A new modern Hydrolithon-like coralline red alga from the Upper Ordovician of Estonia

A new genus and species of encrusting fan-shaped calcareous coralline red algae have been described from the lower Katian of Estonia. The fossils occur on the shell of the brachiopod Porambonites as a crust consisting of radial rows that systematically bifurcate, forming secondary rows that further divide into two distinct structures. The exposed surface of the rows is nearly flat and uniformly perforated with single or double rows of large, circular pores. Antiquifosliella tinnae gen. et sp. nov. stands out among other Ordovician red algae due to its large pores, which may have had a dual function. These pores were most likely involved in both gas exchange and reproduction. A. tinnae gen. et sp. nov. most likely belongs to the stem group of Corallinaceae, though it may also represent an early lineage within the Hydrolithon group. In the latter case, Hydrolithon is a classical “living fossil” showing only a little morphological evolution since the Late Ordovician. The discovery of true Corallinaceae from the Upper Ordovician proves the early Paleozoic origin of the group that was previously considered to be late Mesozoic.