About Trilobite
Trilobites were a very diverse group of extinct marine arthropods. They first appeared in the fossil record in the Early Cambrian (521 million years ago) and went extinct during the Permian mass extinction (250 million years ago). They were one of the most successful early animals on our planet: over 25,000 species have been described, filling nearly every evolutionary niche. Due in large part to their hard exoskeletons (shells), they left an excellent fossil record.
Trilobites Complexity
Many trilobite species, particularly in the Devonian, developed complex arrays of spines. These spines could have served as protection against predators such as fish.
Many types of trilobites could roll into a ball much like "pill bugs" for protection. Remains found in this shape are known as enrolled trilobites.
Nearly all trilobite fossils are fossils of their external exoskeletons, or shells. Only a few localities in the world preserve soft body parts such as legs, antennae, and gills.
Tens of thousands of trilobite species have been described. Some are quite common while others are extremely rare. Only a fraction of the known species have ever been found complete.
Trilobite Eyes
Trilobites were one of the first animals we know of to have eyes. They were unique in that they had multi-faceted eyes made out of calcite lenses.
Trilobites are the most primitive animals known to have vision. Some possessed stalked eyes, while others had turrets full of lenses, and some seem to have no eyes at all! Many trilobites evolved compound eyes with 360 degrees of vision. Some trilobites peered through the most ancient and widespread mode of trilobite vision-holochroal eyes. These eyes were often comprised of thousands of tiny lenses. A single corneal membrane covered all of the lenses of this compound eye. The lenses were packed closely in a hexagonal fashion. The eyes were not covered by the white layer of sclera: this absence is also seen in modern arthropods.
Trilobite Extinction Events
The last trilobites went extinct at the end of the Permian period, over 20 million years before the first dinosaurs appeared on Earth.
The early Cambrian (521 MYA) marks trilobites' first appearance. Fossils from this era already display a high degree of diversity and geographic distribution, so emergence must have been much earlier. Trilobites continued to develop and radiate during the lower Paleozoic until their decline into the Devonian. Proetida is the only trilobite order that survived into the Permian. The Permian mass extinction that occurred 250 million years ago also took out the last trilobites.
They may have been reduced through warming and cooling trends caused by volcanism or the effects of a meteor impact. Very high carbon dioxide levels would have also killed marine species and affected the food chain. Overall, there is no definitive conclusion as to what eliminated these exotic treasures from the Earth.