The environmental and ecological context of the rise of the Ediacara Biota
Professor Mary Droser, University of California, Riverside, USA
The advent and evolution of complex life on Earth is interpreted largely from the fossils of the Precambrian soft-bodied Ediacara Biota, which appeared and evolved during a time of dynamic biogeochemical and environmental fluctuation in the global ocean. The Ediacara Biota is historically divided into three successive Assemblages—the Avalon, the White Sea, and the Nama—which are marked by the appearance of novel biological traits and ecological strategies. Recent research on all three assemblages at multiple localities has begun to clarify the significance of the Ediacara biota to our understanding of the development of Phanerozoic and Modern ecosystems. Heterogeneous seafloors – or patchiness – was on par with modern oceans during the reign of the Avalon assemblage and continued through the White Sea Assemblage. This unusually variable diversity-abundance structure is likely due both to their preservation as near-snapshots of benthic communities and to original ecological differences, in particular the paucity of motile taxa and the near-lack of predation and infaunalization. The younger White Sea and Nama Assemblages further record a “second wave” of ecological innovations, including the development of bilaterian-grade animals and Phanerozoic-style ecological innovations, such as scavenging, complex reproductive strategies, increased ecospace utilization and motility. Evidence from both the fossil record as well as geochemical data suggests that there was an extinction of some taxa between the White Sea and Nama assemblages. However, emerging data suggests that a number of Ediacaran body plans survived into the Cambrian.
Clay minerals and the fossilisation of early complex life
Dr Ross P Anderson, University of Oxford, UK
Proterozoic fossils provide the only direct evidence of early eukaryotic life. Yet they are rare, restricted to rocks where non-biomineralised remains are conserved. Compilations of Proterozoic eukaryotic fossil occurrences suggest most are found in mudstones—a clay-rich lithology known for iconic Cambrian soft-tissue fossilisation (Burgess Shale-type [BST]). Here we compare the role of clays in Cambrian BST and Proterozoic fossilisation. Experimental data suggest both berthierine and kaolinite are toxic to decay-bacteria and may promote fossilisation. X-ray diffraction (XRD) confirms BST fossils are found in rocks rich in berthierine. Moreover, novel selected-area XRD shows kaolinite to be intimately associated with BST fossil tissues. Its association with these tissues hints at early clay-organic interactions that likely promoted organic polymerisation. XRD of a similar compilation of Proterozoic fossil bearing shales reveals a contrasting pattern. Fossils with the highest preservation quality are associated with high illite content versus berthierine, suggesting that the main fossilisation control may be burial rate rather than clay mineralogy, and that most Proterozoic microfossils do not require BST conditions for fossilisation—likely a function of algae and protists being more resistant to decay than Cambrian animals. However, elemental/mineral distributions over cross-sections of fragile eukaryotic fossils and surrounding matrix from three Proterozoic localities reveal kaolinite enrichments adjacent to fossil cell-walls, similar to the association between kaolinite and BST fossils. These data suggest the conditions for Proterozoic fossilisation might be more ubiquitous than previously thought. However, to fossilise delicate forms, a small subset of Proterozoic fossil localities exhibit characteristics of BST fossilisation.
The rise of bioturbation: tracking and modelling the development of the sedimentary mixed layer
Dr Lidya Tarhan, Yale University, USA
Bioturbation—sediment mixing by burrowing animals—critically shapes seafloor ecology and sediment properties, as well as global marine biogeochemical cycling. Observation of strong bioturbation-biogeochemical feedbacks in modern marine environments suggests that the evolutionary development of bioturbation should have profoundly impacted contemporaneous biogeochemical (e.g., C, P, O and S) cycling. Stratigraphic archives indicate that the early Palaeozoic development of bioturbation was a protracted process, and that the appearance of intensively and deeply mixed sediments lagged significantly behind relatively early advances in infaunal seafloor colonization. Recent modelling work has suggested that even limited bioturbation may nonetheless have initiated an early Palaeozoic productivity crisis and ocean-wide deoxygenation. However, the precise biogeochemical impact of early Palaeozoic bioturbation has remained debated. To further address this question, I explore a new and more fully parameterized multi-component reaction-transport diagenetic model. This approach indicates that the relationship between bioturbation and both C-P-O and S cycling is complex and non-linear, and that not only intensity but style of bioturbation (e.g., biodiffusion vs. bioirrigation) influence the magnitude of P recycling and S oxidation. In this light, early Palaeozoic bioturbation—which was likely bioirrigation-dominated and characterized by relatively muted and shallow biodiffusional sediment mixing—may have initially only weakly influenced net S oxidation, while simultaneously mediating increased P recycling. Moreover, porosity—a parameter that, although rarely explored in diagenetic models, is substantially impacted by bioturbation—strongly influences both these systems. Lastly, in contrast to previous studies, I find that bioturbation amplifies the sensitivity of the coupled C-P-O cycle to environmental perturbations.
Survivorship and selection bias in the Cambrian explosion and their role in its structure
Professor Graham Budd, Uppsala University, Sweden
Big evolutionary events such as the Cambrian Explosion have inspired many attempts at explanation – why do they happen when they do? What shapes them, and why do they eventually come to an end? Or, more generally, simply what causes them? However, much less attention has been paid to the idea of a “null hypothesis” – that certain features of such diversifications arise simply through their statistical structure. When we look back from our own perspective to the origins of large groups such as the arthropods, or even the animals themselves, we will see many features that look causal but are in fact inevitable. For example, such large clades tend to be characterised by a burst of morphological innovation at their base, which has then often (and arguably invalidly) been used as an explanation for the subsequent success of the group (the “key innovations” concept). This is not necessarily to say that such events do not have causes, but that we need to be rather careful in trying to understand what it is we can actually determine merely from the patterns we see in the fossil record.
Typical sorts of features that might be affected by such biases include the early rates of diversification (the so-called “push of the past”), the rate of establishment of “body plans” and the overall timing of such events. The Cambrian explosion exemplifies many of these issues, and understanding them is therefore essential to perceiving what its fossil record may (and may not) be telling us.