Graptolites are extinct marine Paleozoic organisms (like trilobites) that built small dendritic or saw-blade like colonial exoskeleton of chitinous material.
Graptos = writing
They are preserved as carbonized remains of the
original chitinous exoskeleton resembling writing.
Graptolites live in colonies called rhabdosome;
each rhabdosome consists of a number of branches
Called stipe.
Graptolites with one branch are called monograptus,
Two are called didymograptus, four are called
tetragraptus, and eight are called dichograptus.
Each branch (stipe) consists of a number of tubes called theca; the primitive tube is the sicula. The sicula is prolonged as a chitinous thread called nema which is used for attachment. The colony increases by budding.
Habit and Habitat:
Graptolites are either planktonic or benthonic and are marine.
Geologic History:
Graptolites appeared in the Middle Cambrian and
reached their acme in the Ordovician and Silurian.
They started to decline in the Mississippian (Early
Carboniferous).
They are common in black shale but they are also
present in shale of other colors as well as sandstone
and limestone and in chert nodules.
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