Of all the fossil arthropod
s, Trilobites have been given the most attention. This is mainly because they are abundant
rocks, useful for correlation
of location and time and attractive in appearance.
Complete sections of trilobite fossils are rare, because like all arthropods they were composed of rigid segments of exoskeleton joined by flexible organic connections that decayed after the death of the animal.
Although complete exoskeletons are uncommon, the seperated parts of the carapace are common fossils in Cambrian, Ordovician and Silurian rocks, and are proof of the abundance of these animals in the early Paleozoic seas. The carapaces of trilobites were highly calcified and are preserved as calcite: they are assumed to have contained some organic component, probably chitinous, during life.
Except in extraordinary conditions, the only part of the trilobite that is preserved is the exoskeleton that covered the back or dorsal side. The ventral side and the appendages (much like that of a crab: spiny walking legs) were not calcified and are known only from a few specimens in which this soft tissue was preserved as a thin film, or replaced by pyrite.
Trilobites are also the first organism known to have eyes. These were compound, made of many individual lenses, much like a modern insect.