Late Ordovician calcified peyssonnelialean red algae: systematics and evolutionary significance

The extant Order Peyssonneliales (Rhodymeniophycidae, Rhodophyta) includes calcareous crustose forms that are widely distributed in shallow marine environments and broadly resemble crustose coralline algae in their ecology. Current molecular clock analyses suggest that the Peyssonneliales diverged from its sister group Gigartinales around 300 Ma. However, until now, the earliest undisputed fossil peyssonnelialeans have been Lower Cretaceous (early Aptian, 121.4-117 Ma) in age. Here we report peyssonnelialean red algae, Paleometapeyssonnelia gen. nov., including two new species, P. gracilis sp. nov. and P. crassa sp. nov., from shallow marine carbonates of the Upper Ordovician Lianglitag Formation (approx. 453 Ma) in the Tarim Basin of northwestern China. Paleometapeyssonnelia has pseudoparenchymatous organization with dividing filaments that show fountain-like arrangement in longitudinal section. Its preservation suggests original aragonite replaced by calcite. Crustose habit, pseudoparenchymatous organization and aragonite calcification are all characteristics of extant Peyssonneliales. Upper Ordovician Paleometapeyssonnelia extends the current fossil record of Peyssonneliales by approximately 335 Myr. This supports the divergence time of Rhodymeniophycidae proposed by current molecular phylogenetic analysis and provides a fossil base for its origination and evolutionary divergence in the early Palaeozoic.