Arthropods

Arthropods

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.

Trilobites

Tri =three

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

Longitudinally:

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

Transversely:

The body is divided into three parts; head (cephalon),

thorax and pygidium.

Head:

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.

Thorax:

The thorax consists of a variable number of segments,

each segment is provided with a pair of appendages.

The thoracic segments are never fused.

Pygidium

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.

Habit and Habitat:

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 rugose corals, crinoids, brachiopods and cephalopods.

Geologic History:

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.