Palaeoconchocelis starmachii, a carbonate boring microfossil from the Upper Silurian of Poland (425 million years old): implications for the evolution of the Bangiaceae (Rhodophyta)

A 425 million-year-old organically preserved endolithic microfossil, Palaeoconchocelis starmachii Campbell, Kazmierczak, and Golubic (1979), from sedimentary strata of the Upper Silurian of Poland, is identified as the conchocelis phase of a bangiacean rhodophyte. Documentation of the fossil's biological identity is based on several properties shared by the fossil and its living counterpart, conchocelis: their vegetative filaments penetrate carbonate and form a network below the substrate surface (a fossil crinoid columnal and a Recent shell); bulbous swellings occur in series along a vegetative filament; conchosporangial branches develop from the vegetative filaments; conchospores form in the cells of conchosporangial branches; pit connections link cells of the conchocelis. All structural elements of the fossil are equivalent in size, shape, and position within the substrate to modern representatives of the conchocelis phase of Porphyra nereocystis. The extreme plasticity of the biphasic life cycle as well as advantages conferred by the endolithic habit of the conchocelis may be responsible for the unchanged persistence of the bangiacean conchocelis morphology from the Silurian to the present.