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

Carbon isotopic abundances have been measured for more than one hundre d samples of fossil plants ranging in age from middle Triassic to late Tertiary. Most of the plant fossils were identified at the specific o r generic level and were selected as representing a variety of contine ntal environments, including xeric and humid habitats. Material analys ed included numerous fragments of flowers, seeds, fruits, leaves and w ood, as well as a single amorphous lignite sample. The analyses perfor med for the plant fragments indicate relatively constant isotopic comp ositions during this time interval, with plant deltaC-13 values rangin g between -28 and -20 parts per thousand. These values are within the range for living terrestrial plants with C3 photosynthesis, although v alues more positive than -23 parts per thousand are rare in C3 plants and typically found in plants growing under environmental stress. Lowe r deltaC-13 values might have been expected owing to the much higher C O2 levels of the Cretaceous atmosphere that have been inferred from ma rine carbonates. No fossILs with values indicating C4 photosynthesis w ere discovered. FossIL plants from inferred mesic environments showed deltaC-13 values ranging ;between -26.7 and-24.1 parts per thousand. H ighest deltaC-13 values in angiosperms (up to -20.1 parts per thousand ) were measured for Late Cretaceous combretaceous flowers from Portuga l. Some cheirolepidiaceous conifers from the Early Cretaceous also sho wed high deltaC-13 values. Values measured for Pseudofrenelopsis varia ns and Glenrosa texensis were -21.9 parts per thousand, and values of gymnosperm wood, probably of cheirolepidiaceous affinity, were -19.0 p arts per thousand. These high values are in accordance with inferred e cological conditions for the fossil plants. They may suggest a tendenc y 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 Cretac eous (Maastrichtian) age. The nature of this plant fragment is unknown , but the result suggests that C4-like photosynthesis was present at l east in some latest Cretaceous vegetation. A hadrosaurian dinosaur wit h well-preserved collagen-like organic matter from the same deposit sh owed deltaC-13 values around -16 parts per thousand, which also sugges ts the presence of CAM or even C4 plants in the latest Cretaceous.