GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 39-13
Presentation Time: 9:00 AM-5:30 PM


MCMENAMIN, Mark A.S., RABINOW, Sophie A., CURTIS-HILL, Aurora B., MARTIN, Kalyndi and TRELOAR, Destiny, Geology and Geography, Mount Holyoke College, South Hadley, MA 01075

We report here the determinate conidium (porospore) of an ascomycete fungus (cf. Alternaria alternata) from the black chert stromatolitic facies of the Gunflint Iron Formation (~1.9-2.0 Ga) of southern Ontario, Canada. The fossil is extremely rare in the Gunflint microbiota, and is reported here from a single specimen whose size sets it apart from other, comparatively tiny, Gunflint microfossils. The specimen is unquestionably embedded in chert, is the same color as other organic matter in thin section, and is thus not a modern contaminant. The conidium is 200 microns in length and shows both transverse and longitudinal septa. Individual spores are 30-46 microns in greatest dimension. A collapsed conidiophore (stalk) is visible. The apical spore is cap-shaped, resembling that of modern Alternaria. This fossil thus provides the earliest evidence for fungi in the fossil record, and also provides the first evidence for asexual reproduction in a eukaryote by means of mitospores. The rarity of this fossil indicates that the conidium may have experienced long-distance transport by wind or currents before settling on the stromatolite-generating biofilm. This isolated fossil occurrence suggests the presence of an Alternaria-like fungus on land by 2.0 Ga, and also indicates an origin of Kingdom Fungi at the older end of the age range (0.67-2.15 Ga) inferred from molecular clock estimates.