Abstract The Arctic Ocean is unique among the world’s oceans because of its perennial ice cover. The geologic and climatologic factors that contributed to development of the Arctic Ocean ice cover are understood in a general way, even though the precise mechanism and time during the Cenozoic that the first ice cover formed are not known. Data concerning climatological processes that encouraged development of an Arctic Ocean ice cover have developed from the general understanding of the paleogeographic sequence of events since the last major time of ice-free conditions during the Cretaceous and early Cenozoic. The lack of facts concerning the precise time, and to some extent the mechanism, of ice cover origin is largely the result of an inadequate data base in the Arctic Ocean. For example, no long sediment core with middle Cenozoic sediment that may represent the time of the initial ice-cover development has been collected. Unfortunately, no research ship with capability for recovery of long sediment cores has been designed for work in the area of year-round Arctic pack ice. Therefore, the only sediment record for the central Arctic Ocean is that recovered from drifting ice stations such as the U.S. T-3 program and the Canadian LOREX and CESAR projects. Offshore drilling on the continental slope of Alaska and Canada has penetrated a more complete Cenozoic section. The sediment is largely non marine and, in the shallow Beaufort Sea area, consists of thick deltaic sediment. Detailed paleoclimato- logic study of this sediment has not been accomplished, but

Abstract High-latitude polar regions of the Earth have experienced cold, cool, and temperature paleoclimates in the course of their geologic history, but they have probably always been colder than low-latitude continents and oceans. Extreme climates leading to development of extensive frozen and ice-covered regions at high latitudes can, however, only be documented for a few, relatively short intervals of the Earth’s history, separated by long time spans with little or no ice (Frakes, 1979). The Cenozoic evolution of glacial-type climates during the past 30 to 40 m.y. is the most recent period of extreme climate, and differs from the preceding ones. During the Cenozoic, plate-tectonic processes generated climatically isolated land areas and ocean basins in both the Southern and Northern Hemispheres, which were repeatedly affected by glaciations. For glacial-type paleoclimates older than the Cenozoic, we have only been able to document unipolar glaciation because the opposite high-latitude area was situated in wide and deep ocean basins and was probably relatively ice free due to advection of warmer surface water from lower latitudes. Despite the apparent similarity of Quaternary high-latitude paleoclimates, the development of glacial-type paleoceanographies of the northern and southern polar oceans have revealed important differences, and they are not easily compared with each other. Our understanding of Cenozoic Southern Hemisphere paleoclimates is much more advanced than it is for the Northern Hemisphere. It is particularly intriguing that the available data appear to indicate that the Southern Hemisphere may have become cold more than 20 m.y. earlier than its northern counterpart.

Most biostratigraphic schemes for conodonts (and other fossils) are based on a combination of geographically separated stratigraphic sequences that contain abundant fossils. In developing biostratigraphy, generally little attention is given to identification of lithofacies that yield the fossils. Many sections in western North America include a mixture of sediments that represent a variety of marine environments. Such a stratigraphic mix of environment types yields a vertical sequence of fossils that represent a range of environments. Geographically separated but chronologically similar sections containing different environmental sequences may produce different sequences of fossils within a similar stratigraphic framework. Because of this, biostratigraphic schemes for similar age sequences may differ. Lower Triassic biostratigraphy in western North America is based on study of rock sequences that represent a variety of transgressional and progradational sediments. The “standard” Lower Triassic conodont zonation that has emerged is real, even usable, but includes conodont species representing a range of environments from shallow inner shelf to deeper basinal. This ecologic mix has utility simply because most comparable Triassic rock sequences are also a mix of lithofacies, and at least some of the conodont species in the “standard” can be found in any other section. Because certain conodont species probably were more sensitive to a narrow set of environmental parameters than has been realized, application of the ecostratigraphy concept may develop actual conodont biofacies biostratigraphy, a different biostratigraphy for each definable position along an environmental gradient.

A computerized file of approximately twenty thousand records of conodont occurrences was used in a quantitative study of conodont provincialism. Although biases in the fossil record, in specimen collection, and in data collection preclude any rigid statistical testing, study of quantitative measures of similarity between faunas, when combined with paleogeographic reconstructions, can give insight into provincial patterns and their possible causes. Conodonts showed strong provinciality three times during Paleozoic time. In each instance, temperature is a plausible control of the provincial distribution. In the Ordovician, one fauna inhabited the low to mid latitudes in Laurentia, China, Siberia, and northern Gondwana. Another fauna inhabited high latitudes in Baltica. Cooler high-latitude temperatures as compared to warmer low-latitude temperatures could have been the factor controlling distribution. In the Early Devonian, the Aurelian-province fauna (in present-day Europe and Turkey) inhabited a semirestricted seaway, while the Tasman-Cordilleran-province fauna (in present-day western North America, Siberia, and Australia) occupied the shores of a larger ocean. Eastern North America had a seemingly transitional fauna. These provinces were all in low to mid latitudes, but reconstructed current patterns suggest a warmer temperature in the Aurelian seaway than in the larger ocean. In the Pennsylvanian and Permian, the fauna in western Pangea (present-day North America) differed from that in eastern Pangea (present-day Eurasia). Again both provinces were in low to mid latitudes, but a stronger westward equatorial current due to the Pennsylvanian-Permian glacial episode could have contributed to a warming of the eastern (Tethyan) coast relative to the western coast.

