ORDOVICIAN “SPORE-THALLI” FROM THE KANOSH SHALE AT FOSSIL MOUNTAIN, UTAH

The Dapingian-Darriwilian shales at Fossil Mountain contain a mixed palynological assemblage that includes acritarchs, isolated cryptospore tetrads and dyads, and geometrically regular sheets of spore-like dyads. These sheets retain evidence of their formation via successive orthogonal divisions to produce somewhat regular, unilayered packets of spore-like cells. Morphologically, these packets of dyads resemble the vegetative thalli of the extant chlorophyte, Prasiola, but, the overall shape of the most complete thalli mimic the extant charophyte, Coleochaete. TEM analysis by W. A. Taylor (University of Wisconsin Eau Claire) shows no internal wall structure, as the dyad bodies are completely fused together. This condition reflects the high thermal alteration state (TAI~4) of organic matter throughout the section.

Loosely aligned, unilayered, packets of the cryptospore Adinosporus voluminosus, have been recovered in the Conasauga Group (Cambrian, Series 3) in eastern Tennessee, but they do not form the mutually-compressed, geometrically-regular dyads of these Ordovician forms. Still, this observation does provide a connection with earlier cryptospores and helps in assessing their potential relation to the evolving charophyte lineage prior to the canalization of sporogenic meiosis by Darriwilian time. The antithetic origin of the plant sporophyte requires the de novo origination of complex plant tissue from a zygote that divided mitotically. The “spore-thalli” presented here fit such an evolutionary scenario, but not in the way that led ultimately to the axial plant sporophyte. Thus, the early Ordovician may have been a time of active selection in terrestrial landscapes as charophytic algae evolved toward the embryophytic condition.