Trilobite associations from the Chouteau Formation (Kinderhookian) of central Missouri

Abstract

Three distinct trilobite associations can be recognized in the Chouteau Formation (Tournaisian, Early Carboniferous) of central Missouri. These associations appear to be lithologically and environmentally sensitive. Association A, the stratigraphically lowest association, contains six genera and is among the most diverse Carboniferous trilobite associations known in North America. This association is dominated numerically by Proetides colemani and exhibits a restricted geographic and stratigraphic distribution. The association is found only in the basal 20 cm of the Chouteau Formation at one locality, in strata representing transgressive subtidal deposits. Association B is found within subtidal and open shelf lithologies and is dominated by two trilobite species, Breviphillipsia sampsoni and Breviphillipsia? swallowi. Breviphillipsia sampsoni, an apparently eurytopic species, is found in both shaly and carbonate strata but is predominately recovered from the shaly partings and interbeds. Breviphillipsia? swallowi, a more stenotopic species, occurs primarily in the more carbonate-rich intervals. In addition, five trilobite species, subordinate in numbers, are only locally common in Association B. The sparse trilobite fauna of Association C is dominated numerically by Namuropyge? armata, and is contained within open shelf sediments that interfinger with mud biostrome lithologies.Early Carboniferous adaptive radiation of trilobites resulted from the creation of new ecospace by transgression. This allowed initially eurytopic trilobites to move into and occupy certain niches that may have been unavailable to them prior to the Frasnian extinctions. With continued deepening, niche-specialized associations developed in more offshore settings.