Six stratigraphically distinct trilobite faunas are recognized in the Lower Mississippian strata of the central United States. These faunas range in age from earliest Kinderhookian to Meramecian, and are, in ascending order: Pudoproetus missouriensis, Comptonaspis swallowi, Proetides insignis-Perexigupyge, Breviphillipsia semiteretis, Exochops portlockii, and Hesslerides bufo. Trilobite species diversity waxed and waned through the early Mississippian of the central United States, but reached its maximum during the late Kinderhookian within the Comptonaspis swallowi fauna. The Comptonaspis swallowi, Breviphillipsia semiteretis, and Exochops portlockii faunas of the mid-continent can be correlated with the C. swallowi, B. semiteretis, and Hesslerides arcentensis faunas of the Caballero and Lake Valley Formations of New Mexico. The vertical distribution, composition, and diversity variations among individual faunas suggest that they are evolutionarily discrete and therefore of biostratigraphic utility. Their stratigraphic distribution appears to be controlled by sea level and climatic fluctuations.

New Osagean trilobites identified and described are Exochops burlingtonensis n. sp. and Richterella carteri n. sp. from the Burlington Formation of Missouri, Australosutura osagensis n. sp. from the Keokuk Limestone of Missouri and the Arcente and Dona Ana Members of the Lake Valley Formation of New Mexico, and Spergenaspis boonensis n. sp. from the Boone Formation of Oklahoma.

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