Arthropods are organisms possessing a hard outer coat or exoskeleton and jointed appendages, which they use for movement, and feeding. They include insects, crustaceans and spiders as well as extinct trilobites and eurypterids (the giant Paleozoic water scorpions).

Arthron =joint Podus =foot

Fossil arthropods are common, most important of these are trilobites which are good index fossils.

The name trilobites refers to the fact that the body is divided longitudinally as well as transversely into three portions.

The body is divided into three lobes: axial lobe in the middle and two lateral (pleural) lobes on both sides.

The body is divided into three parts; head (cephalon), thorax and pygidium.

The head consists of a glabellum in the center and two cheeks on both sides called pleural (lateral) cheeks. Each cheek carries a compound eye. Some trilobites are however blind. Each cheek is divided by a facial suture into a free cheek and a fixed one.

The thorax consists of a variable number of segments, each segment is provided with a pair of appendages. The thoracic segments are never fused.

The pygidium consists of a number of segments either distinct or fused.

In primitive (Early Cambrian) trilobites, there is no pygidium, these are called apygous.

Forms with small pygidium are called micropygous and those having a pygidium equal in size to the head are called isopygous.

Trilobites are either free-swimming (nektonic) or crawling on the bottom of the ocean. The crawling forms may have been blind.

They are exclusively marine and are found with corals, crinoids, brachiopods and cephalopods.

Trilobites are very ancient organisms; they appeared in Early Cambrian and died out at the end of Paleozoic (Late Permian).

They are good index fossils for Paleozoic.

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 colony increases by budding. The sicula is prolonged as a chitinous thread called nema which is used for attachment.

Graptolites are either planktonic or benthonic and are marine.

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.