DIFFERENTIAL PRESERVATION OF PROTOPLASM IN FOSSIL ANGIOSPERM LEAF TISSUES

Abstract

Examination of Miocene angiosperm leaf compressions from the Clarkia and Succor Creek localities of Idaho and Oregon reveals a characteristic and predictable pattern of protoplasmic deterioration. Remnants of chloroplasts are preserved in 63.9% of the 8,800 randomly selected cells, while mitochondria (18.6%) and nuclei (11.3%) are less commonly preserved. This pattern appears to be independent of the presumed environment of deposition associated with fossilization. Leaf tissues from sediments thought to be derived from shallow and deeper waters of the Miocene Clarkia lake (Chloro = 34.6%, Mito < 3%, Nuc < 1% and Chloro = 90.1%, Mito = 26%, Nuc = 4.3%, respectively) show similar patterns of ultrastructural preservation to leaves preserved in volcanic ash‐fall deposits (Chloro = 80%, Mito = 45.4%, Nuc = 39.5%). Analyses of the data for the relative abundance of various organelles per cell (e.g., in living plants Chloro/Nuc = 250) in fossil tissues and referable modern taxa indicate that the chances of the observed preferential degradation of nuclei being the results of chance is <0.001.The data are interpreted to be evidence for the preferential preservation of chloroplasts and degeneration of nuclei. The pattern of protoplasmic degeneration, reconstructed from the frequencies of organelle preservation, is in contrast to that of the pattern observed in modern plant cell necrosis or senescence where nuclei are the most stable organelle and persist until the death of the cell. Preliminary data indicate that regimes of dehydration and compression may duplicate the pattern of protoplasmic degeneration observed in fossil plant cells.