|The trilobite Phacops rana (about 50 mm long) from the Devonian of the USA. The compound eyes of this specimen are very well preserved.|
Trilobites are a long extinct group of marine arthropods, distantly related to crustaceans. Most lived on the sea floor, probably as active predators on smaller soft-bodied animals, but some of the smallest trilobites were planktonic, floating near the surface of the sea. Some are thought to have lived in deep water and have large sophisticated eyes. They first appeared in the early Cambrian (560 million years ago) and became extinct at the end of the Permian (250 million years ago). During the early part of their history they were among the most fearsome predators, with the largest examples more than half a metre long.
Like all arthropods, trilobites grew by moulting their old shell when it became too small and growing a larger one. Each of these moulted shells potentially could become a fossil, so one trilobite could produce perhaps a dozen or more fossils in its lifetime!
Trilobites are not very common in Northern Ireland. Small examples are sometimes found in Lower Carboniferous rocks in counties Armagh, Fermanagh and Tyrone. Only in the shallow-water Ordovician (about 440 million years old) sandstones of parts of Co. Tyrone, such as near Pomeroy, are they abundant and diverse, but here there is very little rock exposed.
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