First appearance datum

A first appearance datum (FAD) is a special term scientists use to mark when a new kind of plant or animal first shows up in rocks from long ago. Scientists find the oldest fossil they can of that species and that moment is called the first appearance datum. This helps them understand how old the Earth is and how life has changed over time.

FADs are very important for making a timeline of Earth's history. They can help decide when one part of the timeline begins or ends. For example, a certain kind of tiny sea creature called Iapetognathus fluctivagus marks the start of a time called the Tremadocian Stage, which is part of the Ordovician Period. This happens in a place called the Green Point section in western Newfoundland, and in rocks from other parts of the world too.

But sometimes, the same species might show up at different times in different places, which can make it tricky to match up rock layers from far apart. Scientists then use other clues, like special measurements of rocks, to make sure they are matching the right layers together.