CRYPTOSPORES FROM FOSSIL MOUNTAIN AND BOWER'S ANTITHETIC ORIGIN OF PLANTS

In 1908, F. O. Bower predicted that the transition from green algae to land plants involved the interpolation of a diploid sporophyte phase in a gametophyte-dominated life cycle. He referred to the gametophyte as "amphibious" in character with reference to the requirement in plants for a film of water for fertilization. The sporophyte was considered the first plant phase that was truly subaerial. His antithetic hypothesis made some specific predictions about hypothetical initial sporophytes: 1) the first cells of the diploid generation would be sporogenous tissues, 2) that the first vegetative tissues of the sporophyte phase would have resulted from the subsequent loss of spore-generating capacity. Thus he predicted an initial phase of multicellular spore-generating tissue as defining the first subaerial sporophyte. We have recovered sheets of spore-like cells from the Kanosh Shale at Fossil Mountain Utah. These Ordovician cryptospores pre-date the first reported occurrences of trilete spores and tetrahedrally-arranged tetrads that mark a transition from sporogenesis via successive meiosis to simultaneous cytokinesis that characterizes land plants today. The microfossils are composed of highly organized unilayered sheets of cell packets, formed via divisions in two planes. Each of the individual packets is similar to the recently described Upper Cambrian cryptospore, Agamachates casearius Taylor & Strother. These microfossils appear to show thalloid, vegetative morphology yet they are composed entirely of spore-like cells. Thus, they fit entirely within Bower's prediction for the first transitional phase of the antithetic origin to the plant sporophyte.