals with backbones, such as bones and teeth, which are typically hard and can be preserved over time.
- **Invertebrate Fossils**: These are remains of organisms without backbones. Since they do not have bones, their exoskeletons (the hard outer structure) are often what gets preserved.
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