Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae.

Stefan Bengtson,Veneta Belivanova,Therese Sallstedt,Martin Whitehouse,David Penny

doi:10.1371/journal.pbio.2000735

Stefan Bengtson, Veneta Belivanova + Show 3 more

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https://doi.org/10.1371/journal.pbio.2000735

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Abstract

The ~1.6 Ga Tirohan Dolomite of the Lower Vindhyan in central India contains phosphatized stromatolitic microbialites. We report from there uniquely well-preserved fossils interpreted as probable crown-group rhodophytes (red algae). The filamentous form Rafatazmia chitrakootensis n. gen, n. sp. has uniserial rows of large cells and grows through diffusely distributed septation. Each cell has a centrally suspended, conspicuous rhomboidal disk interpreted as a pyrenoid. The septa between the cells have central structures that may represent pit connections and pit plugs. Another filamentous form, Denaricion mendax n. gen., n. sp., has coin-like cells reminiscent of those in large sulfur-oxidizing bacteria but much more recalcitrant than the liquid-vacuole-filled cells of the latter. There are also resemblances with oscillatoriacean cyanobacteria, although cell volumes in the latter are much smaller. The wider affinities of Denaricion are uncertain. Ramathallus lobatus n. gen., n. sp. is a lobate sessile alga with pseudoparenchymatous thallus, “cell fountains,” and apical growth, suggesting florideophycean affinity. If these inferences are correct, Rafatazmia and Ramathallus represent crown-group multicellular rhodophytes, antedating the oldest previously accepted red alga in the fossil record by about 400 million years.

Highlights

Multicellular eukaryotes rose to prominence around the Proterozoic–Phanerozoic transition coupled to evolving ecological interactions between megascopic autotrophs and heterotrophs
Estimates of the age of the last eukaryote common ancestor, LECA, vary between extremes of about 0.8 Ga [10] and 2.3 Ga [11], though most proposed dates based on modern molecular-clock work land within the Mesoproterozoic [7,12,13,14,15,16] or late Palaeoproterozoic [17]
The considerable interval of uncertainty is due to difficulties in establishing the origination time of fossil taxa used for calibration as well as to other problems involved in molecular-clock dating

Summary

Introduction

Multicellular eukaryotes rose to prominence around the Proterozoic–Phanerozoic transition coupled to evolving ecological interactions between megascopic autotrophs and heterotrophs. The other major opisthokont multicellular clade, are important heterotrophic consumers, but their role in Phanerozoic ecosystem evolution is generally considered to be focused on the terrestrial biosphere [2,3,4]. The three major lineages of multicellular eukaryotes—Plantae, Fungi, and Metazoa—diversified already during the Proterozoic, but the time of eukaryote origin is under dispute (reviews by [5,6,7,8,9]). The considerable interval of uncertainty is due to difficulties in establishing the origination time of fossil taxa used for calibration as well as to other problems involved in molecular-clock dating. One major issue is the lack of robust phylogenies in the base of the eukaryotic tree [6,18,19,20,21,22,23]

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