Algae from the Lower Devonian Rhynie chert: Populations of a probable saccoderm desmid (Mesotaeniaceae, Zygnematales) preserved in a microbial mat

Abstract

Green algae must have been taxonomically diverse in the Early Devonian Rhynie ecosystem; however, documented evidence of this is rare. Rhyniotaenium velatum gen. et sp. nov. is a unicellular organism that occurs in small populations in a microbial mat from the Rhynie chert. It consists of a straight, oblong cell (up to 14.5 μm long and 9 μm wide) surrounded by a stratified mucilaginous envelope; well-preserved cells contain a single, axile plate-like structure interpreted as a chloroplast. Populations are embedded in confluent mucilage. A high level of morphological correspondence exists between R. velatum and present-day saccoderm desmids of the genera Mesotaenium and Serritaenia (Mesotaeniaceae, Zygnematales, Zygnematophyceae), and thus can be used to interpret the fossil as a member of this grouping of streptophyte green algae. Uniformly aligned chloroplasts in one population suggest that R. velatum was capable of profile-to-face chloroplast movement. Pigmented or otherwise darkened envelopes and peculiar inclusions at the cell poles in several R. velatum specimens resemble a photoprotective adaptation recently described in Serritaenia. Rhyniotaenium velatum broadens our knowledge of the green algal component of early terrestrial environments, but is also significant because fossil evidence of Zygnematophyceae older than Carboniferous is exceedingly scant.