BOYCE, C. KEVIN1*, ANDREW H. KNOLL1, GEORGE D. CODY2, MARILYN L. FOGEL2, and ROBERT M. HAZEN2. 1Harvard Botanical Museum, 26 Oxford St., Cambridge MA 02138 USA; 2Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd., Washington D.C. 20015. - The evolution of tracheids and lignification in early land plants.
Anatomically preserved land plant fossils from the Lower Devonian
Rhynie Chert contain conducting tissues that range from dark colored,
elongated cells without secondary wall thickenings to tracheids
similar to those of extant plants. Observed differences in conducting
cell differentiation may or may not correspond to the physiological
innovations of lignin biosynthesis and deposition. In order to
evaluate the degree to which the morphological and physiological
evolution were coupled in the tissues of early land plants, three
analytical techniques were employed to study anatomically preserved
fossils: electron microprobe analysis to obtain micron-scale
resolution of elemental abundances, isotope ratio mass spectrometry to
obtain millimeter-scale resolution of carbon isotopic abundances, and
X-ray microscopy in conjunction with nuclear magnetic resonance
spectroscopy to obtain micron-scale resolution of the preservation of
organic molecular functionality. Studies demonstrate that original
organics are not homogenized during diagenesis and are preserved in
place with sufficient spatial resolution for paleobiological
investigations. Tissue-specific isotopic and organic chemistry of
Rhynie Chert plants suggests that the anatomical evolution of tracheid
secondary thickenings and the biochemical evolution of lignin
synthesis were originally independent: the earliest vascular
thickenings were unlignified and cell wall lignification may have
first appeared in other tissues. Only later were the anatomical and
biochemical characteristics coupled to produce true tracheids like
those of all living vascular plants.
Key words: geochemistry, lignin, paleobotany, Paleozoic, Rhynie Chert