| 1. |
- Donoghue, Philip C. J., et al.
(författare)
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Synchrotron X-ray tomographic microscopy of fossil embryos
- 2006
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Ingår i: Nature. - 0028-0836. ; 442:7103, s. 601-718
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Tidskriftsartikel (refereegranskat)abstract
- Fossilized embryos from the late Neoproterozoic and earliest Phanerozoic have caused much excitement because they preserve the earliest stages of embryology of animals that represent the initial diversification of metazoans1, 2, 3, 4. However, the potential of this material has not been fully realized because of reliance on traditional, non-destructive methods that allow analysis of exposed surfaces only1, 2,3, 4, and destructive methods that preserve only a single two-dimensional view of the interior of the specimen5, 6. Here, we have applied synchrotron-radiation X-ray tomographic microscopy (SRXTM)7, obtaining complete three-dimensional recordings at submicrometre resolution. The embryos are preserved by early diagenetic impregnation and encrustation with calcium phosphate, and differences in X-ray attenuation provide information about the distribution of these two diagenetic phases. Three-dimensional visualization of blastomere arrangement and diagenetic cement in cleavage embryos resolves outstanding questions about their nature, including the identity of the columnar blastomeres. The anterior and posterior anatomy of embryos of the bilaterian worm-like Markuelia confirms its position as a scalidophoran, providing new insights into body-plan assembly among constituent phyla. The structure of the developing germ band in another bilaterian, Pseudooides, indicates a unique mode of germ-band development. SRXTM provides a method of non-invasive analysis that rivals the resolution achieved even by destructive methods, probing the very limits of fossilization and providing insight into embryology during the emergence of metazoan phyla.
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| 2. |
- Huldtgren, Therese
(författare)
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Cleavage patterns in Neoproterozoic and Cambrian fossil embryos
- 2007
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Ingår i: 51st Annual Meeting of the Palaeontological Association.
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Konferensbidrag (populärvet., debatt m.m.)abstract
- The Neoproterozoic-Cambrian phosphoritic layers in southern China hold exceptionally preserved embryos of early animals. The specimens, preserved as diagenetically phosphatized replacements and encrustations, represent different developmental stages from early cleavage through hatching. Recent studies, where fossil embryos were analysed with synchrotron-based X-ray tomographic microscopy (srXTM), have yielded details of cellular and sub-cellular information of the internal embryonic structures. In this study, fossil embryos from both the Early Cambrian Kuanchuanpu Member of the Dengying Formation and the Neoproterozoic Doushantuo Formation have been identified for analysis of their complete three-dimensional morphology. The aim of this study is to depict the different cleavage patterns and the cell geometry of the fossil embryos from the two localities. Thus far, the apparent difference, when comparing the fossil embryos found in the early Cambrian with the Neoproterozoic ones, is that the Cambrian embryos have preserved the developmental stages from blastula to hatching, whereas the Neoproterozoic ones generally seem to be restricted to early cleavage embryos that show only pre-blastula features.
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| 3. |
- Huldtgren, Therese
(författare)
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Fossilized cleavage embryos from Early Cambrian Dengying Formation and the Neoproterozoic Doushantuo Formation, China
- 2008
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Ingår i: Geology and Hydrocarbon Potential of the Neoproterozoic-Cambrian Basins in India, Pakistan and the Middle East.
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Konferensbidrag (refereegranskat)abstract
- The Cambrian Period, the earliest period of the Palaeozoic Era and of the Phanerozoic Eon, saw the end of the evolutionary event commonly referred to as the Cambrian explosion (e.g., Conway Morris, 2000). This event is described as the sudden appearance of a large variety of complex body plans (in particular the metazoans) and new life strategies in an apparently brief span of geological time (Valentine et al., 1999). Over a period of a few tens of millions of years almost all modern phyla were established, although these animals were not the crown-group forms seen today (Budd and Jensen, 2000). Even so, the early history of animals is still very imperfectly known, and there is a great deal of speculation as to what caused or triggered the metazoan radiation (e.g., Conway Morris, 2003), as well as why it happened when it did.Discoveries during the past ten years have revealed exquisitely preserved fossil embryos in Cambrian and Late Neoproterozoic deposits. In 1994 the first article regarding egg-like structures was published, in which Zhang and Pratt identified what they considered to be traces of cell boundaries on the surface of Middle Cambrian globular microfossils, which they interpreted as arthropod eggs. Bengtson and Yue (1997) subsequently discovered that embryos of several kinds of Early Cambrian marine metazoans had been preserved through fossilization by early diagenetic phosphatization. The specimens showed the full developmental sequence from early cleavage through hatching. These first findings, in the lowermost Cambrian limestones/phosphorites of southern Shaanxi, China, and the Aldan River basin, Siberia, quickly led to other discoveries of phosphatized embryos in the Neoproterozoic Doushantuo limestones/phosphorites of