The north american occurrence of the algal coenobium plaesiodictyon’. paleogeographic, paleoecologic, and biostratigraphic importance in the Triassic

Abstract

The distinctive coenobial chlorophycean alga (Chlorococcales) Plaesiodictyon has been recovered from Upper Triassic subsurface samples of Cass County, Texas. Specimens have been assigned to Plaesiodictyon mosellaneum ssp. variable and Plaesiodictyon mosellaneum ssp. bullatum ssp. nov. This is the first illustrated record of Plaesiodictyon from North America. The presence of the palynomorphs Brodispora striata, Patinasporites densus, P. toralis and Pyramidosporites traversei suggests a Carnian age for this occurrence. A review of published information indicates that Plaesiodictyon has a wide geographic breadth in the Middle/Upper Triassic. Conjectural evidence suggests this alga lived in fresh‐brackish water areas and could have been transported to marine environments via a fluvial plume. The wide, and relatively rapid, distribution of Plaesiodictyon may be related to aerobiological dispersal as evidenced by recent studies which have recovered viable algae (including chlorococcalean) at high altitudes and great distances from freshwater sites. Aeroalgal dissemination gives forms like Plaesiodictyon the capacity for wide biogeographic dispersal and colonization independent of streams and animal vectors.