Neoproterozoic Geobiology and Paleobiology

Abstract

The Neoproterozoic Era (1000–542 million years ago) is a geological period of dramatic climatic change and important evolutionary innovations. Repeated glaciations of unusual magnitude occurred throughout this tumultuous interval, and various eukaryotic clades independently achieved multicellularity, becoming more complex, abundant, and diverse at its termination. Animals made their first debut in the Neoproterozoic too. The intricate interaction among these geological and biological events is a centrepiece of Earth system history, and has been the focus of geobiological investigations in recent decades. The purpose of this volume is to present a sample of views and visions among some of the growing numbers of Neoproterozoic workers. The contributions represent a cross section of recent insights into the field of Neoproterozoic geobiology. Chapter One by Porter gives an up-to- date review of Proterozoic heterotrophic eukaryotes, including fungi and various protists. Heterotrophs are key players in Phanerozoic ecosystems; indeed, most Phanerozoic paleontologists work on fossil heterotrophs. However, the fossil record of Proterozoic heterotrophs is extremely meagre. Why? Porter believes that preservation is part of the answer. Chapter Two by Huntley and colleagues explore new methods of quantifying the morphological disparity of Proterozoic and Cambrian acritarchs, the vast majority of which are probably autotrophic phytoplankton. They use non- metric multidimensional scaling and dissimilarity methods to analyze acritarch morphologies. Their results show that acritarch morphological disparity appears to increase significantly in the early Mesoproterozoic, with an ensuing long period of stasis followed by renewed diversification in the Ediacaran Period that closed the Neoproterozoic Era. This pattern is broadly consistent with previous compilation of acritarch taxonomic diversity, but also demonstrates that initial expansion of acritarch morphospace appears to predate taxonomic diversification. Using similar methods, Xiao and Dong in Chapter Three analyze the morphological disparity of macroalgal fossils, which likely represent macroscopic autotrophs in Proterozoic oceans. The pattern is similar to that of acritarchs: stepwise morphological expansions in both the early Mesoproterozoic and late Neoproterozoic separated by prolonged stasis. What might have caused the morphological stasis of both microscopic and macroscopic autotrophs? The authors speculate that it might have something to do with nutrient limitation. oproterozoic animals, or at least fossils that have been interpreted as animals. Chapter Four by The following two chapters review the depauperate fossil record of Bottjer and Clapham places emphasis particularly on the evolutionary paleoecology of benthic marine biotas in the Ediacaran Period. They interpret the paleoecology of Ediacaran fossils in light of increasing evidence of a mat-based world. These authors are particularly intrigued by the non-random association of certain Ediacara fossils (e.g., fronds vs. bilaterians) and the contrasting ecological roles between bilaterian and non-bilaterian tierers in Ediacaran epibenthic communities. They notice that the Avalon (575–560 Ma) and Nama (549– 542 Ma) assemblages appear to be dominated by non-bilaterian fronds that stood as tall tierers above the water-sediment interface, while the White Sea assemblage (560–550 Ma) seems to be characterized by flat-lying Ediacara organisms, including such forms as Dickinsonia that may be interpreted as mobile animals. It is still uncertain whether all or most Ediacara fossils can be interpreted as animals, but it is clear that evidence of animal activities is preserved as trace fossils in the last moments of Ediacaran time. Jensen, Droser, and Gehling take a step further in Chapter Five to comprehensively review the Ediacaran trace fossil record. The interpretation of Ediacaran trace fossils is not as straightforward as one would think. Many Ediacaran body fossils are morphologically simple spheres, discs, tubes, or rods. In many cases, these simple fossils, particularly when preserved as casts and molds, mimic the morphology of trace fossils such as tubular burrows or cnidarian resting traces. Jensen and colleagues do a heroic job of critically reviewing most published claims of Ediacaran trace fossils. They found that many Ediacaran trace fossil-like structures lack the diagnostic features (e.g., sediment disruption) of animal activities, biologists and may be play a alternatively interpreted as body fossils. Thus, although there are bona fide animal traces in the White Sea and Nama assemblages, they conclude that previous estimates of Ediacaran trace fossil “diversity” have been unduly inflated. Developmental and molecular distinct role in understanding animal evolution. In Chapter Six, Erwin takes an