British & USA Trilobite Fossils
The earliest trilobites known from the fossil record are redlichiids and ptychopariid bigotinids (yes try saying that after a drink or two) dated to some 540 to 520 million years ago. For such a long-lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished while surviving groups diversified to fill ecological niches with comparable or novel adaptations. Generally, trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn-out decline in the Devonian culminating in final extinction of the last few survivors at the end of the Permian period. Trilobite means "Three lobed".
Exactly why the trilobites became extinct is not clear; with repeated extinction events (often followed by apparent recovery) throughout the trilobite fossil record, a combination of causes is likely. After the extinction event at the end of the Devonian period, what trilobite diversity remained was bottlenecked into the order Proetida. Decreasing diversity of genera limited to shallow-water shelf habitats, coupled with a drastic lowering of sea level (regression) meant that the final decline of trilobites happened shortly before the end of the Permian mass extinction event. With so many marine species involved in the Permian extinction, the end of nearly 300 million successful years for the trilobite would not have been unexpected at the time. The closest extant relatives of trilobites may be the horseshoe crabs
British Trilobites
Most commonly available are Ogyginus, Ogygiocarella and Trinucleus from the Ordovician Welsh Mountains.
USA Trilobites
Most commonly available species are Elrathia and Peronopsids (Itagnostus). All are Cambrian age.
