Ediacara growing pains: modular addition and development in Dickinsonia costata

Constraining patterns of growth using directly observable and quantifiable characteristics can reveal a wealth of information regarding the biology of the Ediacara biota—the oldest macroscopic, complex community-forming organisms in the fossil record. However, these rely on individuals captured at an instant in time at various growth stages, and so different interpretations can be derived from the same material. Here we leverage newly discovered and well-preserved Dickinsonia costata Sprigg, 1947 from South Australia, combined with hundreds of previously described specimens, to test competing hypotheses for the location of module addition. We find considerable variation in the relationship between the total number of modules and body size that cannot be explained solely by expansion and contraction of individuals. Patterns derived assuming new modules differentiated at the anterior result in numerous examples in which the oldest module(s) must decrease in size with overall growth, potentially falsifying this hypothesis. Observed polarity as well as the consistent posterior location of defects and indentations support module formation at this end in D. costata. Regardless, changes in repeated units with growth share similarities with those regulated by morphogen gradients in metazoans today, suggesting that these genetic pathways were operating in Ediacaran animals.

Developmental processes in Ediacara macrofossils

The Ediacara Biota preserves the oldest fossil evidence of abundant, complex metazoans. Despite their significance, assigning individual taxa to specific phylogenetic groups has proved problematic. To better understand these forms, we identify developmentally controlled characters in representative taxa from the Ediacaran White Sea assemblage and compare them with the regulatory tools underlying similar traits in modern organisms. This analysis demonstrates that the genetic pathways for multicellularity, axial polarity, musculature, and a nervous system were likely present in some of these early animals. Equally meaningful is the absence of evidence for major differentiation of macroscopic body units, including distinct organs, localized sensory machinery or appendages. Together these traits help to better constrain the phylogenetic position of several key Ediacara taxa and inform our views of early metazoan evolution. An apparent lack of heads with concentrated sensory machinery or ventral nerve cords in such taxa supports the hypothesis that these evolved independently in disparate bilaterian clades.

Ediacaran diversity and paleoecology from central Iran

The late Ediacaran (Nama) Fossil Assemblage from the Kushk Series in the Kushk and Chahmir areas of Central Iran highlights a diverse community of globally distributed, soft-bodied (non-skeletonized) Ediacara biota coexisting with skeletonized tubular forms of likely metazoan affinities. Several biostratigraphically and biogeographically important taxa are reported (i.e., erniettomorphs, rangeomorphs, cloudinomorphs, kimberellomorphs, Chuaria, Corumbella), including Convolutubus dargazinensis new genus new species, a new organic-walled tubular organism, allowing for paleoecological studies to be performed. This study highlights the need for continued investigations into the late Ediacaran of Iran, and suggests a biosphere in transition, with a shift in diversity and abundance from large Ediacara biota to organic-walled and skeletonized tubular organisms at the dawn of the Cambrian Explosion. UUID: http://zoobank.org/350bb98c-5322-488d-a9a5-22c911ab7e53

The Shibantan Lagerstätte: insights into the Proterozoic–Phanerozoic transition

The Shibantan Lagerstätte (551–543 Ma) in the Yangtse Gorges area in South China is one of the best-known examples of terminal Ediacaran fossil assemblages preserved in marine carbonate rocks. Taxonomically dominated by benthic organisms, the Shibantan Lagerstätte preserves various photoautotrophs, biomineralizing tubular fossils, Ediacara-type macrofossils (including rangeomorphs, arboreomorphs, erniettomorphs, palaeopascichnids, a possible dickinsoniomorph, the mobile bilaterian Yilingia and soft-bodied tubular fossils), abundant ichnofossils and a number of problematic and dubious fossils. Shibantan fossils provide intriguing insights into ecological interactions among mobile bilaterians, sessile benthic Ediacara-type organisms and microbial mats, thus offering important data to test various hypotheses accounting for the decline of the Ediacara biota and the concurrent expansion of bilaterian bioturbation and mobility across the Proterozoic–Phanerozoic transition.

