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THE LATE CRETACEOUS AND CENOZOIC HISTORY OF VEGETATION AND CLIMATE AT NORTHERN AND 168 SOUTHERN HIGH LATITUDES: A COMPARISON dispersed plant cuticle fragments. Deciduousness can often be inferred from fossil leaf mats representing autumnal fall, dehiscence structures on conifer short shoots, etc. It is not reliable to extrapolate past leaf retention-shedding from habits of modern analogues, because at least some plants (e.g., Sequoia, which was possibly ancestrally deciduous; Spicer, 1987) may not be obligate evergreen or deciduous taxa, but capable of either habit over time, depending on ambient light related to latitude. Microphyllous-xeromorphic (e.g., small-leaved, needle-leaved) plants can retain their leaves and survive winter cold. In extreme cold, evergreenness is not detrimental because metabolic processes are essentially shut down. In warmer greenhouse winters, however, leaf shedding was suggested as a better strategy to conserve stored metabolites (Spicer, 1987; Spicer and Chapman, 1990). Northern Cretaceous high latitude conifers and angiosperms were almost entirely deciduous, and in the Late Cretaceous the geographic boundary between deciduous (poleward) and evergreen floras coincided approximately with the paleo-Arctic Circle (66°Nâlimit of 24-hr light-dark regime; Wolfe, 1985). Deciduousness still typifies present mid- to high latitude northern vegetation, along with an evergreen mixed coniferous zone. Deciduousness in the north evolved at low to midlatitudes in response to seasonally arid conditions (like modern savanna). Large leaves (for higher productivity) could be produced only if they were shed in the dry season. In conifers, inefficient vascular systems necessitated small leaves, even under relatively favorable conditions, although many conifers were also deciduous. For plant taxa that subsequently grew at high latitudes under a polar light regime, deciduousness was fortuitously advantageous. Extant southern floras are typically evergreen, but it is uncertain what habit prevailed in Cretaceous southern high latitudes. Evergreenness can be advantageous in polar latitudes because less energy is required to produce photosynthetic organs each spring, and a quick start to the short growing season is possible (Spicer and Chapman, 1990). This is true, however, only if freeze damage can be avoided in the winter or, at the other extreme, if warm winter temperatures do not lead to metabolite depletion during respiration. The southern middle to late Cretaceous floras, close to the paleo-Antarctic Circle, included some deciduous taxa. At the end of the Cretaceous, and continuing into the Paleocene, the Seymour area vegetation was evergreen. At those paleolatitudes (60 to 65°S, and this applies to parts of the Antarctic margin in the Paleogene) a prolonged dark period would not occur and an evergreen habit with mild winters would not have been a problem. Thus, warm, moist coastal areas of Antarctica could have supported an evergreen forest. (In northern latitudes, evergreens extended to 70 to 75°N during the Eocene.) There is, however, no fossil evidence to determine which habit predominated in inland Antarctica. There, deciduousness would have been beneficial and perhaps some southern plant taxa assumed a deciduous habit when growing under polar light regimes during greenhouse warmth. However, the greater continentality experienced in the south appears to have given rise to winter temperatures that were low enough to favor evergreenness as a successful strategy (sensu Spicer and Chapman, 1990). Habit of ancestral Nothofagus that evolved under these conditions remains unknown (Dettmann et al. (1990), and extant Nothofagus includes both deciduous (in Patagonia-Tierra del Fuego and Tasmania) and evergreen species. Cenozoic Vegetational Changes In both hemispheres, after a cooler earliest Paleocene, climatic warming into the Eocene was reflected by concomitant changes in high latitude vegetation. Late Paleocene and Eocene floras exhibit increased diversity and complexity and, in both northern and southern high latitudes, include typically warmer-climate (mesothermal) taxa and communities. In the Cenozoic, global climates ultimately were controlled by tectonism that occurred in southern mid- to high latitudes, culminating in the isolation of Antarctica, inception of the Antarctic Circumpolar Current, development of the modern cryospheric ocean, buildup of Antarctic ice, and deterioration into icehouse conditions (references in Kennett and Barker, 1990). Vegetational changes in the north and south correspond to tectonic events and climatic changes (Figures 9.5 and 9.6). Whereas broad dispersal corridors surrounding the North Pole allowed climatically driven northward and southward migrations, post-Eocene Antarctica was completely isolated, resulting in stepwise extinction of taxa with each climatic deterioration. Important sequential cooling steps identified in the Cenozoic southern oceans (summarized by Kennett and Barker, 1990) occurred in the Middle Eocene, near the Eocene-Oligocene boundary, in the middle Oligocene, the Middle Miocene, the early Late Miocene, the latest Miocene, and the Late Pliocene, with some intervening, short-lived warming trends. Southern high latitude floras suffered the loss of many taxa (including mesothermal types) during and at the end of the Eocene. Subsequent loss of most remaining taxa occurred through the Oligocene and Miocene, with an extremely depauperate flora surviving to the Pliocene in coastal refugia. Along ice-free margins, present-day Antarctica supports a cryptogamic flora (e.g., mosses, lichens) with only two vascular plant species found in sheltered areas of the Antarctic Peninsula. A similar diversity decline occurred in northern high latitudes during and at the end of the Eocene with the
