Trilobites lived between 520 to 250 million years ago (mya). Like the vast majority of animals at the time, they went extinct during the Permian Extinction, or “Great Dying.” While all trilobites lived during the Paleozoic Era, which preceded the much better-known Mesozoic Era or “Age of Reptiles,” different genera of trilobites lived during different periods. Over 20,000 species of trilobites have been identified, and they have been placed in the following ten orders: Agnostida, Asaphida, Corynexochida, Harpetida, Lichida, Odontopleurida, Phacopida, Proetida, Ptychopariida, and Redlichiida.
The order Redlichiida was the first order to develop and existed from 520 mya to 501 mya. Eight of the other ten orders also appeared during the Cambrian Period. Redlichiida and Ptychopariida were the most primitive trilobites. The oldest known trilobite genera included the following:
Albertella longwelli, which belonged to the order Corynexochida, appeared in what became Nevada during the middle of the Cambrian Period. It could grow to be over 42 mm (1.7 inches) and is known for the long, flamboyant spines that extend from the cephalon (head shield), third chest segment, and pygidium (tail).
Phacopida was the last trilobite order to appear; the earliest fossils date back to start of the Ordovician Period or 488 mya. Members of the order survived until the end of the Devonian Period.
The late Cambrian and the early Ordovician were the peak periods for trilobite diversity. From 501 mya to 472 mya, there were over 60 families representing the nine remaining orders.
The Ordovician ended with a mass extinction that wiped out 60 to 70 percent of all species, including the trilobite orders Agnostida and Ptychopariida.
Isotelus rex, which is the largest known trilobite, could grow to be 700 mm (28.3 inches) long. It also had a maximum width of 400 mm (15.7 inches) and a maximum height of 70 mm (2.8 inches). It belonged to the order Asaphida, which was the largest order; roughly 20 percent of trilobite species belonged to it.
Acanthalomina minuta, which belonged to the order Lichida, was a small spiny trilobite that was no more than 66 mm (2.6 inches) long. Its fossils have been found in both Spain and the western United States.
Dalmanites limuluris, a member of the order Phacopida, owes its species name to the fact that its discoverer thought it resembled a horseshoe crab (genus Limulus).
The Devonian Period also ended with a mass extinction that wiped out 70 percent of the world’s species, including all of the remaining trilobite orders except Proetida.
Phacops rana, a member of the order Phacopida, is the state fossil of Pennsylvania. Its species name means “frog” and refers to its large eyes.
Dicranurus, which belonged to the order Odontopleurida, had spines extending from its body as well as a pair of “horns” extending back from its head. It provided moviemakers with the inspiration for one of the creatures in the science fiction film “Prometheus.”
The order Proetida, which first appeared 501 mya during the Cambrian Period, was the only trilobite order left by the Carboniferous Period. Proetida was then represented by two superfamilies, Aulacopleuroidea and Proetoidea. Only four families of trilobites still existed in the Carboniferous. One of these families, Phillipsidae, enjoyed a major radiation during the early Carboniferous and produced many new species. Unfortunately, trilobites as a group began declining during the late Carboniferous, and only three families survived into the Permian Period.
The last surviving members of Proetida died during the Permian Extinction 250 mya.
Aulacoplueroidea was represented by a single family, Brachymetopidae, which had only two genera at that time, Brachymetopus and Cheiropyge, the second of which survived to the end of the Permian.
Proetoidea had two surviving families, Proetidae and Phillipsiidae, and some members of Phillipsiidae also survived to the very end. They included the following genera:
https://samnoblemuseum.ou.edu/common-fossils-of-oklahoma/invertebrate-fossils/trilobites/
https://www.zmescience.com/other/feature-post/trilobite-facts/
http://www.fossilmuseum.net/Paleobiology/SilurianFossils.htm
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