The photo above shows 25mm long graptolite
from the Manx Group. Image is reproduced here
by kind permission of Manx National Heritage.
Graptolites are now extinct but during the Lower
Palaeozoic era (around 550-300 million years ago) they lived as
free-floating colonies resembling thin twigs or wishbones. They probably
floated near the surface of the Iapetus
Ocean and when they died they sank to
the sea bed where some were buried and preserved as fossils. These
fossils often appear as flattened grey strips around 3-5 cm long and
a few millimeters wide with saw-tooth serrations along one long edge.
Unfortunately, they can be extremely difficult to spot and relatively
few have been found on the Island. However, the subtle variations
in their shapes due to the effects of evolution have been crucial
in telling us that the Manx
Group was formed around 470
million years ago while the Dalby Group
was deposited around 425
million years ago.
In the absence of more graptolites to help us work out the ages of
the different parts of the Manx
Group, we have to rely on acritarchs. These are fossilized planktonic
algae and look like pollen grains, but being typically around 0.025
mm in diameter, they require the use of a high powered microscope
after dissolving most of the rock with acid.
