If you're interested in the largest trilobites, have a look over at this page on "A Guide to the Orders of Trilobites" or this page at the American Museum of Natural History. The following quote is from the AMNH page.
The size variance shown among trilobite species is nothing less than astonishing. Generally when one considers these long-extinct arthropods, images of small, hard-shelled creatures scurrying across ancient sea floors spring readily to mind. And for the most part, such a perception is accurate. The vast majority of the more than 25,000 scientifically recognized species that existed during the trilobites' 270 million year reign from the Lower Cambrian to the end of the Permian were diminutive -- an inch, or smaller in length. By no means, however, should such information be interpreted as indicating that trilobites didn't grow bigger than that… much bigger!
Perhaps the most notable of these large trilobite species are the Ordovician-age Isotelus maximus from the mid-western United States, a type that grew up to 16 inches in length and became recognized as the State Fossil of Ohio back in 1985. In addition, impressively sized Silurian-age trilobites have been found in western New York State, with species such as Trimerus delphinocephalus attaining lengths up to eight inches. And a variety of species, including Hungoides bohemicus, that on rare occasion exceed 12 inches from head to tail, have been found in the prolific Ordovician quarries near Volongo, Portugal, marking the entire formation as one filled with mega-fauna.
The list of largest trilobites includes:
- Isotelus brachycephalus
- Acadoparadoxides levi-settii - up to 18 inches long
- Acadoparadoxides briareus
- Terataspis grandis
- Uralichas hispanicus - over 12 inches long
- Isotelus maximus - up to 16 inches long and the State Fossil of Ohio
- Isotelus rex - largest found is 27 inches long!
The largest complete trilobite ever found resides at the Manitoba Museum in Manitoba, Canada. It is a specimen of Isotelus rex and is almost twice as big as then next largest specimen. It is over 27 inches long and lived during the Ordovician Period 455 million years old . If we were living in the Ordovician period, we would find these creatures while scuba diving. This individual specimen was found in Churchill, Manitoba, which is a city in northern Manitoba. In it's display, you see it's fossilized track, called Cruziana.
