The dramatic late Early Ordovician radiation of cephalopods on tropical paleocontinents is illustrated by the diverse fauna (21 genera, 30 species) of the Fort Cassin Formation (Floian and lower Blackhillsian Stage) in northeast Laurentia. Cephalopods occur through the thin (ca. 30–65 m) depositional sequence of the Fort Cassin but are most common and diverse in mollusk-rich, trilobite-poor parts of the formation that characterize the thrombolite-bearing intervals in the shoaling part of the highstand systems tract. This lithofacies-biofacies linkage persists from the Tribes Hill and Rochdale Formations (lower and lower upper Tremadocian, and upper Skullrockian and Stairsian Stages, respectively), and suggests that the Early Ordovician radiations of cephalopods took place in shallow-marine, thrombolite reef facies of tropical carbonate platforms. These habitats differed strongly from the near-shore, peritidal habitats of the older Cambrian evolutionary radiation. Genus-level diversity and absolute abundance changed little through the Skullrockian–Blackhillsian, but morphologic diversity and body size increased dramatically by the late Early Ordovician. The morphological diversification suggests cephalopods diversified into a wider variety of macropredators and more complex late Early Ordovician ecosystems. Anrangeroceras whitehallense n. gen. and n. sp. is proposed. The following are emended: the Protocycloceratidae, Centrotarphyceras and C. seelyi, Protocycloceras and P. lamarcki, and Rudolfoceras cornuoryx. The following are indeterminate and abandoned: Baltoceras? pusillumRuedemann, 1906; Cameroceras annuliferumFlower, 1941; Cyptendoceras whitfieldiUlrich et al., 1944; Endoceras? champlainenseRuedemann, 1906; Wolungoceras valcourenseFlower, 1964. BeekmanocerasUlrich and Foerste, 1936 is a gastropod.