The migration of thermophilic marine Ostracoda into the Arctic Ocean during the Pliocene indicates that winter and summer ocean temperatures around Arctic margins were > 0 øC and > 3 øC, respectively, and that ice-free conditions existed for most or all of the Arctic. By at least 3.5-3.0 Ma, probably earlier, the opening of the Bering Strait allowed marine organisms to migrate through the Arctic Ocean, mostly from the Pacific Ocean. Migrant taxa such as Cythere, Hemicythere, and Neomonoceratina are known from Pliocene deposits of Alaska and Canada as well as Neogene deposits of the North Pacific and Atlantic oceans. On the basis of ecological and zoogeographic information on ostracode species from more than 800 modern "core top" samples for the North Atlantic, North Pacific, and Arctic Oceans, we determined winter and summer temperature tolerances for migrant taxa to be at or above about 0 øC and 3 øC. This suggests ice-free summers, and probably, a perennially ice-free Arctic Ocean in some regions. Elevated water temperatures in the Arctic Ocean between 3.5 and 2.0 Ma is supported by evidence for late Pliocene increased meridional heat transport in the North Ariantic Ocean.Paper number 93PA00060. 0883-8305/93/93PA-00060510.00 INTRODUCTIONThe Arctic Ocean, in general, and Arctic sea ice, in particular, play an important but poorly understood role in oceanic circulation and global climate. Raymo et al. [1990] conducted an experiment using the GISS II atmospheric general circulation model (GCM) to examine the influence of reduced Arctic sea ice, a situation postulated for the Pliocene, on global climate. Among their conclusions, they suggested that late Pliocene cooling, well documented in the North Atlantic deep-sea record and many other paleoclimate proxy records, may have been linked in part to a shift in the Arctic Ocean from perennially ice-free to ice-covered conditions. However, they conclude that paleoclimate evidence from the Arctic is still too fragmentary to identify the forcing mechanisms that changed late Pliocene climate.Perhaps equally important as its influence on atmospheric parameters, Arctic Ocean sea ice contributes in a significant way to deepwater formation and the overall thermohaline circulation of the world's oceans [Aagaard, 1981; Aagaard et al., 1991; Rudels et al., 1991]. Freezing of sea ice and brine rejection in Arctic shelf seas leads to the formation of cold, dense water [Aagaard et al., 1985] and the maintenance of the modem Arctic Ocean halocline [Aagaard, 1981], which serves a key role as a heat sink and which buffers Arctic sea ice from underlying warm Arctic Intermediate Water (AIW) = (the Ariantic layer). A catastrophic breakdown of the halocline may have occurred during Termination I when rapid runoff lowered salinity so much that convection and deep ventilation stopped [Rooth, 1982; Broecker et al., 1985], Smaller-scale oscillations in fresh water from sea ice over historical times can similarly alter convection and deepwater formation [Aagaard and Carmack, 1989] because th...

Species of Cythere (Ostracoda, Crustacea) have about one hundred normal pore systems on each valve of the carapace. These can be classified into five types on the basis of morphological features. They were examined in the light of intraspecific variation and interspecific difference. Each of the five pore systems has its own mode of ontogenetic increase in number. Comparison among fourteen Cythere species shows that the distributional patterns are completely identical in the A‐3 stage, but that they are classified into four and eight groups in the A‐2 and A‐1 stages, respectively. In the adult stage the pattern becomes diversified among the fourteen species, though homologous relationships are still recognized between different species. The similarity and difference of the pore patterns, which are traceable until the last ontogenetic stage, can be used to determine phylogenetic relationships. These findings were applied to the reconstruction of phylogeny of the eleven living and three extinct species of Cythere.□Crustacea. Ostracoda, Cythere, Ontogeny. Phylogeny, pore system, pore canal. speciation, laroal morphology.

The genus Semicytherura belongs to the family Cytheruridae, and was distinguished from Cytherura on the basis of carapace features. Species of Semicytherura from Japan and adjacent seas can be divided into two groups. One is represented by Semicytherura miurensis Hanai, 1957, characterized by a thin, oval carapace covered with fine reticulation. The other is represented by Semicytherura henryhowei Hanai & Ikeya, 1977, characterized by a thick sub-rectangular carapace in lateral view. Semicytherura henryhowei, which is distributed from Hokkaido to Okinawa in Japan, has been regarded as having several morphotypes distinguishable on outline and reticulation of carapace. However, as a result of detailed observations on the copulatory organ, carapace outline and distributional pattern of pore systems, remarkable differences are shown to exist between the two most frequently occurring morphotypes. In order to recognize S. henryhowei sensu stricto, the carapace of the holotype was re-examined. Consequently, neither of the two morphotypes are considered to belong to S. henryhowei due to differences of carapace outline and distribution of pore systems. The two morphotypes are here regarded as independent taxa, described as new: S. kazahana n. sp. and S. sasameyuki n. sp. The geographical distributions of the two new species overlap, but their micro-habitats differ from each other; the former lives on calcareous algae on rocky shores, the latter lives on silty sand bottom within the inner bay. A third new species, S. slipperi sp. nov., is also described. In view of their present geographical distributions and fossil records, the origin of this group of species would appear to be the Japanese islands or adjacent areas in and after the Miocene. This group then migrated to the Arctic Ocean and East Pacific Ocean during or before the middle Pliocene.

The pore-systems of 17 extant species of Loxoconcha around Japan were studied in order to understand their phylogeny and evolution. The phylogeny was estimated by two steps. First, the 17 species were divided into two groups, Group A (12 species) and Group B (five species) by Pore pattern Below Eye tubercle (PBE) analysis. Then, intragroup relationships were estimated by Differentiation of Distributional pattern of Pore-system (DDP) analysis. PBE analysis reveals that species of Groups A and B have on average different ecological preferences. Species of Group A, which appeared in the late Pliocene, are more diverse, have both phytal and bottom-dwelling modes of life, possess fewer pore-systems in the ventral area, and inhabit normal marine environments. Species of Group B, whose oldest fossil record is the lower Miocene, are less diverse, have only bottom-dwelling species, possess more pore-systems in the ventral area, and tend to inhabit brackish water environments. The results of this study suggest that the differences in ecology may have had an impact on the late Cenozoic diversification around Japan. The primary invasion of Group B occurred before the lower Miocene,with no subsequent diversification. Group A invaded after the late Pliocene and immediately diversified, which created the present abundance of Loxoconcha species around Japan in both species diversity and variety of modes of life.

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