Micropaleontology of the lower Mesoproterozoic Roper Group, Australia, and implications for early eukaryotic evolution

Emmanuelle J Javaux,Andrew H Knoll

doi:10.1017/jpa.2016.124

Emmanuelle J Javaux, Andrew H Knoll

Open Access

https://doi.org/10.1017/jpa.2016.124

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Abstract

AbstractWell-preserved microfossils occur in abundance through more than 1000 m of lower Mesoproterozoic siliciclastic rocks composing the Roper Group, Northern Territory, Australia. The Roper assemblage includes 34 taxa, five interpreted unambiguously as eukaryotes, nine as possible eukaryotes (includingBlastanosphaira kokkodanew genus and new species, a budding spheromorph with thin chagrinate walls), eight as possible or probable cyanobacteria, and 12 incertae sedis. Taxonomic richness is highest in inshore facies, and populations interpreted as unambiguous or probable eukaryotes occur most abundantly in coastal and proximal shelf shales. Phylogenetic placement within the Eukarya is difficult, and molecular clock estimates suggest that preserved microfossils may belong, in part orin toto, to stem group eukaryotes (forms that diverged before the last common ancestor of extant eukaryotes, or LECA) or stem lineages within major clades of the eukaryotic crown group (after LECA). Despite this, Roper fossils provide direct or inferential evidence for many basic features of eukaryotic biology, including a dynamic cytoskeleton and membrane system that enabled cells to change shape, life cycles that include resting cysts coated by decay-resistant biopolymers, reproduction by budding and binary division, osmotrophy, and simple multicellularity. The diversity, environmental range, and ecological importance of eukaryotes, however, were lower than in later Neoproterozoic and Phanerozoic ecosystems.

Highlights

In classifying the past 541 million years as the Phanerozoic Eon, stratigraphers give formal recognition to the animal fossils commonly found in rocks of this age
We suggest that species should be distinguished on the basis of modal rather than maximum diameter, so that Leiosphaeridia with lanceolate folds and a modal size 70 μm would be placed in L. jacutica
Roper microfossils contribute to an increasingly refined picture of life in Mesoproterozoic oceans, complementing the window opened by microfossils preserved in silicified carbonates

Summary

Introduction

In classifying the past 541 million years as the Phanerozoic Eon, stratigraphers give formal recognition to the animal fossils commonly found in rocks of this age. Molecular clocks, calibrated largely by Phanerozoic fossils, most suggest that crown group eukaryotes emerged during the Proterozoic Eon. The error bars are large, but even so, the oldest Neither early nor late crown group emergence precludes the possibility of a substantial earlier record of stem group eukaryotes (Javaux et al, 2010; Javaux, 2011; Knoll, 2014; Sugitani et al, 2015; Butterfield, 2015). Older microfossils potentially push the eukaryotic crown group still deeper into the past (Moczydłowska et al, 2011; Butterfield, 2015), but only if these fossils can be confidently interpreted as eukaryotes and as crown versus stem group taxa—a potentially challenging distinction in ancient rocks (Knoll, 2014)

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