Modern ecologic models for conodonts were extrapolated principally from experience with North American shallow-water subequatorial faunas. Further evidence can be derived from the calcareous lower part of the Swedish Ordovician. This succession among other things offers uniformity of facies, as well as long-ranging conodont genera. Paleomagnetic data indicate deposition at 60°S, i.e., relatively cool climate and fluctuations in air and shallow-water temperatures. The succession might represent a subantarctic shallow-water carbonate platform. Another interpretation favors depth of 100 to 500 m. The relative frequencies of long-ranging conodont genera were plotted against facies data. All data indicate complexity of interaction of depth, temperature, and current dependent factors that influenced the distribution of conodont genera, in particular during a regressive phase about the Arenigian-Llanvirnian transition. Microzarkodina, Periodon , and Protopanderodus had extended frequency minima during the regression. Paroistodus was abundant before the regression, then apparently disappeared from Europe. In a section at Skövde, formed perhaps in particularly deep water, Baltoniodus has a minimum and Drepanoistodus a maximum that might correspond to the peak of regression, but elsewhere the conditions are either ambiguous or reversed. At least Protopanderodus and Periodon probably were not epibenthic, since they occur with shelly fauna in carbonates as well as with graptolites in dark mudstones and with radiolarians in ophiolite-associated cherts (in Scotland). The importance of sorting by differential transport is stressed throughout the study.

Conodonts are divided into three groups with different histologies: protoconodonts (most primitive), paraconodonts, and euconodonts (most advanced). The first is poorly known, but paraconodonts included a Westergaardodina and a coniform evolutionary lineage, and each was the ancestor of one or more euconodont lineages. Early euconodonts are thus polyphyletic and included the Proconodontus and Tendonitis Lineages, which appeared in the middle Late Cambrian, and the Fryxellodontus and Chosonodina Lineages, which appeared in the Early Ordovician. Major changes in conodont evolution, biofacies adaptation, and development of provincialism coincided with sea-level fluctuations near the end of the Cambrian (here named the Lange Ranch Eustatic Event, or LREE) and similar fluctuations recorded at the Lower/Upper Tremadoc boundary (here named the Black Mountain Eustatic Event, or BMEE). Protoconodonts and paraconodonts were probably pelagic and cosmopolitan. Genera of the Proconodontus Lineage were probably also pelagic. Some genera of the latter lineage are found only in low- to mid-paleolatitude areas; others were cosmopolitan, including Cordylodus . Genera of the Teridontus and Fryxellodontus Lineages may have been nektobenthic. Some were adapted to warm, high-salinity environments that existed during the LREE, but younger genera probably were adapted to normal salinity and were more widely distributed. No apparent provincialism existed until the appearance of euconodonts, after which two broad faunal realms are distinguishable. The warm faunal realm included shallow seas in low to middle paleolatitudes; the cold faunal realm included high-paleolatitude seas and open-ocean areas. Early euconodonts of the Proconodontus Lineage appeared and quickly became dominant in the warm faunal realm during the latest Cambrian. Much of the preexisting protocondont-paraconodont fauna was displaced from the warm faunal realm but continued to dominate the cold faunal realm through the Early Tremadoc. Major faunal changes occurred in the warm faunal realm as a result of the LREE, and after this event conodonts in this ream consisted for the most part of genera from the Teridontus Lineage. During the BMEE a different euconodont fauna of uncertain ancestry became adapted to the cold faunal realm, after which most of the previously dominant primitive fauna became extinct. Cosmopolitan Cordylodus lived in both faunal realms during much of the Tremadoc, but after it became extinct prior to the Arenig, provincialism was extreme because few species were adapted to both faunal realms. Oneotodus tenuis Müller is reclassified as the type species of a new genus, Phakelodus.

R- and Q-mode cluster analysis of data on the occurrence and distribution of 43 conodont species enables delineation in North America of warm-water Red River and Ohio Valley provinces during the Late Ordovician Velicuspis Chron, and suggests recognition of six major biofacies that represent a continuum from nearshore, shallow-water biotopes with numerous endemics to offshore, deeper-water biotopes characterized by more cosmopolitan species. Approximately coeval conodonts from Great Britain, Baltoscandia, and continental Europe are assignable to at least 36 taxa, which are less well known than those of equivalent age in North America but represent cold-water faunas whose Late Ordovician distribution and frequency of occurrence may be used to characterize British, Baltoscandic, and Mediterranean provinces, within which we recognize only three distinct biofacies. Only a third of the taxa in the Late Ordovician cold-water region are also represented in warm-water areas, where they characterize relatively deeper-water biofacies or have a distribution that indicates they were eurythermal cosmopolites. Late Ordovician conodonts are treated as components of warm- and cold-water pelagic faunas, not because their distribution demands that interpretation, but because the pelagic model is simpler than a benthic or nektobenthic one and squares readily with available distributional data.