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Journal of Paleontology

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CEPHALOPODS AND PALEOENVIRONMENTS OF THE FORT CASSIN FORMATION (UPPER LOWER ORDOVICIAN), EASTERN NEW YORK AND ADJACENT VERMONT

¹ Université des Sciences et Technologies de Lille 1, Sciences de la Terre, Laboratoire de Paléontologie et Paléogéographie du Paléozoïque (LP3), Bâtiment SN5, 59655 Villeneuve d'Ascq Cedex, France; current address, Museum für Naturkunde, Humboldt Universität zu Berlin, D–10115 Berlin, Germany <bjoekroe{at}gmx.de>
² New York State Museum, Madison Avenue, Albany, New York 12230 <elanding{at}mail.nysed.gov>

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? pusillum Ruedemann, 1906; Cameroceras annuliferum Flower, 1941; Cyptendoceras whitfieldi Ulrich et al., 1944; Endoceras? champlainense Ruedemann, 1906; Wolungoceras valcourense Flower, 1964. Beekmanoceras Ulrich and Foerste, 1936 is a gastropod.

Key Words: Cephalopoda • Lower Ordovician • New York • Vermont • Fort Cassin Formation • Ordovician Radiation