text: mongst the earliest of all known arthropods are the trilobites.  The first species of trilobite are found in rocks of Lower Cambrian age, above the non-trilobite 'Tommotian' Cambrian fauna.  The last known species to have existed are found in the Late Permian.  Throughout their 350 million years of existence in the ancient seas the overall morphology of the earliest and latest trilobites did not vary greatly, however their individual characteristics had many variations and it is these changes that can be studied and used to determine age ( Levi-Setti, 1975).  However it must be remembered that no individual feature is a sufficient guide for classification on its own, it is only with a collection of characteristics that a reliable classification can be made.\n\n Well over 1500 genera are known (Clarkson, 1994) and there are several thousand species, many of which have a stratigraphic value, especially those found within Cambrian and Ordovician rocks.  It is the fact that trilobites are found world wide which makes some of them serve as excellent index fossils, and it is their short life periods which allows precise dates and correlations to be made about the rocks in which they are found.\n\n  Most trilobites were sea floor dwellers, living in the soft sediments, while others were free swimming.  They appear to have been exclusively marine organisms since the fossilized remains of trilobites are always found in rock containing fossils of other salt-water animals, e.g. brachiopods, crinoids and corals.