Taking the most fossils as the first fossils
- The idea
- Predict
- See it
- Why
- Check
The idea
“Something first appeared where its evidence gets plentiful and clear.”
The layer packed with fossils looks like the start, because the evidence there is hard to miss. The first few traces are easy to skip.
The short version
The mistake: Something first appeared where its evidence gets plentiful and clear.
What's actually true: Something was around at least as early as its oldest evidence, however scarce or faint that evidence is.
- A fossil shows the organism was alive when that rock formed. It may be older still, since most living things never fossilize.
- A new group starts rare and spreads over millions of years. Old rocks are also scarcer and more often eroded, so the earliest traces are few.
- Other evidence counts too. If fossil spores or chemical traces made by the organism are older than its first full fossils, the oldest sets the date.
On the exam: Date a first appearance from its oldest evidence, even one fossil. Where it's common only shows spread.