A Laurentian record of the earliest fossil eukaryotes

A Laurentian record of the earliest fossil eukaryotes

ABSTRACT

The oldest evidence of eukaryotes in the fossil record comes from a recurrent assemblage of morphologically differentiated late Paleo-proterozoic to early Mesoproterozoic microfossils. Although widely distributed, the principal constituents of this Tappania-Dictyosphaera-Valeria 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. 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 Paleo-proterozoic, and a recurrent assemblage of distinctive microfossils from China, Australia, India, and Siberia points to the presence of an ecologically coherent biota extending from approximately one thousand six hundred fifty million years ago to approximately one thousand four hundred million years ago. 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. Older fossils are known from North America, notably Grypania and Horodyskia from the early Mesoproterozoic Belt Supergroup, but neither of these macroscopic forms is unambiguously eukaryotic. Apart from our report of long-ranging Valeria in the Chamberlain Formation, the same is true for spheroidal and filamental microfossils that have been recovered from Belt Supergroup strata. 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.