1. Types of Graptolites

2. GRAPTOLITES
The first graptolites were colonies of animals attached to each other on branches (stipes) and attached to the sea-bed by a hold-fast. They extracted their food from sea-water.
STIPES
HOLD-FAST

3. Tetragraptus
STIPE
As time passed, it became more efficient for graptolite colonies to float freely in the oceans to find food (pelagic floating). The number of stipes reduced and the trilobite animals on each colony decreased in number. Their chambers (thecae) became larger and are clearly seen in this picture.
THECAE

4. Didymograptus murchisoni
TWO STIPES
Didymograptus murchisoni
In the Ordovician period the number of stipes per colony reduced to two. Often the thecae became more complex in structure.
THECA

5. Monograptus
Later, in the Silurian period the two stipes of Didymograptus united and became one, but with thecae on both sides. Monograptus is only found in Silurian rocks and is therefore an excellent zone fossil to correlate rocks of this age.
THECAE ON BOTH SIDES OF THE STIPE
SINGLE STIPE

6. Dictyonema
Climacograptus