Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO2 was 102 PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO2 rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.

https://hal.archives-ouvertes.fr/hal-01765604/document

Snowball Earth climate dynamics and Cryogenian geology-geobiology

https://www.cell.com/current-biology/pdf/S0960-9822(15)01177-X.pdf

Uncertainty in the Timing of Origin of Animals and the Limits of Precision in Molecular Timescales.

Strontium isotopes are a powerful tool for investigating the geographical origins of people and animals but assignment of provenance requires reference maps and databases. This paper presents a map of strontium isotope variations across Britain for use in archaeological and other studies, and is produced by direct measurement of strontium isotope compositions of biosphere components, predominantly plants. The map characterizes the biosphere signature of specific lithologies: a technique that is applicable worldwide. Marine-derived strontium, from both rainwater and coastal sea-splash and spray, is an important contribution to the British biosphere, particularly on the western seaboard, which is subject to high rainfall and prevailing westerly winds. Supplementary material: Methods, full data table, and full-size version of Figure 1b are available at http://www.geolsoc.org.uk/SUP18388.

Spatial variations in biosphere 87Sr/86Sr in Britain

Ninety years after Zahlbruckner, we present the most recent update to the classification of lichen fungi in the Ascomycota and Basidiomycota to genus level, with species numbers and references to changes compared to the 2010 Outline of Ascomycota and other recent classifications. Updated statistics on global species richness of lichen fungi and species richness at family, order and class level are given. The number of accepted species is 19,387 in 995 genera, 115 families, 39 orders and eight classes. Lichenized Basidiomycota amount to 172 species (0.9% of the total), 15 genera (1.5%), five families (4.3%), five orders (12.8%) and one class (12.5%). The most speciose genera are Xanthoparmelia, Lecanora, Arthonia, Cladonia, Pertusaria, Ocellularia, Graphis, Caloplaca, Usnea and Buellia. The average number of species per genus is 19.5 and 256 genera are monospecific. Using newly defined categories, two genera (Xanthoparmelia, Lecanora) are ultradiverse (more than 500 species), 17 hyperdiverse (201–500 species) and 12 megadiverse (101–200). The largest family is Parmeliaceae, with 2,765 species and 77 genera, followed by Graphidaceae (2,161; 79), Verrucariaceae (943; 43), Ramalinaceae (916; 43) and Lecanoraceae (791; 25). The largest order is Lecanorales, with 6,231 species and 234 genera, followed by Ostropales (3,261; 138), Arthoniales (1,541, 103), Peltigerales (1,301; 67) and Caliciales (1,276; 55). The largest class is Lecanoromycetes, with 15,131 species and 701 genera, followed by Arthoniomycetes (1,541; 103), Eurotiomycetes (1,203; 63), Dothideomycetes (812; 39) and Lichinomycetes (390; 50). A total of 751 out of 995 genera (75%) have molecular data. Fifty-nine genera remain in unresolved positions at the family, order or class level. The phylogenetic position of the 39 orders containing lichenized fungi suggests 20–30 independent lichenization events during the evolution of higher Fungi, 14–23 in the Ascomycota and 6–7 in the Basidiomycota. The following names are validated: Candelariomycetidae Miądl. et al. ex Timdal & M.Westb. subcl. nov., Cystocoleaceae Locq. ex Lucking, B.P.Hodk. & S.D.Leav. fam. nov, Letrouitineae Gaya & Lutzoni subordo nov., Rhizocarpales Miądl. & Lutzoni ordo nov. and Teloschistineae Gaya & Lutzoni subordo nov. Lectotypes are designated for Clathroporinopsis M.Choisy and Protoschistes M.Choisy, making both synonyms of Gyalecta Ach., and Stromatothelium Trevis., making it a synonym of Pyrenula Ach. Members of Cyphobasidiales, which are here interpreted as hyperlichenized fungi, as well as fossil lichen fungi, are added in additional classifications in two appendices.

https://bioone.org/journals/The-Bryologist/volume-119/issue-4/0007-2745-119.4.361/The-2016-classification-of-lichenized-fungi-in-the-Ascomycota-and/10.1639/0007-2745-119.4.361.pdf

The 2016 classification of lichenized fungi in the Ascomycota and Basidiomycota – Approaching one thousand genera

/pdf/depositional-sedimentary-environments-106r28he2c.pdf

Depositional Sedimentary Environments

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.

Giving the early fossil record of sponges a squeeze.

/pdf/remarkable-preservation-of-microbial-mats-in-neoproterozoic-3p9w8imbov.pdf

Remarkable preservation of microbial mats in Neoproterozoic siliciclastic settings: Implications for Ediacaran taphonomic models

Genesis and character of thin-bedded turbidites associated with submarine channels

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 pre-date firmground assemblages of trace fossils that include Planolites and Trichophycus. The tops of cracked mudstone beds contain up to 2·1 wt% total organic carbon, relative to underlying mudstone beds that contain around 0·5 wt% total organic carbon. High-resolution carbon isotope analyses reveal low δ13Corg values (−27·6‰) on bed tops compared with sandy intervals lacking cracks (−24·4 to −24·9‰). 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.

Microbial mats implicated in the generation of intrastratal shrinkage (‘synaeresis’) cracks

A unifying model is presented that explains most of the major changes seen in fossil preservation and redox conditions across the Precambrian–Cambrian transition. It is proposed that the quality of cellular and tissue preservation in Proterozoic and Cambrian sediments is much higher than it is in more recent marine deposits. Remarkable preservation of cells and soft tissues occurs in Neoproterozoic to Cambrian cherts, phosphates, black shales, siliciclastic sediments and carbonates across a wide range of environmental conditions. The conditions for remarkable preservation were progressively restricted to more marginal environments through time, such as those now found in stagnant lakes or beneath upwelling zones. These paradoxes can no longer be adequately explained by recourse to a series of ad hoc explanations, such as those involving unusually tough organic matter in the Ediacaran, or unusual seawater chemistry, or even the role of microbial biofilms alone. That is because the exceptions to these are now too many. Instead, we suggest that elevated pore water ion concentrations, coupled with the almost complete lack of infaunal bioturbation, and hence the lack of a sediment Mixed-layer, provided an ideal environment for microbially-mediated ionic concentrations at or near the sediment–water interface. These strong ionic gradients encouraged early cementation and lithification of sediments, often prior to complete decomposition of delicate organic structures. Seen in this way, not only did the biosphere evolve across the Precambrian–Cambrian transition. Fossilization itself has evolved through time, and never more dramatically so than across this interval.

Richard H. T. Callow

Papers

Giving the early fossil record of sponges a squeeze.

Remarkable preservation of microbial mats in Neoproterozoic siliciclastic settings: Implications for Ediacaran taphonomic models

Genesis and character of thin-bedded turbidites associated with submarine channels

Microbial mats implicated in the generation of intrastratal shrinkage (‘synaeresis’) cracks

Evolutionary Trends in Remarkable Fossil Preservation Across the Ediacaran–Cambrian Transition and the Impact of Metazoan Mixing