An international team of paleontologists has discovered 1,000- to 900-million-year-old microfossils of a fungus in estuarine shale of the Grassy Bay Formation in Arctic Canada. These multicellular organic-walled microfossils are more than half a billion years older than previously reported occurrences of fungi.

“Fungi are essential components of modern ecosystems and are among the first traces of life to colonize the continents,” said University of Liège researcher Corentin Loron and colleagues.

“To date, the earliest fossil fungi are 410-million-year-old specimens from Scotland and spores of glomeromycotan fungi from Wisconsin that date to 450 million years ago, in the Ordovician period.”

“The 1-0.9-billion-year-old (Proterozoic era) fossil fungi from the Grassy Bay Formation are older than these previously reported fossils by more than half a billion years.”

The paleontologists discovered abundant microfossils of a fungus named Ourasphaira giraldae.

These fossilized specimens have a wall made of chitin, a fibrous compound that forms fungal cell walls.

“These organic-walled microfossils consist of multicellular, branching filaments with terminal spheres,” the scientists said.

“Transmitted-light and scanning electron microscopy (SEM) examinations show smooth unornamented walls of filaments and spheres.”

“SEM images also reveal the presence of locally well-preserved and intertwined (approximately 15–20-nm thick) microfibrils, which make up the walls.”

“Ultrastructural analyses using transmission electron microscopy show that the flattened microfossils are hollow, with a bilayered wall that consists of an electron-dense thick inner layer and a thin electron-tenuous outer layer.”

This combination of complex morphology, right-angle branching, multicellularity, bilayered wall ultrastructure, compositional recalcitrance and relatively large size permits the unambiguous placement of Ourasphaira giraldae among eukaryotes.”

“Together they indicate the presence of a complex cytoskeleton, which is absent in prokaryotes.”

Extant fungi are mostly terrestrial, although some marine forms are known.

Because Ourasphaira giraldae is preserved in shallow-water estuarine shale of the Grassy Bay Formation, this fungus may have lived in an estuarine environment. The fungus may also have been transported into this estuarine setting from land or marine niches.

“The later colonization of terrestrial settings by fungi may have preceded and aided the colonization of land by plants through symbioses and through soil processing, which would have provided ecological niches, improved the substrate, nutrient uptake and increased aboveground productivity,” the researchers said.

“As multidisciplinary studies of Proterozoic fossil assemblages progress, we predict that more fossil fungi and other early eukaryotes will be discovered and will improve our understanding of the evolution of the early biosphere.”

The discovery is reported in a paper published this week in the journal Nature.

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Corentin C. Loron et al. Early fungi from the Proterozoic era in Arctic Canada. Nature, published online May 22, 2019; doi: 10.1038/s41586-019-1217-0