Middle Ordovician facies and trilobite faunas in N America

Abstract

SummaryThe generic composition of trilobite faunas around the perimeter of North America during the Medial Ordovician is related to the regional palaeogeography deduced from field evidence. Major divisions into shelf, shelf-edge (biohermal mounds) and slope (upper and lower) can be recognized. The Caradoc immersion of the N American plate resulted in landward invasion of pre-adapted slope forms. Some of these genera had European origins and their appearance is due to migration in deeper water facies accompanying closure of the Proto-Atlantic ocean. There is no obvious repeat of Cambrian biomere patterns in the Ordovician although some later Ordovician trilobites, especially shelf inhabitants, had their origins in earlier Ordovician biohermal or slope communities. The origin and composition of the Remopleuridid faunal Province is briefly discussed.