BIOTIC CHANGES IN PLATFORM COMMUNITIES ACROSS THE PRECAMBRIAN PHANEROZOIC BOUNDARY

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

The authors thank Duncan McIlroy and Alex Liu for their discussions, help, comments and field support, the National Trust for access to Longmyndian localities, and the staff of the British Geological Survey Palaeontology unit and the Oxford University Museum of Natural History for their assistance with access to materials. The comments and suggestions of two anonymous reviewers and Nora Noffke significantly improved the manuscript.Peer reviewedPostprin

Cryptic Disc Structures Resembling Ediacaran Discoidal Fossils from the Lower Silurian Hellefjord Schist, Arctic Norway

The Hellefjord Schist, a volcaniclastic psammite-pelite formation in the Caledonides of Arctic Norway contains discoidal impressions and apparent tube casts that share morphological and taphonomic similarities to Neoproterozoic stem-holdfast forms. U-Pb zircon geochronology on the host metasediment indicates it was deposited between 437 ± 2 and 439 ± 3 Ma, but also indicates that an inferred basal conglomerate to this formation must be part of an older stratigraphic element, as it is cross-cut by a 546 ± 4 Ma pegmatite. These results confirm that the Hellefjord Schist is separated from underlying older Proterozoic rocks by a thrust. It has previously been argued that the Cambrian Substrate Revolution destroyed the ecological niches that the Neoproterozoic frond-holdfasts organisms occupied. However, the discovery of these fossils in Silurian rocks demonstrates that the environment and substrate must have been similar enough to Neoproterozoic settings that frond-holdfast bodyplans were still ecologically viable some hundred million years later

Multiple global radiations in tadpole shrimps challenge the concept of ‘living fossils’

‘Living fossils’, a phrase first coined by Darwin, are defined as species with limited recent diversification and high morphological stasis over long periods of evolutionary time. Morphological stasis, however, can potentially lead to diversification rates being underestimated. Notostraca, or tadpole shrimps, is an ancient, globally distributed order of branchiopod crustaceans regarded as ‘living fossils’ because their rich fossil record dates back to the early Devonian and their morphology is highly conserved. Recent phylogenetic reconstructions have shown a strong biogeographic signal, suggesting diversification due to continental breakup, and widespread cryptic speciation. However, morphological conservatism makes it difficult to place fossil taxa in a phylogenetic context. Here we reveal for the first time the timing and tempo of tadpole shrimp diversification by inferring a robust multilocus phylogeny of Branchiopoda and applying Bayesian divergence dating techniques using reliable fossil calibrations external to Notostraca. Our results suggest at least two bouts of global radiation in Notostraca, one of them recent, so questioning the validity of the ‘living fossils’ concept in groups where cryptic speciation is widespread

The origin of animals: can molecular clocks and the fossil record be reconciled?

The evolutionary emergence of animals is one of the most significant episodes in the history of life, but its timing remains poorly constrained. Molecular clocks estimate that animals originated and began diversifying over 100 million years before the first definitive metazoan fossil evidence in the Cambrian. However, closer inspection reveals that clock estimates and the fossil record are less divergent than is often claimed. Modern clock analyses do not predict the presence of the crown-representatives of most animal phyla in the Neoproterozoic. Furthermore, despite challenges provided by incomplete preservation, a paucity of phylogenetically informative characters, and uncertain expectations of the anatomy of early animals, a number of Neoproterozoic fossils can reasonably be interpreted as metazoans. A considerable discrepancy remains, but much of this can be explained by the limited preservation potential of early metazoans and the difficulties associated with their identification in the fossil record. Critical assessment of both records may permit better resolution of the tempo and mode of early animal evolution.JAC and AGL acknowledge support from Natural Environment Research Council (NERC) Fellowships [grant numbers NE/J018325/1 and NE/L011409/1]. SB and JAC acknowledge funding from the Danish National Research Foundation [DNRF53] and the Swedish Research Council [2013-4290]. PCJD was supported by a Royal Society Wolfson Merit Award, a Leverhulme Trust Research Fellowship and a NERC standard grant [NE/F00348X/1]

Bootstrapping the energy flow in the beginning of life.

This paper suggests that the energy flow on which all living structures depend only started up slowly, the low-energy, initial phase starting up a second, slightly more energetic phase, and so on. In this way, the build up of the energy flow follows a bootstrapping process similar to that found in the development of computers, the first generation making possible the calculations necessary for constructing the second one, etc. In the biogenetic upstart of an energy flow, non-metals in the lower periods of the Periodic Table of Elements would have constituted the most primitive systems, their operation being enhanced and later supplanted by elements in the higher periods that demand more energy. This bootstrapping process would put the development of the metabolisms based on the second period elements carbon, nitrogen and oxygen at the end of the evolutionary process rather than at, or even before, the biogenetic even

MicroRNAs and metazoan macroevolution: insights into canalization, complexity, and the Cambrian explosion

One of the most interesting challenges facing paleobiologists is explaining the Cambrian explosion, the dramatic appearance of most metazoan animal phyla in the Early Cambrian, and the subsequent stability of these body plans over the ensuing 530 million years. We propose that because phenotypic variation decreases through geologic time, because microRNAs (miRNAs) increase genic precision, by turning an imprecise number of mRNA transcripts into a more precise number of protein molecules, and because miRNAs are continuously being added to metazoan genomes through geologic time, miRNAs might be instrumental in the canalization of development. Further, miRNAs ultimately allow for natural selection to elaborate morphological complexity, because by reducing gene expression variability, miRNAs increase heritability, allowing selection to change characters more effectively. Hence, miRNAs might play an important role in shaping metazoan macroevolution, and might be part of the solution to the Cambrian conundrum

A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale

Author Posting. © Nature Publishing Group, 2006. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 442 (2006): 159-163, doi:10.1038/nature04894.Odontogriphus omalus was originally described as a problematic non-biomineralized lophophorate organism. Here we reinterpret Odontogriphus based on 189 new specimens including numerous exceptionally well-preserved individuals from the Burgess Shale collections of the Royal Ontario Museum. This additional material provides compelling evidence that the feeding apparatus in Odontogriphus is a radula of molluscan architecture comprising two primary bipartite tooth rows attached to a radular membrane and showing replacement by posterior addition. Further characters supporting molluscan affinity include a broad foot bordered by numerous ctenidia located in a mantle groove and a stiffened cuticular dorsum. Odontogriphus has a radula similar to Wiwaxia corrugata but lacks a scleritome. We interpret these animals to be members of an early stem-group mollusc lineage that likely originated in the Neoproterozoic Ediacaran Period, providing support for the retention of a biomat-based grazing community from the late Precambrian until at least the Middle Cambrian.Our research was in part supported by a Post-Doctoral Natural Sciences and Engineering Research Council of Canada grant (to JBC-2005) and by a Swedish Research Council grant (to CS)