During the Paleozoic Era (541 to 252 million years ago) fish diversified, fossils include trilobites and cephalopods such as crinoids, as well as insects and ferns. The greatest mass extinction in Earth's history ended this era.
Crinoids reached their highest generic richness and overall abundance during the Mississippian
, which thus has been dubbed the Age of Crinoids. The causes are hypothesized to be from the coincidence of two factors. First, in the wake of the Late Devonian mass-extinction event, the five major crinoid groups recovered and radiated in the Early Mississippian. The advanced cladids continued to radiate from their origin in the Early Devonian and reached a peak in the Middle Mississippian (Visean) that was not exceeded again until the Middle Pennsylvanian
(Moscovian). Second, the Late Devonian mass-extinction event destroyed the extensive coral-stromatoporoid platform-edge reefs that had restricted circulation on carbonate platforms and limited the abundance of crinoids, which are stenohaline. The resulting carbonate ramps during the Mississippian had improved circulation, producing stenohaline conditions that resulted in an abundance peak for crinoids, recorded by widespread regional encrinites on multiple continents. This increased habitat space was ideal for camerate crinoids and resulted in a new radiation of camerate crinoids. The simultaneous radiation of pinnulate cladids and the short resurgence of camerates were responsible for the biodiversity spike in the Mississippian. The Age of Crinoids ended with a major drop in sea level at the end of the Mississippian as massive glaciers formed on Gondwana and epicontinental seas were drained.