4.6.3.5 Fossils

Question 1

Fossils:

Answer 1

the ‘remains’ of organisms from millions of years ago, which are found in rocks

Question 2

When are fossils formed?

Answer 2

• from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent
  
  • parts of organisms that have not decayed because oxygen or moisture were not present, meaning that the microbes that cause decay could not survive
• when parts of the organism are replaced by minerals as they decay forming a rock structure of the original part e.g. teeth, shells, bones etc.
• as preserved traces of organisms e.g. footprints, burrows and rootlet traces due to the ground hardening around them and forming a cast

Question 3

How are fossils formed?

Answer 3

• remains of dead organism covered in sediment
• sediment builds up compressing dead organism
• soft parts of organism decay or are eaten
• bones / hard parts do not decay
• so minerals enter bones and replace the bone turning it to stone

Question 4

Why are there no/not many fossils of early forms of life?

Answer 4

• many-early forms of life were soft-bodied and therefore decayed completely so there are few fossils of them
• any traces left have been destroyed by geological activity
• this is why scientists cannot be certain about how life began on Earth

Question 5

What can we learn from fossils?

Answer 5

• we can learn from fossils how much or how little different organisms have changed as life developed on Earth
• used to show how the anatomy of organisms has changed over time
• can be used to compare how closely related 2 organisms are through looking at the number of similarities they have
  
  • this information is used to create evolutionary trees

Question 6

Why is the fossil record incomplete?

Answer 6

• fossils formed under v. specific conditions
• remains of the organism usually washed into water and buried by mud or silt
  
  • if there is any O2 at all the remains would decay and there would be nothing left
• hard parts of organism e.g. shells, bones, teeth etc. replaced by minerals
  
  • if the organisms are completely soft-bodied like an earthworm - no hard parts so no fossils formed
• very rare for all conditions to be met
• even if fossils form they still need to be found
  
  • only weathering + erosion (beneath sea or river bed) can expose them (in rocks etc.) which takes a long time