TAYLOR, WILSON A. Department of Biology, University of Wisconsin-Eau Claire, Eau Claire, WI 54702-4004. - Assessing the earliest putative land plant remains - Cambrian cryptospores.
The pace of discovery of early land plant remains has broadened the
temporal gap between the first appearance of microfossils and that of
megafossils. Representatives of all major categories of cryptospores
(monads, dyads and tetrads) as well as minute trilete spores have been
recovered from Middle Cambrian sediments of terrestrial provenance
(conservatively, a fifty million year gap). What clues do we have to
the affinity of these microfossils? Those that have been examined so
far with the TEM (Bright Angel Shale) possess a three-layered wall
that surrounds each member of the unit. While this wall is
morphologically comparable to that seen in some modern green algal
cysts (e.g., Mychonastes desiccatus, Chlorococcales), the
thickness of the individual sporopollenin lamellae differs by a factor
of two. The presence of thinner fossil lamellae in somewhat younger
specimens suggests that preservation is not a limitation. Comparisons
with other fossil and modern sporopollenin-containing propagules as
well as less equivocal cryptospore remains from the Middle Cambrian
Rogersville Shale may provide some of the first clues to the possible
identity of these earliest land plant pioneers.
Key words: Cambrian, cryptospores, microfossils, Paleozoic, palynology, spores