490 TERTIARY TO PRESENT/Pliocene played a major role in the onset of northern hemisphere glaciation by redirecting surface currents within the Atlantic Ocean since 4.6 Ma and by influencing salinity levels The resulting intensification of the Gulf Stream increased the transport of warm saline surface water to northern high latitudes and, consequently, the formation of North Atlantic deep water, which in turn, ensured increased import of warmer South Atlantic surface water into the northern hemisphere (Figure 1) Intensification of this marine ‘conveyor belt’ increased the potential for evaporation at high northern latitudes, thus supplying the additional moisture that would facilitate ice-sheet growth The gradual shoaling of the Panama isthmus and the associated changes in ocean currents and salinity gradients did not in themselves cause the onset of large-scale northern hemisphere glaciation Initially, the enhanced Gulf Stream and increased transport of warm surface water into the high northern latitudes pushed global climates towards warmer conditions, culminating in the mid-Pliocene warm interval The emergence of the Panama isthmus contributed significantly to creating the necessary preconditions for the initiation of large-scale northern hemisphere glaciation However, other factors may have been involved (e.g deepening of the Bering Straits, increased river discharge of fresh water into the Arctic Ocean from the Palaearctic) Continued Pliocene uplift or rejuvenation of various mountain ranges and plateaus across the world has also been suggested as a cause for the deterioration of Late Neogene climates Invoked mechanisms include the influence of high elevation areas on atmospheric circulation as well as the reduction of atmospheric CO2 concentrations through the weathering of newly exposed silicates and the resulting formation of carbonates, the increased burial of organic carbon due to increased rates of erosion, or increased ocean productivity through an increased delivery of phosphorus to the ocean, again as the result of increased erosion It is likely that several of these factors combined to create the preconditions necessary for the onset of northern hemisphere glaciation The ultimate trigger, however, lay with the nature of fluctuations in the Earth’s obliquity (see Earth: Orbital Variation (Including Milankovitch Cycles)) Low tilt angles of the Earth’s rotational axis result in cold northern hemisphere summers Just prior to the onset of northern hemisphere glaciation, 4.5–3.1 Ma, there was a prolonged period during which the amplitude of Earth’s obliquity fluctuated only minimally Records of d18O indicate that during this time, limited glaciations of the Arctic were initiated several times, but failed to substantiate From around 3.0 Ma the gradual increase of the amplitude of obliquity fluctuations set the final stage for the current pattern of pronounced glacial and inter-glacial periods, which have prevailed since the end of the Middle Pliocene Biotic Events The Great American Interchange The emergence of the isthmus of Panama in the Middle Pliocene resulted in the first contact of South America with another continent since the opening of the Drake Passage finalized its separation from West Antarctica toward the end of the Eocene Having been rifted from Gondwanaland $100.0 Ma and spent most of the Cenozoic in total isolation, South America was inhabited by a distinct biota with a large proportion of endemic taxa In contrast, the North American continent had previously had a long history of intermittent connections and resulting floral and faunal interchange with the Old World The emergence of a permanent land connection between these two sub-continents resulted in a substantial level of terrestrial biotic interchange However, not all taxa proved equally successful at dispersing and the biotas of the two sub-continents were not equally affected With respect to long-term effects, the influence that southern taxa have had on North American biotas is near negligible In contrast, the long-term effect that the great American interchange has had on the South American mammal fauna is substantial Today nearly half of the families and genera of South American mammals are members of groups that emigrated from North America since the emergence of the Panama land bridge This does not appear to be due to a substantially larger number of species initially dispersing from north to south as opposed to from south to north In fact, when differences in source area are taken into account, the extent of north to south dispersal was similar to that from south to north The difference in the proportions of modern representatives arose through the fact that North American immigrants to the south underwent substantial diversification in their new environments, whereas South American immigrants in the north did not The reasons for the substantial differences in success rates between North American and South American immigrants are not clear Movements across the Panamanian isthmus were not restricted to mammals, but data for other groups are scarcer For birds the available data suggest a net movement of taxa from north to south, whereas the majority of dispersals of reptiles and amphibians appear to have been from south to