Trilobites are an extinct group of marine arthropods.

The name Trilobite means in translation “three lobes”. The term refers to the visual three rounded lobes that the upper part of the exoskeleton of the Trilobite shows from one side to the other. Two are representing the left and the right pleural lobes on both sides of the body. The third is the thorax lobe in the middle of the thorax.

The exoskeleton of Trilobites is also further divided in three parts, the cephalon(head), thorax(body) and the pygidium(tail).

They first appeared in the the early Cambrian (lower Atdabanian), which dates to about 521 mio years ago. For almost 300 mio years they showed up in the fossil records all over the world. In the Devonian mass extinction they had a major set back. Only one of their eleven orders survived the event. The order Proetida continued through the Carboniferous and into the Permian. Trilobites disappeared from the fossil records during the Permian extinction.

Trilobites are found in marine deposits. It is believed, that they have lived on the seafloors but some could swim up. Some may also have entered the land on shorelines, at least at times.

Trilobites evolved from eggs and developed through three stages, protaspid, meraspid and holaspid. The first two are larval stages. The holaspid stage is the fully developed mature stage. In the holaspid stage Trilobites did not change their appearance, but could shed their skin(molt), to grow in size. All three stages can be found in the fossil records. At times the adult looks completely different than the larvae.

Trilobites have been scavenging, filterfeeding, hunting, and algae eating. They have have been found on fossilised trackways and burrowings.

Their appearance changed steady. They resemble one of the most amazing group of fossil animals.

Great statement from the AMNHcan be found here:
https://www.amnh.org/research/paleontology/collections/fossil-invertebrate-collection/trilobite-website/the-trilobite-files/curating-a-trilobite-collection

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