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The evolutionary divergence of cnidarian and bilaterian lineages from their remote metazoan ancestor occurred at an unknown depth in time before the Cambrian, since crown group representatives of each are found in Lower Cambrian fossil assemblages. We report here a variety of putative embryonic, larval, and adult microfossils deriving from Precambrian phosphorite deposits of Southwest China, which may predate the Cambrian radiation by 25-45 million years. These are most probably of cnidarian affinity. Large numbers of fossilized early planula-like larvae were observed under the microscope in sections. Though several forms are represented, the majority display remarkable conformity, which is inconsistent with the alternative that they are artifactual mineral inclusions. Some of these fossils are preserved in such high resolution that individual cells can be discerned. We confirm in detail an earlier report of the presence in the same deposits of tabulates, an extinct crown group anthozoan form. Other sections reveal structures that most closely resemble sections of basal modern corals. A large number of fossils similar to modern hydrozoan gastrulae were also observed. These again displayed great morphological consistency. Though only a single example is available, a microscopic animal remarkably similar to a modern adult hydrozoan is also presented. Taken together, the new observations reported in this paper indicate the existence of a diverse and already differentiated cnidarian fauna, long before the Cambrian evolutionary event. It follows that at least stem group bilaterians must also have been present at this time. Key Words: cnidarian; fossil; Precambrian; embryo.

Archaeocyathan sponges were the dominant metazoan framework builders during Series 2 of the Cambrian. After their near extinction during the Toyonian stage (middle Stage 4), this important ecological role was eventually filled by robustly skeletonized lithistid sponges. However, the exact timing of ecological restructuring is not well understood and was likely not contemporaneous across different paleocontinents. For example, reefs from the Wuliuan of China appear to show rapid replacement of archaeocyaths with lithistid sponges, yet the earliest occurrence of lithistids in Laurentia is not until the early Furongian. In this study, we explore the Mule Spring Limestone of Nevada, which contains shallow water carbonate environments from the immediate aftermath of the regional archaeocyathan extinction, for signs of reef-building activity. Within this formation, we find evidence of sparse microbial-built leolites and some potentially poorly preserved metazoan organisms. However, the totality of our field observations and thin-section point counts suggest that there was no substantial reef-building activity by either microbial or metazoan organisms within our study locality. Our data suggest the occurrence of a local reef eclipse during this interval for the locality investigated. We also incorporate geochemical proxies to determine paleoredox conditions, which suggest well-oxygenated marine conditions through the period of interest. Lack of hardground substrate is proposed as the cause for this gap in the reef record. These results show that a temporary loss of framework-building activity occurred after the regional extinction of archaeocyaths and demonstrates the ecological impact of losing framework builders on a reef environment. [ABSTRACT FROM AUTHOR]

Bioturbation in neritic siliciclastic settings during the Proterozoic-Phanerozoic transition increased in depth and intensity, causing a change in substrates from the matgrounds characteristic of the Proterozoic to the mixgrounds characteristic of the Phanerozoic. This change in bioturbation increased the water content of surficial layers of sediment and blurred the sediment-water interface, leading to the first appearance of a mixed layer. Development of a mixed layer throughout neritic environments would have had a strong impact on any benthic metazoans, particularly sessile suspension feeders, that were well adapted for survival on relatively unbioturbated Proterozoic substrates. The impact of this substrate transition on benthic metazoans has been termed the 'Cambrian substrate revolution.' The unusual Early Cambrian helicoplacoid echinoderms were well adapted for survival on typical Proterozoic-style substrates. The examination of new helicoplacoid specimens collected during this study, combined with extensive study of the rocks in which they are preserved, indicate that helicoplacoids lived as sediment stickers on a muddy substrate that underwent only low to moderate levels of strictly horizontal bioturbation and did not have a mixed layer. The significant increase of bioturbation through the Cambrian in neritic siliciclastic settings is likely to have led to the extinction of the helicoplacoids. Other similarly adapted sessile suspension-feeding echinoderms may have also been driven to extinction by the effects of the Cambrian substrate revolution. The co-existence during the Cambrian of organisms adapted to the variety of substrates characteristic of this transitional period may also have contributed to the high degree of perceived morphological disparity during the Cambrian 'explosion.' Keywords: helicoplacoids, Cambrian, echinoderms, bioturbation.

The Copper Harbor Conglomerate is a Mesoproterozoic (1.09 Ga) freshwater sedimentary sequence that outcrops in the Upper Peninsula of Michigan. The formation was deposited during infilling of the failed Midcontinent Rift and contains fluvial, lacustrine, and alluvial fan facies. This study describes and analyzes the formation of small domal structures preserved in fluvial sandstone facies within the lower portion of the formation. These domal structures range from millimeters to several centimeters in diameter and height, and are preserved in convex epirelief on fine-grained sandstone beds. The structures have a pustulose texture and a patchy distribution on bedding planes. Slabs containing the structures were collected in the field and analyzed in the laboratory through inspection of cut slabs, petrographic thin sections, X-radiographs, and RAMAN spectroscopy. Results of these analyses reveal that the domal structures often contain weak, wavy horizontal bedding and laminae, and lack any vertical structures. These results support a biogenic origin of the domal structures instead of through abiogenic processes such as loading, sand volcanoes, or adhesion warts. These structures are akin to what were traditionally labeled as 'sand stromatolites', but are now known as 'domal sand structures'. Along with previous descriptions of carbonate stromatolites, organic-rich paleosols, and microbial sand structures, our findings provide further evidence that mat-forming microbial communities thrived in the late Mesoproterozoic freshwater systems of the Midcontinent Rift. [ABSTRACT FROM AUTHOR]

The increase in the depth and intensity of bioturbation through the Proterozoic-­Phanerozoic transition changed the substrates on which marine benthos lived from being relatively firm with a sharp sediment-water interface to having a high water content and blurry sediment-water interface. Additionally, microbial mats, once dominant on normal marine Proterozoic seafloors, were relegated to stressed settings lacking intense metazoan activity. This change in substrates has been termed the 'agronomic revolution', and its impact on benthic metazoans has been termed the 'Cambrian substrate revolution'. The shallow marine phosphorites of the Lower Cambrian Meishucun Formation of southwest China contain evidence suggestive of the presence of seafloor microbial mats. This evidence includes abundant and distinctive red-colored bedding planes enriched in heavy iron minerals and mica, interpreted as resulting from mat-decay mineralization and mica trapping by microbial mats. The radular grazing trace fossil Radulichnus is also found in this formation, indicating a firm, microbial mat-bound substrate. These radular scratches are always preserved with circular impressions around 10 cm in diameter, possibly the fossils of soft-bodied organisms. The first relatively intense bioturbation in this region is found in this formation and is dominated by horizontal Thalassinoides burrows, which could represent undermat mining behavior. The evidence for the presence of microbial mats in the Lower Cambrian Meishucun Formation, and for metazoan lifestyles associated with such mat-bound seafloors, reveals that normal marine environments dominated by typical Proterozoic-style soft substrates still existed during the Cambrian substrate revolution. [ABSTRACT FROM AUTHOR]