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An enigmatic thallophyte ( Litothallus ganovex gen. et sp. nov.) occurs in the form of compressions with cellular preservation and associated sheets of complete cells in non-marine Triassic deposits in the Transantarctic Mountains, Antarctica. A combination of microscopic techniques (i.e. bright field and epifluorescence microscopy, SEM) was used to document details of this organism. The thalli are flat and appear to be composed of one to several congruently superimposed cell sheets that together form a plectenchyma or pseudoparenchyma. This discovery represents the youngest record of sheets of fossil cells, which have heretofore exclusively been reported from the Proterozoic and Palaeozoic, and is one of the few examples of cell sheets that can be attributed to macroscopic remains with confidence. We interpret the thalloid organism from Antarctica as the remains of a crustose freshwater macroalga based on its occurrence in overbank deposits of a braided fluvial environment and its morphological similarity to thalli of extant freshwater representatives of the red algal order Hildenbrandiales.