During their successful evolutionary history, they survived four mass extinction events. Ultimately, they only succumbed to the mass extinction at the Permian-Triassic boundary about 252 million years ago. In the nearly 270 million years, trilobites had conquered almost every marine ecosystem. There are even indications that trilobites partially colonized brackish water areas. Within the nine orders of the class Trilobita, more than 150 families with a total of over 5000 genera have been described so far. More than 15000 species of trilobites have been scientifically described. And new species are still being discovered. Here is a small overview of the trilobite orders:

Order	Typical families	Short description	Typical representative
Agnostida Salter, 1864	Agnostidae, Peronopsidae, Eodiscidae	Very small and smallest trilobites, usually only a few millimeters long. Age: Lower Cambrian to Upper Ordovician.	Itagnostus interstricta (WHITE, 1874)
Redlichiida Richter, 1932	Redlichiidae, Olenellidae, Paradoxididae	Very old trilobites with many thoracic segments ending in pleural spines. Age: Lower Cambrian to Middle Cambrian.	Paradoxides gracilis (BOECK, 1827
Corynexochida Kobayashi, 1935	Corynexochidae, Ellipsocephalidae, Dorypygidae	Trilobites with a large glabella and a mostly smooth thorax. Age: Lower Cambrian to Upper Ordovician.	Ellipsocephalus hoffi (Schlotheim, 1823)
Lichida MOORE, 1959	Lichidae, Odontopleuridae, Acanthopygidae	Trilobites with a large glabella and a mostly smooth thorax. Age: Lower Cambrian to Upper Ordovician.	Arctinurus boltoni(BIGSBY, 1825
Phacopida SALTER, 1864	Phacopidae, Cheiruridae, Calymenidae	Trilobites with large eyes and a mostly smooth or grooved thorax. Age: Middle Ordovician to Upper Devonian.	Phacops araw CHATTERTON et al., 2009
Proetida Fortey & Owens, 1975	Proetidae, Phillipsiidae, Bathyuridae	Trilobites with a small glabella and a mostly smooth or grooved thorax. Age: Lower Ordovician to Upper Permian.	Gerastos cuvieri (STEININGER, 1831)
Asaphida Salter, 1864 (emend. Fortey & Chatterton, 1988)	Asaphidae, Trinucleidae, Raphiophoridae	Trilobiten mit einem großen Glabella und einem meist glatten oder gerillten Thorax. Zeitalter: Unteres Ordovizium bis Oberes Silur.	Nobiliasaphus delessei (DUFET 1875)
Harpetida SALTER, 1864	Harpetidae, Entomaspididae, Dikelocephalidae	Trilobites with a large glabella and a mostly smooth or grooved thorax. Age: Lower Ordovician to Upper Silurian.	Harpes macrocephalus (GOLDFUSS 1839)
Ptychopariida Swinnerton, 1915	Ptychopariidae, Solenopleuridae, Olenidae	Trilobites with a medium-sized glabella and a mostly smooth or grooved thorax. Age: Lower Cambrian to Upper Ordovician.	Elrathia kingi (Meek,1870)
Nektaspida Størmer, 1943	Naraoiidae, Emucarididae, Liwiidae	Trilobites without eyes and without calcification of the exoskeleton. Age: Lower Cambrian to Upper Silurian.	Naraoia spinosa Zhang & Hou, 1985

It should be noted that the order Nektaspida is only tentatively assigned to the actual trilobites. After all, the describer of the class Trilobita, Johann Ernst Immanuel Walch, had already pointed out the remarkable armor of the trilobites in 1771.Please note also this table uses a strong generalization. There will a lot  exceptions to the short description for each Trilobite Order.  In summary, it can be said that trilobites occur worldwide throughout the entire Paleozoic era and are also widely distributed as fossils today. There are only a few countries where no fossils of trilobites have been found.