evo-devo approach to reconstruct what the “urbilaterian”—the common ancestor of protostome and deuterostome animals—would look like. Did it have a segmented body with anterior-posterior, dorsal-ventral, and left-right differentiation? Did it have eyes to see the ancient world? Did it have a through gut system to leave fecal strings in the fossil record? Did it have legs to make tracks? In principle, one can at least achieve a partial reconstruction of the urbilaterian bodyplan based on a robust phylogeny and the phylogenetic distribution of key genetic toolkits. In reality, however, the presence of genetic toolkits does not guarantee the expression of the structures. Fortunately, xi corresponding morphologies, and homologous genetic toolkits can be recruited to code functionally related, but morphologically distinct and evolutionarily convergent the absence of certain critical genetic toolkits means the absence of corresponding morphologies. Thus, by figuring out what genetic toolkits might have been present in the urbilaterian, Erwin presents a number of ideas about how complex the urbilaterian could have possibly been, thus sheding light on a maximally complex urbilaterian. This is useful for paleontologists who have been searching for the urbilaterian without a search image, but it does not tell paleontologists what geological period they should focus on in their search. Molecular biologists believe that they can fill this gap by comparing homologous gene sequences of different organisms, based on the assumption that divergence at the molecular level follows a clock-like model. Hedges and colleagues present such a molecular timescale in Chapter Seven. Hedges and colleagues summarize the molecule-derived divergence times of major clades, including oxygen-generating cyanobacteria and methane- generating euryarchaeotes that have shaped the Earth’s surface. In addition, they also present a eukaryote timetree (phylogeny scaled to evolutionary time) in the Proterozoic and give a critical review of the ever complicated models and methods devised to account for the stochastic nature of molecular clocks. Overall, Hedges and colleagues believe that many eukaryote clades, including animals, fungi, and algae, may have a deep history in the Mesoproterozoic–early Neoproterozoic. And they found possible temporal matches between the evolution of geobiologically important clades (e.g., land plants, fungi, etc.) and geological events (e.g., Neoproterozoic ice ages). The field of molecular clock study is still in its infancy, and one would expect more exciting advancements and improvements as it matures over the coming decades. Another way to date evolutionary and geological events is to correlate relevant strata with geochronometrically constrained rock units. Because index fossils are rare in the Neoproterozoic Era, chemostratigraphic methods using stable carbon isotopes, strontium isotopes, and more recently sulfur isotopes, have been used to correlate Neoproterozoic rocks. In Chapter Eight, Halverson presents a Neoproterozoic carbon isotope chemostratigraphic curve based on four well-documented sections. This curve provides a basis on which he considers several key geobiological questions in the Neoproterozoic, including the number and duration of glaciations, and the relationship between widespread ice ages and evolution. In addition to chemostratigraphic data, some distinct sedimentary features have also been used in Neoproterozoic stratigraphic correlation. For example, an enigmatic carbonate is typically found atop Neoproterozoic glacial deposits, and it is characterized by a suite of unusual sedimentary features thought to be useful stratigraphic markers. In particular, Marinoan- style cap carbonates characterized by such features as tepee-like structures, sheet cracks, barite fans, and negative carbon isotope values, are thought to be associated with a synchronous deglaciation event following the Marinoan glaciation in Australia, the Nantuo glaciation in South China, the Ghaub glaciation in Namibia, or the Icebrook glaciation in northwestern Canada. While radiometric dating suggests that some of these cap carbonates may indeed be synchronous, Corsetti and Lorentz in Chapter Nine argue that Marinoan-style cap carbonates may be facies variants that occur repeatedly in Neoproterozoic time. Thus, these authors urge caution to be exercised when using cap carbonates as correlation tools. This is by no means a comprehensive review of recent advancements made by Neoproterozoic workers. Nor does it represent a consensus view of the Neoproterozoic community—or, for that matter, among the contributors to this volume. Diverse opinions and interpretations are the hallmark of a young and vigorous science, and we feel strongly that healthy discussion among different investigators with different world views is an important key to the maturation of Neoproterozoic geobiology.