02683nas a2200181 4500008004100000245012000041210006900161300001200230490000800242520208800250653002202338653001402360653002502374653002002399100001502419700001602434856005102450 2010 eng d00aEarly Ordovician Community Evolution with Eustatic Change Through the Middle Beekmantown Group, Northeast Laurentia0 aEarly Ordovician Community Evolution with Eustatic Change Throug a174-1880 v2943 a

The Beekmantown Group records the important early interval of the Ordovician Radiation. This Upper Cambrian–Middle Ordovician, carbonate-dominated, tropical succession was deposited near the eastern passive margin of the Laurentian platform. This depositional setting remained remarkably stable although the craton was flooded repeatedly with eustatic rises and unconformity-bound, macroscale sedimentary cycles were deposited as successive geological formations. The individual depositional cycles (i.e., formations) show a nearly identical vertical succession with a type 1 sequence boundary, a basal conglomerate, transgressive sandstones, locally a subtidal shale-dominated unit that marks the deepest facies, and a highstand carbonate facies with thrombolite buildups in its middle part. The thrombolitic buildups of each depositional cycle contain a mollusc-dominated macrofauna that changed remarkably from cycle to cycle. In the limestones of the Upper Cambrian Ritchie and Rathbunville School members, the macrofauna is very rare and of low diversity. By comparison, the absolute abundance of macrofossils is high throughout the Lower Ordovician thrombolitic limestones. The genus-level diversity of brachiopods, trilobites, gastropods, and cephalopods increased moderately during the three Lower Ordovician depositional sequences. Dramatic changes in cephalopod disparity, body size, and biomass indicate significant paleoecological changes at the top of the ecosystem food chains, and are an indication of community evolution and intrinsic evolutionary processes. Increased coiling and ornamentation in cephalopods and an increasing number of large gastropod genera with thick shells indicate an escalation among predators. We interpret these changes as evidence for a rise in competition within ecological guilds by a continuing increase in internal differentiation of the food web. Increased organismal interaction and the differentiation of the food web (i.e., community evolution) are regarded as a major driving mechanism early in the Ordovician Radiation.

10aBeekmantown Group10aLaurentia10aOrdovician Radiation10aPalaeodiversity1 aKroger, B.1 aLanding, E. uhttp://dx.doi.org/10.1016/j.palaeo.2009.11.02502634nas a2200205 4500008004100000245013100041210006900172300001200241490000700253520198300260653001602243653002602259653002102285653001302306653002502319653001202344100001502356700001602371856004102387 2009 eng d00aCephalopods and Paleoenvironments of the Fort Cassin Formation (upper Lower Ordovician), Eastern New York and Adjacent Vermont0 aCephalopods and Paleoenvironments of the Fort Cassin Formation u a664-6930 v833 a

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.

10aCephalopoda10aFort Cassin Formation10aLower Ordovician10aNew York10aOrdovician Radiation10aVermont1 aKroger, B.1 aLanding, E. uhttp://www.jstor.org/stable/20627655