Experimental evidence supports early silica cementation of the Ediacara Biota

Casts and molds of soft-bodied organisms in Ediacaran sandstones (“Ediacara-style” fossilization) have played an important role in reconstruction of the emergence and radiation of early complex macroscopic life. However, the preservational processes responsible for the Ediacara fossil record are still vigorously debated. Whereas classic studies proposed fossilization via rapid sulfide mineralization of carcass and matground surfaces, a more recent view posits silica as the key mineral involved in their preservation. We performed experiments in which a variety of soft-bodied organisms were exposed to silica-rich solutions at concentrations considered characteristic of Ediacaran seawater (2 mM). Our results document continuous precipitation of amorphous silica onto the surfaces of these organic tissues under constant and normal marine pH values (7.8). Mineral formation was accompanied by a progressive decrease in the dissolved silica (DSi) concentration of the experimental solution to levels well below amorphous silica saturation. Additionally, we find that the magnitude of silica precipitation is correlated to each organism’s functional-group chemistry, as measured by potentiometric acid-base titrations. We suggest that a wide range of soft-bodied organisms were prone to silicification in Ediacaran marine environments characterized by anactualistically high DSi concentrations. This provides further support for the model that the extraordinary moldic preservation of the Ediacara Biota was promoted by early silica cementation and that this mode of preservation can offer an accurate glimpse into the composition of those early animal ecosystems.

BIOLOGICAL AND ECOLOGICAL INSIGHTS FROM THE PRESERVATIONAL VARIABILITY OF FUNISIA DOROTHEA, EDIACARA MEMBER, SOUTH AUSTRALIA

ABSTRACT The Ediacara Biota represents a turning point in the evolution of life on Earth, signifying the transition from single celled organisms to complex, community-forming macrobiota. The exceptional fossil record of the soft-bodied Ediacara Biota provides critical insight into the nature of this transition and into ecosystem dynamics leading up to the so-called “Cambrian Explosion”. However, the preservation of non-biomineralizing organisms in a diversity of lithologies goes hand-in-hand with considerable taphonomic complexity that often shrouds true paleoecological and paleobiological signatures. We address the nature of this taphonomic complexity within the fossiliferous sandstones of the Ediacara Member in South Australia. Utilizing the most fossiliferous outcropping of the Ediacara Member, located at the Nilpena Station National Heritage Ediacara Fossil Site, we conduct a focused, taxon-level biostratinomic characterization of the tubular organism Funisia dorothea. Funisia is the most abundant body fossil in the Ediacara Member, making the characterization of its preservational variability essential to the accurate interpretation of regional paleobiology and paleoecology. We describe remarkable biostratinomic complexity in all Funisia populations at Nilpena, identifying four distinct preservational variants of internal and external molds and four additional successive biostratinomic grades corresponding to loss of external characters. Synthesis of these observations identify the most robust preservational forms of Funisia for use in paleobiological interpretation and highlight the important impact that Funisia's high abundance had on regional paleoecology and on population-scale preservation in the Ediacara Member.

Supplemental Material: Experimental evidence supports early silica cementation of the Ediacara Biota

Detailed methods, supplemental tables, and acid-base titration curves.<br>

Supplemental Material: Experimental evidence supports early silica cementation of the Ediacara Biota

Detailed methods, supplemental tables, and acid-base titration curves.<br>

Biostratinomy of the Ediacara Member (Rawnsley Quartzite, South Australia): implications for depositional environments, ecology and biology of Ediacara organisms

The Precambrian Ediacara Biota—Earth's earliest fossil record of communities of macroscopic, multicellular organisms—provides critical insights into the emergence of complex life on our planet. Excavation and reconstruction of nearly 300 m 2 of fossiliferous bedding planes in the Ediacara Member of the Rawnsley Quartzite, at the National Heritage Ediacara fossil site Nilpena in South Australia, have permitted detailed study of the sedimentology, taphonomy and palaeoecology of Ediacara fossil assemblages. Characterization of Ediacara macrofossils and textured organic surfaces at the scale of facies, bedding planes and individual specimens has yielded unprecedented insight into the manner in which the palaeoenvironmental settings inhabited by Ediacara communities—particularly hydrodynamic conditions—influenced the aut- and synecology of Ediacara organisms, as well as the morphology and assemblage composition of Ediacara fossils. Here, we describe the manner in which environmental processes mediated the development of taphofacies hosting Ediacara fossil assemblages. Using two of the most common Ediacara Member fossils, Arborea and Dickinsonia , as examples, we delineate criteria that can be used to distinguish between ecological, environmental and biostratinomic signals and reconstruct how interactions between these processes have distinctively shaped the Ediacara fossil record.