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      Disc-shaped fossils resembling porpitids or eldonids from the early Cambrian (Series 2: Stage 4) of western USA

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          Abstract

          The morphology and affinities of newly discovered disc-shaped, soft-bodied fossils from the early Cambrian (Series 2: Stage 4, Dyeran) Carrara Formation are discussed. These specimens show some similarity to the Ordovician Discophyllum Hall, 1847; traditionally this taxon had been treated as a fossil porpitid. However, recently it has instead been referred to as another clade, the eldonids, which includes the enigmatic Eldonia Walcott, 1911 that was originally described from the Cambrian Burgess Shale. The status of various Proterozoic and Phanerozoic taxa previously referred to porpitids and eldonids is also briefly considered. To help ascertain that the specimens were not dubio- or pseudofossils, elemental mapping using energy dispersive X-ray spectroscopy (EDS) was conducted. This, in conjunction with the morphology of the specimens, indicated that the fossils were not hematite, iron sulfide, pyrolusite, or other abiologic mineral precipitates. Instead, their status as biologic structures and thus actual fossils is supported. Enrichment in the element carbon, and also possibly to some extent the elements magnesium and iron, seems to be playing some role in the preservation process.

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          Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale

          Organic preservation of non-mineralizing animals constitutes an important part of the paleontological record, yet the processes involved have not been investigated in detail. Organic-walled fossils are generally explicable as a coincidence of original, relatively recalcitrant, extra-cellular materials and more or less anti-biotic depositional circumstances. One of the most pervasive natural inhibitors of biodegradation results from substrate and enzyme adsorption onto, and within, clay minerals; such interactions are likely responsible for many of the organic-walled fossils preserved in clastic sediments. Close examination of the fossilLagerstätteof the Burgess Shale (Middle Cambrian, British Columbia) reveals that most of its so-called soft-bodied fossils are composed of primary (although kerogenized) organic carbon. Their preservation can be attributed to pervasive clay-organic interactions as the organisms were transported in a moving sediment cloud and buried with all cavities and spaces permeated with fine grained clays. The organic-walled Burgess Shale fossils were studied both in petrographic thin section and isolated from the rock matrix, following careful acid maceration. Isotopic analysis of bulk organic and carbonate carbon yielded values consistent with a normal marine paleoenvironment. Anatomical and histological consideration of the enigmatic Burgess wormAmiskwiasuggest that it may in fact be a chaetognath, while the putative chordatePikaiaappears not to be related to modern cephalochordates.
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            Cambrian burgess shale animals replicated in clay minerals

            Although the evolutionary importance of the Burgess Shale is universally acknowledged, there is disagreement on the mode of preservation of the fossils after burial. Elemental mapping demonstrates that the relative abundance of elements varies between different anatomical features of the specimens. These differences reflect the compositions of the minerals that replicated the decaying organism, which were controlled by contrasts in tissue chemistry. Delicate morphological details are replicated in the elemental maps, showing that authigenic mineralization was fundamental to preserving these fossils, even though some organic remains are also present.
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              The first named Ediacaran body fossil,Aspidella Terranovica

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                Author and article information

                Contributors
                Journal
                PeerJ
                PeerJ
                peerj
                peerj
                PeerJ
                PeerJ Inc. (San Francisco, USA )
                2167-8359
                6 June 2017
                2017
                : 5
                : e3312
                Affiliations
                [1 ]Department of Ecology & Evolutionary Biology, University of Kansas , Lawrence, KS, United States of America
                [2 ]Biodiversity Institute, University of Kansas , Lawrence, KS, United States of America
                [3 ]Pangaea Fossils , San Francisco, CA, United States of America
                [4 ]Microscopy and Analytical Imaging Laboratory, University of Kansas , Lawrence, KS, United States of America
                [5 ]Department of Geography, Edge Hill University , Ormskirk, United Kingdom
                Article
                3312
                10.7717/peerj.3312
                5463991
                c53f5054-e095-4541-901a-cbb5db854537
                ©2017 Lieberman et al.

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ) and either DOI or URL of the article must be cited.

                History
                : 6 January 2017
                : 13 April 2017
                Funding
                Funded by: National Science Foundation
                Award ID: EF-1206757
                This research was supported by the National Science Foundation (EF-1206757). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Paleontology
                Taxonomy
                Zoology

                porpitid,hydrozoa,cnidaria,cambrian,burgess shale type fossil,elemental mapping,eldonid

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