Twenty candidate fossils with claim to be the oldest representative of the Phylum Porifera have been re-analysed. Three criteria are used to assess each candidate: (i) the diagnostic criteria needed to categorize sponges in the fossil record; (ii) the presence, or absence, of such diagnostic features in the putative poriferan fossils; and (iii) the age constraints for the candidate fossils. All three criteria are critical to the correct interpretation of any fossil and its placement within an evolutionary context. Our analysis shows that no Precambrian fossil candidate yet satisfies all three of these criteria to be a reliable sponge fossil. The oldest widely accepted candidate, Mongolian silica hexacts from c. 545 million years ago (Ma), are here shown to be cruciform arsenopyrite crystals. The oldest reliable sponge remains are siliceous spicules from the basal Cambrian (Protohertzina anabarica Zone) Soltanieh Formation, Iran, which are described and analysed here in detail for the first time. Extensive archaeocyathan sponge reefs emerge and radiate as late as the middle of the Fortunian Stage of the Cambrian and demonstrate a gradual assembly of their skeletal structure through this time coincident with the evolution of other metazoan groups. Since the Porifera are basal in the Metazoa, their presence within the late Proterozoic has been widely anticipated. Molecular clock calibration for the earliest Porifera and Metazoa should now be based on the Iranian hexactinellid material dated to c. 535 Ma. The earliest convincing fossil sponge remains appeared at around the time of the Precambrian-Cambrian boundary, associated with the great radiation events of that interval.

Submarine channel related thin-bedded turbidites are deposited in environments such as external levees, internal levees, depositional terraces and at times of channel abandonment.Thin-bedded turbidites are defined as beds that are less than 10cm thick, but the described environments can at times contain beds up to 50cm thick which would be classified as medium or thick-bedded (Boggs, 2006). This paper addresses many examples of these environments from the modern seafloor, outcrop and the subsurface to suggest criteria that assist in the differentiation of levees and terraces from an architectural, sedimentological, M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 2 ichnological and hydrocarbon reservoir perspective. External levees confine channel belts and are elongate sedimentary deposits that are a product of over-spill of turbidity currents from the channel belt they confine. External levees often have predictable vertical, lateral and downstream changes but can commonly be modified by collapse of the inner external levee into the channel, collapse on the outer external levee, sediment waves, and interaction of external levees with topographic features such as other channels, other external levees, basin margins or previous slump/slide blocks, which can greatly modify the sand distribution within them.A combination of internal levees, depositional terraces and slide blocks of external levee sediment make up thin-bedded turbidites within channel belts. We differentiate between wedge shaped internal levees and topographically flat or subdued depositional terraces whose differing geometries and sand distribution reflect the fact that the flow processes involved in the formation of these deposits are different. The characteristic wedge shape of an internal levee requires sufficient space within the channel belt for the over-spilling current to decelerate and deposit the majority of its suspended sediment before reaching the bounding topography of the channel belt. In the case of depositional terraces the space available in the channel belt is insufficient for the current to deposit the majority of its sediment before reaching the bounding topography of the channel belt, creating confined sheet-like deposits.External levees, internal levees and depositional terraces have distinct sedimentological characteristics such as sand bed thickness trends and sedimentary structures that can be used to distinguish them. Together with sedimentological characteristics, in some systems these thin-bedded turbidite deposits contain distinctive trace fossil assemblages, where channel proximal deposits such as proximal external levees, internal levees and depositional terraces M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 3 can have much higher biodiversity than sand-rich channel axes and more mud-dominated outer external levees.The depositional sites for internal levees and depositional terraces within channel belts can be formed by various processes such as entrenchment, point bar accretion, meander bend cutoff, ...

It is beyond doubt that the appearance of infaunal bioturbation and metazoan biomineralization across the Ediacaran-Cambrian transition irreversibly affected the nature of marine sediment architecture and biogeochemistry. Here we review those changes in relation to their likely effect upon the processes of fossil preservation, especially within siliciclastic sediments. Processes of soft-tissue preservation in siliciclastic settings from the Ediacaran Period, including microbes and microbial mats as well as Ediacaran macrofossils, are here reviewed within this context. Highlighted examples include the exceptional preservation of microbes found in association with wrinkle structures and Ediacaran macrofossils in England and Newfoundland (replicated by silicate minerals) and in the White Sea region of Russia (replicated by iron sulphide). These occurrences show that soft-tissue preservation in siliciclastic settings went well beyond that typical for Ediacaran macrofossils alone and also extended to similar modes of preservation in associated microbes. Using these new observations it can be argued that several existing explanations for Ediacaran fossil preservation can be united within a biogeochemical model that involves evolution of the sediment Mixed Layer across this transition.

