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Trilobites are one of the most well-known fossil groups, and highly diverse, and are found in many different locations. Over 17,000 species are known from fossils, which are usually just of the animal's exoskeleton, although occasionally soft bodies are also found in fossils. The fact that Trilobites are so widespread and diverse, and that they evolved rapidly over time, often allows their use as "index fossils" - that is to say that rocks can be dated based on the particular type of Trilobites found within them.
During life, all Trilobites are believed to have been exclusively marine organisms, although they do appear to have led many different lifestyles. Trilobites lived in both shallow and deep water, and although most lived in the seas, there is some evidence (such as trace fossils) that some species may have been able to live in freshwater. Various different types of Trilobites are believed to have been swimming plankton feeders, predators, scavengers, and in some cases, to have even got their food from symbiotic sulfur-eating bacteria.
One final interesting fact about Trilobites is that they are not thought to have been able to cross deep oceans. Thus, the presence of two distinct types of Trilobites (as well two distinct types of Graptolites) of Trilobites in the rocks of Europe and North America is suggestive that an ocean (known as the "Iapetus Ocean") once divided the two continents. The Iapetus Ocean is thought to have closed-up again when all the continents joined together in the super-continent of Pangaea - when a new ocean (the Atlantic Ocean), separating Europe and North America, did subsequently form when Pangaea broke up, it did so along a different line of separation.
Trilobites were arthropods that lived in the seas between 530 and 248 million years ago
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