The systematic position of the Ordovician calcareous microfossil HalysisHøeg, 1932 has long been uncertain. Only known from thin sections, its morphology has been suggested to be either a single chain of cells or a series of tubes and it has been regarded as a green alga or cyanobacterium. Here we propose that Halysis represents a single sheet of cells. This new morphological interpretation accounts for Halysis's appearance in thin section as an extended flexuous series of cells, some of which are not seen to be in mutual contact, exhibiting nonlinear cell-size variation. It is also consistent with the absence of tubiform sections unequivocally attributable to Halysis. This reassessment suggests comparisons between Halysis and Mesozoic–Cenozoic thin laminar unistratose coralline red algae. Halysis cells are relatively large (40–210 μm), but their lower range is comparable to cells of corallinaceans such as Lithoporella (Foslie) Foslie, 1909. Applanate thallus morphology in Halysis resembles that of thin laminar species of LithophyllumPhilippi, 1837 that were traditionally included in TitanodermaNägeli, 1858. Interpretation of Halysis as a coralline-like alga strengthens the likelihood that a variety of corallines was present in the Ordovician, more than 300 Ma prior to the currently recognized major diversification of this important group of red algae in the Cretaceous.

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