Complex Unicellular Microfossils from the 1.9 Ga Gunflint Chert, Canada
Abstract
The presence of eukaryotic life during the early Paleoproterozoic has been a matter of debate because well-preserved fossils older than 1.8 Ga rarely exhibit eukaryotic cellular microstructures. In this study, microfossils from the 1.9 Ga Gunflint Chert were studied using the extended-focal-depth imaging technique, combined with scanning electron microscopy, resulting in recognition of three types of large (10–35 μm diameter) complex unicellular bodies (CUBs) and one type of “multicellular body” (< 50 μm diameter). The CUBs show the following eukaryotic cyst-like structures: (1) radially arranged internal strands similar to those in some acritarchs and dinoflagellates; (2) regularly spaced long tubular processes, stubby pustules, and/or robust podia on the cell surface; (3) reticulate cell-wall sculpturing such as pits, ridges, and scale-like ornaments; and (4) internal bodies that may represent membrane-bounded organelles. These morphological features provide strong evidence for the presence of protists in the late Paleoproterozoic.
Among the three types of CUBs from the Gunflint microbiota, a new species, Germinosphaera gunflinta sp. nov., was recognized. This species has the diagnostic characteristics of Germinosphaera, such as a subrounded to an ellipsoidal cyst, a robust main podium (up to 15 μm), multiple smaller processes, and scale-like ornaments on the surface. Within a broadly continuous lineage of Germinosphaera from the Paleoproterozoic to the early Cambrian, there is a clear increase in cell size from the Paleoproterozoic to the Mesoproterozoic, with the Gunflint species being the smallest and oldest with complex, eukaryote-like, surface ornaments that are well preserved.