Trilobites are one the of earliest known arthropods. The are believed to have existed for about 270 million years and are considered to be one of the most successful early animals. Thanks to their hard exoskeletons, they where able to be fossilized in perfect condition, making them easier to study.
Scientists consider trilobites to be the single most diverse class of extinct organisms. The smallest known trilobites are under a millimeter long, while the largest are over two feet long. All trilobites have a similar body with a head, thorax, and tail piece similar to insects. They also all have a central axial lobe that runs down the middle of their body like a spine, as well as a pleural lobe on either side. Beyond this basic body plan, there is incredible diversity among the different species.
The majority of early trilobites are believed to be predators. They are believed to have eaten invertebrates such as worms. Evidence of this can be found in trace fossils which show trilobite trails suddenly stopping above worm burrows, suggesting that the trilobite was on the hunt for worms and stopped to catch one when they came across a burrow. This theory is further supported both by evidence of predatory trilobite ancestors and the fact that larger species of trilobites had larger digestive tracts, perfect for breaking down large chunks of prey.
The earliest known trilobites were believed to have existed between 540 and 520 million years ago. Trilobites are thought to have originated from present-day Siberia and had radiated outward. They were believed to have originally had no eyes or facial structures, but evolved to have them later on.
Trilobites developed one of the first sophisticated visual systems in prehistoric animals. There are currently three official types of eyes found in trilobites: holochroal, schizochroal, and abathochroal. The most common type for early trilobites to have was holochoral. As with the rest of the trilobite body, mutations were rampant among the animals, leading to many variations in eyes. Phacops trilobites had extremely well-developed eyes, giving them a perfect field of vision. Opipeuter trilobites had eyes that were so large, they covered a majority of the head. Several bottom-feeding trilobites had underdeveloped eyes or lost their eyes all together.
The first time trilobites appear on the fossil record is during the pre-Cambrian era which was about 700 million years ago. At first, they appear to all be fairly uniform in shape and size, with little diversity or evolution. They seem to have existed in modern-day Siberia just before the breakup of the super continent Pannotia. After the breakup of Pannotia and the major shift in tectonic plates, trilobites were displaced from Siberia, which allowed them to migrate to different parts of the planet and to diversify.
During the Cambrian era, which lasted for about 56 million years, trilobites rapidly diversified into their major orders: Redlichiida, Ptychopariida, Agnostida, and Corynexochida. In the middle of the Cambrian, trilobites had their first extinction crisis. The trilobites that survived developed different body shapes and thicker shells for defense against predators. There was a second, more intense, mass extinction event at the end of the Cambrian that is attributed to major environmental changes such as the loss of continental shelf area of the Laurentia continent, which makes up modern-day North America. This extinction event wiped out all Redlichiida as well as the majority of trilobites in the other orders.
The Ordovician era lasted 41.2 million years and is marked by the movement of many brachiopods to new areas and many new appearances on the fossil record. Life flourished during this era, as it had done during the Cambrian era. Arthropods such as trilobites dominated the oceans.The Great Ordovician Biodiversification Event is credited with the rapid increase of diversification of all life on Earth. New forms of trilobites such as Phacopida, Trinucleioidea, and Agnostida took over where the Cambrian forms had gone extinct. Trilobites had evolved to live in new environments such as reefs and were continuing to evolve at an unusual rate. At the end of the Ordorvian, climate temperatures began to cool, with oceans going from 113 degrees fahrenheit to temperatures similar to modern oceans. Sea levels dropped as a result of glacial formations as well. While trilobites faired through the mass extinction better than many other organisms, they weren’t completely unscathed. Both Trinucleioidea and Agnostida went extinct as well as many other early-Ordovician animals.
The Silurian era lasted 24.6 million years. The Silurian began just after the mass extinction of 60% of Ordovician marine life. Virtually no early Ordovician trilobites survived the mass extinction, but about 74% of the late Ordovician trilobites did, which accounted for all Silurian trilobite ancestors. One major evolutionary milestone of the Silurian was the evolution of jawed fish, a new predator for trilobites. The stable ocean temperature allowed trilobites to become very diverse and abundant. While the ocean temperature stabilized during the Silurian, the carbon cycle and weather didn’t. The silurian had a higher concentration of isotopic excursions than any other era. Thanks to rapid sea-level change occurring together with these isotopic excursions, trilobites were particularly impacted by bursts of mass extinction.
The Devonian era lasted 60 million years and is characterized by the rapid evolution and migration of life on land. While other arthropods began to develop on land, trilobites remained in the seas and flourished. The sea levels remained high, which didn’t make it necessary for trilobites to move to land. Lichida and Phacopida were the dominating trilobite species. The end of the Devonian era marked the worst mass extinction of trilobites. The cause of the mass extinction is still being debated, but there is no doubt that marine life was the most severely-effected by it. Trilobites in particular were severely impacted, with three orders and all but five families becoming extinct. Only the order Proetida survived.
The end of the trilobites came near the end of the Permian era. Proetida, the only surviving order of trilobites from the Devonian era, lacked diversity and halted future evolution. The Permian era ended with the largest mass extinction in Earth’s history. This extinction primarily effected marine life with calcium carbonate skeletons, as the trilobites had. It was during this extinction event that all trilobites became extinct.
Today, isopods are the closest living things related to the ancient trilobites. They belong to the same phylum of hard-shelled, segmented, multi-legged animals called Arthropoda that trilobites did. These isopods exist both in the sea, as trilobites had, and on land.