Trilobites (meaning three-lobed) are a class of the phylum arthropoda. Composed of 10 orders, over 150 families, approximately 5,000 genera and nearly 20,000 described species, this diverse group inhabited the oceans of the Paleozoic Era. All trilobites are believed to be marine, as no trilobite fossils have yet been found in freshwater sediments. Trilobites first appear in the fossil record as early as the Lower Cambrian, although by this time they are fairly diverse and widespread. This early diversity leads many paleontologists to hypothesize that the ancestral / first trilobites appeared much earlier. Why haven't fossils of the first trilobites been found yet? We don't know, although many reasons could exist including:
1.) They had a limited environmental area / ecological niche...meaning that no one has looked in the right location for them yet.
2.) They did not readily fossilize. Known trilobites had a shell composed of calcium. It has been theorized that the first trilobites may not have had a shell composed of calcium, and therefore, not easily have been preserved in the fossil record.
Whatever the reason for the current lack of ancestral trilobite remains, at the time of their first appearance in the fossil record some 521 million years ago at the beginning of the Atdabanian Stage of the Early Cambrian, trilobites had already undergone significant diversification and were living on many continents. High diversity levels continued throughout the Cambrian and Ordovician. Trilobites were active participants in the Ordovician radiation event, where they experienced an even greater diversity while successfully exploiting new environments, most notably reefs. The Ordovician, although, marks the last major diversification of the trilobites, and numerous groups go extinct at the end of the period during a mass extinction event, presumably limiting most surviving genera to shallow-water shelf habitats. Trilobites begin their drawn-out decline starting in the Devonian, where a series of Middle and Late Period extinctions wipe out further species until only Proetida apparently survives. The last of the trilobites live through the next couple of periods until their eventual demise at the end of the Permian. According to current knowledge of the fossil record, trilobites left no descendants following their over 350 million years of history.
THE Trilobites
Class trilobita
Among the most easily recognized of fossils by seasoned geology enthusiasts as well as amateurs alike, trilobites find their way into all of our collections. Here's a brief introduction into this fascinating group of early arthropods.
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Agnostid trilobite - Middle Cambrian
Eudolatites trilobite - Ordovician, Ktaoua, South Morocco