75. Niklas, K. J., Brown, Jr. R. M., Santos,
R., and B. Vian. 1978. Ultrastructure and cytochemistry of Miocene
angiosperm leaf tissues. Proc. Nat. Acad. Sci. U.S.A. 75:3263-3267.
75. Abstract
Angiosperm leaf fossils (16.7-25 x 106
years by potassium/argon dating) referable to Zelkova were
isolated from pyroclastic deposits in the Succor Creek Formation,
Oregon. These fossils reveal a three-dimensional structure in
mesophyll cell layers and vascular bundles, while transmission
electron microscopy of fixed tissues reveals well-preserved chloroplasts
with grana stacks and starch, as well as nuclei with condensed
chromatin. Transmission electron microscopy indicates that cell
walls retain a cellulosic microfibrillar architecture, while ultrastructural
and cytochemical confirmation is presented for chloroplast starch
and the presence of native cellulose. The preservation of cell
walls and cytologic structures in fossil materials of this age
indicates an unusually mild fossilization process attending the
post-depositional maturation of associated sediments. The preservation
observed appears to be associated with rapid physical dehydration
coupled with endogenous fixation by organic acids (possibly tannins
and/or chlorophyll derivatives).