A Laurentian record of the earliest fossil eukaryotes

The oldest evidence of eukaryotes in the fossil record comes from a recurrent assemblage of morphologically differentiated late Paleoproterozoic to early Mesoproterozoic microfossils. Although widely distributed, the principal constituents of this Tappania-DictyosphaeraValeria assemblage have not hitherto been recognized on Laurentia. We have recovered all three taxa from a shallow-water shale succession in the early Mesoproterozoic Greyson Formation (Belt Supergroup, Montana, USA). An exceptionally preserved population of Tappania substantially expands the morphological range of this developmentally complex organism, suggesting phylogenetic placement within, or immediately adjacent to, crown-group eukaryotes. Correspondence with Tappania-bearing biotas from China, India, Australia, and Siberia demonstrates an open-ocean connection to the intracratonic Belt Basin and, along with broadly co-occurring macrofossils Grypania and Horodyskia, supports the recognition of a globally expressed biozone. The Greyson Formation, along with contiguous strata in Glacier National Park, is unique in preserving all currently confirmed taxa of early eukaryotic and macroscopic fossils. INTRODUCTION The fossil record of eukaryotes extends back to at least the late Paleoproterozoic (Butterfield, 2015), and a recurrent assemblage of distinctive microfossils from China (Ruyang Group; Yin, 1997; Yin et al., 2005), Australia (Roper Group; Javaux et al., 2001; Javaux and Knoll, 2016), India (Semri Group; Prasad et al., 2005), and Siberia (Kamo Group; Nagovitsin, 2009) points to the presence of an ecologically coherent biota extending from ca. 1650 Ma to ca. 1400 Ma. Typified by process-bearing Tappania, reticulate-walled Dictyosphaera/Shuiyoushaeridium, and concentrically striated Valeria, these earliest fossil eukaryotes offer key insights into the early evolution of the clade, and enable practical applications to paleobiogeography and biostratigraphic correlation. Proterozoic eukaryotes are also richly represented on Laurentia; however, their oldest confirmed records on this craton are of late Mesoproterozoic age (Butterfield and Chandler, 1992; Butterfield, 2000; Hofmann and Jackson, 1994). Older fossils are known from North America, notably Grypania and Horodyskia from the early Mesoproterozoic Belt Supergroup (Walter et al., 1976; Fedonkin and Yochelson, 2002), but neither of these macroscopic forms is unambiguously eukaryotic (Butterfield, 2009). Apart from our report of long-ranging Valeria in the Chamberlain Formation (Adam et al., 2016), the same is true for spheroidal and filamentous microfossils that have been recovered from Belt Supergroup strata (Horodyski, 1980). Here we report a full Tappania-Dictyosphaera-Valeria assemblage from the Greyson Formation of the lower Belt Supergroup, filling a major gap in the early eukaryotic record.