CARBON ISOTOPIC ABUNDANCES IN MESOZOIC AND CENOZOIC FOSSIL PLANTS - PALEOECOLOGICAL IMPLICATIONS

Carbon isotopic abundances have been measured for more than one hundred samples of fossil plants ranging in age from middle Triassic to late Tertiary. Most of the plant fossils were identified at the specific or generic level and were selected as representing a variety of continental environments, including xeric and humid habitats. Material analysed included numerous fragments of flowers, seeds, fruits, leaves and wood, as well as a single amorphous lignite sample. The analyses performed for the plant fragments indicate relatively constant isotopic compositions during this time interval, with plant deltaC-13 values ranging between -28 and -20 parts per thousand. These values are within the range for living terrestrial plants with C3 photosynthesis, although values more positive than -23 parts per thousand are rare in C3 plants and typically found in plants growing under environmental stress. Lower deltaC-13 values might have been expected owing to the much higher CO2 levels of the Cretaceous atmosphere that have been inferred from marine carbonates. No fossILs with values indicating C4 photosynthesis were discovered. FossIL plants from inferred mesic environments showed deltaC-13 values ranging ;between -26.7 and-24.1 parts per thousand. Highest deltaC-13 values in angiosperms (up to -20.1 parts per thousand) were measured for Late Cretaceous combretaceous flowers from Portugal. Some cheirolepidiaceous conifers from the Early Cretaceous also showed high deltaC-13 values. Values measured for Pseudofrenelopsis varians and Glenrosa texensis were -21.9 parts per thousand, and values of gymnosperm wood, probably of cheirolepidiaceous affinity, were -19.0 parts per thousand. These high values are in accordance with inferred ecological conditions for the fossil plants. They may suggest a tendency for C4-like photosynthesis, although the data are equivocal. Higher values (-17.3 parts per thousand) clearly falling outside the C3 range were, however, obtained from a single lignite fragment of Late Cretaceous (Maastrichtian) age. The nature of this plant fragment is unknown, but the result suggests that C4-like photosynthesis was present at least in some latest Cretaceous vegetation. A hadrosaurian dinosaur with well-preserved collagen-like organic matter from the same deposit showed deltaC-13 values around -16 parts per thousand, which also suggests the presence of CAM or even C4 plants in the latest Cretaceous.