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By applying modern biological criteria to trace fossil types and assessing burrow morphology, complexity, depth, potential burrow function and the likelihood of bioirrigation, we assign ecosystem engineering impact (EEI) values to the key ichnotaxa in the lowermost Cambrian (Fortunian). Surface traces such as Monomorphichnus have minimal impact on sediment properties and have very low EEI values; quasi-infaunal traces of organisms that were surficial modifiers or biodiffusors, such as Planolites, have moderate EEI values; and deeper infaunal, gallery biodiffusive or upward-conveying/downward-conveying traces, such as Teichichnus and Gyrolithes, have the highest EEI values. The key Cambrian ichnotaxon Treptichnus pedum has a moderate to high EEI value, depending on its functional interpretation. Most of the major functional groups of modern bioturbators are found to have evolved during the earliest Cambrian, including burrow types that are highly likely to have been bioirrigated. In fine-grained (or microbially bound) sedimentary environments, trace-makers of bioirrigated burrows would have had a particularly significant impact, generating advective fluid flow within the sediment for the first time, in marked contrast with the otherwise diffusive porewater systems of the Proterozoic. This innovation is likely to have created significant ecospace and engineered fundamentally new infaunal environments for macrobiotic and microbiotic organisms alike.

Thin-bedded turbidites deposited by sediment gravity flows that spill from submarine channels often contain significant volumes of sand in laterally continuous beds. These can make up over 50% of the channel-belt fill volume, and can thus form commercially important hydrocarbon reservoirs. Thin-bedded turbidites can be deposited in environments that include levees and depositional terraces, which are distinguished on the basis of their external morphology and internal architecture. Levees have a distinctive wedge shaped morphology, thinning away from the channel, and confine both channels (internal levees) and channel

Intrastratal shrinkage (often termed 'synaeresis') cracks are commonly employed as diagnostic environmental indicators for ancient salinity-stressed, transitional fluvial-marine or marginal-marine depositional environments. Despite their abundance and use in facies interpretations, the mechanism of synaeresis crack formation remains controversial, and widely accepted explanations for their formation have hitherto been lacking. Sedimentological, ichnological, petrographic and geochemical study of shallow marine mudstone beds from the Ordovician Beach Formation of Bell Island, Newfoundland, has revealed that crack development (cf. synaeresis cracks) on the upper surface of mudstone beds is correlated with specific organic, geochemical and sedimentological parameters. Contorted, sinuous, sand-filled cracks are common at contacts between unbioturbated mudstone and overlying sandstone beds. Cracks are absent in highly bioturbated mudstone, and are considered to predate firmground assemblages of trace fossils that include Planolites and Trichophycus. The tops of cracked mudstone beds contain up to 2Á1 wt% total organic carbon, relative to underlying mudstone beds that contain around 0Á5 wt% total organic carbon. High-resolution carbon isotope analyses reveal low d 13 C org values (À27Á6&) on bed tops compared with sandy intervals lacking cracks (À24Á4 to À24Á9&). Cracked mudstone facies show evidence for microbial matgrounds, including microbially induced sedimentary structures on bedding planes and carbonaceous laminae and tubular carbonaceous microfossils in thin section. Non-cracked mudstone lacks evidence for development of microbial mats. Microbial mat development is proposed as an important prerequisite for intrastratal shrinkage crack formation. Both microbial mats and intrastratal shrinkage cracks have broad palaeoenvironmental distributions in the Precambrian and early Phanerozoic. In later Phanerozoic strata, matgrounds are restricted to depositional environments that are inhospitable to burrowing and surface-grazing macrofauna. Unless evidence of synaeresis (i.e. contraction of clay mineral lattices in response to salinity change) can be independently demonstrated, the general term 'intrastratal shrinkage crack' is proposed to describe sinuous and tapering cracks in mudstone beds.