Trilobites are a group of marine arthropods that lived in the Palaeozoic. Their body was divided into three longitudinal lobes – hence the name – and into three sections: the head (cephalon) with compound eyes, the middle (thorax) formed by several articulated segments, and a “tail” (pygidium). They had an exoskeleton, or external support structure, which was mineralised on the dorsal side and non-mineralised on the ventral side. Fragile protuberances on the ventral side, preserved only in very rare cases, worked essentially as limbs. These predominantly benthic animals, meaning that they lived on the seabed, were an extremely important element of the Palaeozoic fauna, and came in very strange and unusual forms. The fossil remains of these animals range from the Early Cambrian (from about 521 million years ago) to the Late Permian (about 250 million years ago). They reached their highest peak of development in the Ordovician (about 480 to 450 million years ago): more than 1,500 genera and thousands of species are known, many of which are of great stratigraphic value, particularly for the study of the Early Palaeozoic. Indeed, their well-defined presence over time makes them excellent ‘guide fossils’, useful in dating the rocks in which they are found. In addition to fossils and the remains of their moulted exoskeleton, trilobites have also left traces of their activity, for example at rest and in movement. In Italy, fossils of trilobites can be found in Sardinia, the Carnic Alps and Sicily.
