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Paleontology (continued) Paleoclimatology Fossils provide very significant clues to ancient climates. The presence of growth rings in ancient woods, for example, provides evidence of seasonality, and the shapes of tree leaves prove to be a sensitive gauge of temperature—leaves with smooth margins are typical of warmer climates, whereas those with ragged or serrated margins typify cooler temperatures. The ratio of the two leaf types can be used to hindcast ancient mean annual temperatures to within 1°C. In a similar way, the coiling directions (left-hand versus right-hand) in certain planktonic foraminifera (shelled protozoans) are temperature- dependent (more left-handed coiling in cooler temperatures) and the left–right ratio can be used to hindcast temperatures accurately, at least for the last few million years. Furthermore, the geochemistry of fossil skeletons may en- code information about paleotemperatures. For example, the oxygen isotopic ratios (180/160) of calcium carbonate (calcite or aragonite) of normal marine shells are dependent upon the temperature at which they were secreted. Thus, the 180/160 ratio may be utilized indirectly to reconstruct the temperature of the water from which they were secreted. Paleogeography Fossils are indispensable guides for determining the posi- tions of continents and seas in former times. Organisms isolat- ed by barriers (such as deep ocean basins) evolve, although in some cases with parallel evolution to those in other areas, lead- ing to geographically distinct provinces. The history of chang- ing fossil assemblages provides evidence for the development and breakdown of barriers. The formation of the Isthmus of Panama can be dated, for example, by studying the distribu- tions of marine and terrestrial fossils. Before North America and South America were connected by this land bridge, Atlantic and Pacific marine faunas were very similar, but the mammals of the two continents were completely different. However, in South American rocks deposited after the Isthmus was formed, there are fossils of North American mammals, which had mi- grated over the newly formed land. Likewise, Atlantic and Pa- cific marine faunas, isolated from each other by the rise of the Isthmus, began to evolve in different directions; this increasing difference can be traced in the fossils of successively younger rocks. Fossils also provide evidence that continental drift has occurred. If South America and Africa were once united, their faunas should be similar during that time. As they drifted apart, there should be stronger and stronger faunal differences. Biological aspects The key biological aspects of paleontology support the evo- lutionary processes of life throughout geologic time. Evolutionary process and life history The most fundamental fact of paleontology is that organ- isms have changed throughout the history of Earth and that each geological period has its characteristic forms of life. When Charles Darwin published his theory of evolution in 1859, pale- ontological evidence for evolution was meager, but it began to accumulate quickly. The "feathered reptile" (Archaeopteryx), an almost perfect link between dinosaurs and birds, was discov- ered early in the 1860s, and the first fossil humans were found a few years later. In the light of this evidence and the purview of a more scientific culture, evolution was accepted as the only reasonable explanation for change in the history of life. To a pa- leontologist, life's outstanding attribute is its evolution (Fig. 5). + ward ' s science Fig. 5: Processes of evolution. (a) Heterochrony illustrated by the retention of larval features (neoteny) in evolution of the axolotl with adult gills from a normal salamander; (b) adaptive radiation of specialized descendants from a common ancestor; (c) gradual evolutionary trends in fossil oysters; (d) branching evolutionary patterns involving cladogenesis in horse evolution; note brushy rather than straight-line evolutionary patterns; (e) mass extinction illustrated by the Cretaceous/Tertiary boundary extinction; (f) adaptive transition involving modification (exaptation) of tetrapod limbs from lobed fins of fish; and (g) evolutionary convergence (parallelism) in marsupial and placental mammals. (Credit: Adapted from an original drawing by S. M. Stanley)