5 The importance of the fossil record

1. Tracing the appearance and development of life

• Sedimentary rock layers are deposited one on top of another, so in an undisturbed sequence the deeper layers are older than the ones above.
• Organisms whose fossils are found in deeper layers appeared on Earth earlier than those found in higher layers.

Example: fossils of invertebrates are found in deeper layers than fossils of vertebrates, so most invertebrates appeared before vertebrates.

2. Identifying mass extinctions

• A mass extinction is an event in which a huge number of species die out in a relatively short time.
• The fossil record shows that certain layers contain many kinds of fossils, but the layers above them lack those fossils.
• For example, about 66 million years ago a mass extinction wiped out about 75% of all species on Earth, including most dinosaurs.

Hypotheses for mass extinctions:

1. cosmic catastrophes such as the impact of a large meteorite (asteroid);
2. severe climate change and changes in the composition of the atmosphere;
3. massive volcanic activity;
4. changes in sea level and the changing positions and shapes of continents.

Cause and effect. Cosmic catastrophes, severe climate change or massive volcanic activity in ancient times (causes) led to the extinction of many organisms (effect).

3. Identifying evolutionary relationships between ancient and modern organisms

• Palaeontology (the study of fossils) works with anatomy and genetics to link ancient organisms to those alive today and shows how species branched from common ancestors.
• Example: the modern Asian elephant is the closest living relative of the extinct woolly mammoth, since both come from a common ancestor.

4. Inferring ancient environments and climate

• Skeletons of primitive whales in Wadi Al-Hitan (Fayoum) show that the area was once covered by an ancient sea.
• The Petrified Forest in Cairo shows that the region was once a humid forest, and the climate there has changed from humid to dry.

• Worked reasoning example 1. Petrified tree trunks are found in a dry desert.
• Trees need water, so the region was once moist with plenty of rain and rivers (a forest).
• Since then the climate changed to dry.

• Worked reasoning example 2. Marine shells are found in rocks high on a mountain.
• Marine organisms live only in seas; so the rock formed on an ancient sea floor, and was later lifted up when plates collided.