Fossils

what are fossils?

they are remains of organisms from many years ago → often found in rocks

what do fossils show?

they provide evidence that organisms lived years ago → they tell us how much an organism has evolved as life om Earth developed

what are the three different ways in which fossils form?

1) from gradual replacement by minerals

2) from casts and impressions

3) from preservation in places where no decay happens

how are fossils formed by gradual replacement by minerals?

things like teeth, shells and bones → don’t decay easily → and are usually replaced by minerals as they decay → this forms rock-like substances shaped like the original hard part

• the surrounding sediment also turns to rock → but the fossil stays distinct inside the rock

how are fossils formed by casts and impressions?

• when an organism is buried in soft material, such as clay

• they clay later hardens around it and the the organism decays → leaving a cast of itself

what can be preserved as casts?

an animal’s burrow or a plants rootlet traces can be preserved as casts

how are impressions formed and what can be preserved as impressions?

things like footprints cam be pressed into soft materials → leaving an impression when the material hardens

how are fossils formed by preservation in places where no decay happens? And for what reasons does this type of formation occur?

why:

• the microbes involved in decay need the right conditions in order to break down material → plenty of oxygen, enough moisture and the right temp and pH is needed for their enzymes to work properly

  • if conditions aren’t right → decay won’t take place and the dead organism’s remains will be preserved

explain why there is a lack of valid evidence to support any hypothesis about how life on Earth began - and what does this ultimately mean?

• scientists believe many organisms were soft-bodied → and soft tissue tends to decay away completely → without forming fossils

• fossils that did form million years ago may have been destroyed by geological activity → e.g. the movement of tectonic plates may have crushed fossils

  • means that the fossil record is incomplete