Dating Rocks using Fossils

Learning Objectives• To know what cross dating is and how

it can be used to date rocks• To be able to name some common

index fossil and explain why they are good for dating rocks.

• To be able to describe what a suture is and how its shape has evolved over time.

What is a Fossil?• A fossil is the remains of a once living

organism or evidence of its existence from a previous geological age.

Any Examples?• Trace Fossils – Footprints, burrows,

tracks• Coprolites – Fossilised Droppings• Body Fossils – Teeth, Bones, Hair,

Scales, Shells, Vegetation

Cast and Mould FossilsMould – A negative impression left after the organism has decayed away.Cast - A model of the organism in the form of a mineral. The mineral is produced by filling a mould with minerals in solution: calcite, quartz and iron pyrite are the most common

A Mould of a Trilobite

A Cast of a Trilobite

Dating Rocks Using FossilsHow?• Rocks can be dated using fossils, this is known as

cross dating.

• We can date the rocks using fossils because we think know when different species existed and so can give a relative date to the rocks that they are found in.

Why?Some rocks of the same age can look completely different due to the minerals that they are made of (e.g. clays or chalky limestone's).

If they contain fossils of the same species then we can determine that they must be of a similar geological age.

Why would this be a problem?

Can You Use Any Kind of Fossil?Some organisms evolved more rapidly than others and so provided much smaller timescales during which the rocks that they are found within must have formed and thus more accurate dates. The fossil species also needed to be widespread geographically allowing for many different rocks to be compared from different locations.

Fossils used to date rocks are known as Index Fossils.

The most useful fossils for cross dating are graptolites or cephalopods e.g.

Nautilus

Cuttlefish

Giant Squid

Dwarf Cuttlefish

Cephalopods

• The class cephalopoda includes sub classes such as Ammonites and Nautiloids.

• Over time these organisms went through several evolutionary changes including the amount of ornamentation to their shell, the type whorl, if they were coiled or uncoiled and the shape of their suture.

• The main evolutionary changed that is used for cross dating is the shape of the organisms suture.

So What is a Suture?• Ammonite fossils have intricate patterns on their outer

surface called Sutures.

• Sutures are found beneath the shell wall, and are often visible if the fossil has been subject to weathering or polishing.

• Scientists believe that the older the ammonites are, the less complicated the suture lines.

Changing Sutures• Goniatite suture line - 358.9 million years ago

• Ceratite suture line – 200 million years ago

• Ammonite suture line – 66 million years ago

Saddle and LobeThe saddle and lobe are the crest and trough of the ‘wave form’ of the suture line.