Weng’an, Guizhou, China (Li et al., 1998; Xiao et al., 1998). Since then, a large number of papers concerning fossilized embryos have been published (e.g., Yue and Bengtson, 1999; Kouchinsky et al., 1999; Chen et al., 2000; Chen et al., 2004b; Dong et al., 2004; Dong et al., 2005; Donoghue et al., 2006; Hagadorn et al., 2006; Steiner et al., 2004 a,b; Yin et al., 2007).Previous work on fossilized embryos has been limited to analysis of their three-dimensional external morphology using scanning electron microscope (e.g., Bengtson and Yue, 1997) or their two-dimensional internal morphology using thin-section observation (e.g., Chen et al., 2004a). Recent studies, where fossil embryos were analysed with synchrotron-based X-ray tomographic microscopy (srXTM), have yielded details of cellular and sub-cellular information of the internal embryonic structures (e.g., Donoghue et al., 2006). The srXTM slices (pictures) prove that the preservation of these embryo fossils is quite extensive and not only limited to surface structures, but also that it is possible to distinguish between taphonomic and diagenetic features.In this study, fossil embryos from both the Early Cambrian Kuanchuanpu Member of the Dengying Formation and the Neoproterozoic Doushantuo Formation have been identified for analysis of their complete three-dimensional morphology. The specimens, preserved as diagenetically phosphatized replacements and encrustations, represent different developmental stages from early cleavage through hatching.The aim of this study is to depict the different cleavage patterns and the cell geometry of the fossil embryos from the two localities. Thus far, the apparent difference, when comparing the fossil embryos found in the early Cambrian with the Neoproterozoic ones, is that the Cambrian embryos have preserved the developmental stages from blastula to hatching, whereas the Neoproterozoic ones generally seem to be restricted to early cleavage embryos that show only pre-blastula features (Yin et al., 2001).The results from these ongoing experiments confirm that synchrotron-based X-ray tomographic microscopy is a powerful tool, non-destructively yielding three-dimensional information at micrometer resolution and in so, revealing internal details of sub-cellular and other embryonic structures. Clearly this technique has the potential to increase our understanding of developmental cycles of the early animals, and thereby providing valuable new insights in phylogenetic and evolutionary studies.
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| 4. |
- Huldtgren, Therese, et al.
(författare)
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Fossilized nuclei and germination structures identify Ediacaran ‘animal embryos’ as encysting protists.
- 2011
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Ingår i: Science. - Washington, DC : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 334:6063, s. 1696-1699
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Tidskriftsartikel (refereegranskat)abstract
- Globular fossils showing palintomic cell cleavage in the Ediacaran Doushantuo Formation, China, are widely regarded as embryos of early metazoans, although metazoan synapomorphies, tissue differentiation, and associated juveniles or adults are lacking. We demonstrate using synchrotron-based x-ray tomographic microscopy that the fossils have features incompatible with multicellular metazoan embryos. The developmental pattern is comparable with nonmetazoan holozoans, including germination stages that preclude postcleavage embryology characteristic of metazoans. We conclude that these fossils are neither animals nor embryos. They belong outside crown-group Metazoa, within total-group Holozoa (the sister clade to Fungi that includes Metazoa, Choanoflagellata, and Mesomycetozoea) or perhaps on even more distant branches in the eukaryote tree. They represent an evolutionary grade in which palintomic cleavage served the function of producing propagules for dispersion.
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| 5. |
- Huldtgren, Therese, et al.
(författare)
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Response to comment on “Fossilized nuclei and germination structures identify Ediacaran ‘animal embryos’ as encysting protists”.
- 2012
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 335:6073, s. 1169d-
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Tidskriftsartikel (refereegranskat)abstract
- The objections of Xiao et al. to our reinterpretation are based on incorrect assumptions. The lack of nanocrystals lining the nuclear membrane is consistent with membrane fossilization, and nucleus volume through development is correlated to cytoplasm volume and fully consistent with sizes of eukaryote nuclei. Identical envelope structure unites the developmental stages of the fossils, and 2n cleavage and Y-shaped junctions are holozoan symplesiomorphies.
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| 6. |
- Huldtgren, Therese
(författare)
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Subcellular structures in Ediacaran cleavage embryos
- 2008
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Ingår i: The 33rd International Geology Congress, Olso 2008 6–14 August.
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Konferensbidrag (refereegranskat)abstract
- The Ediacaran Doushantuo phosphoritic deposits in southern China hold exquisitely preserved embryos of early animals and during the past ten years, a large number of papers have been published regarding these investigations. Recent studies, in which fossil embryos were analysed with synchrotron-based X-ray tomographic microscopy (srXTM), have shown that with this technique it is possible not only to distinguish between taphonomic and diagenetic features (Donoghue et al. 2006, Nature 442: 680–683), but also to depict cellular and subcellular details of the internal embryonic structures (Hagadorn et al. 2006, Science 314: 291–294). The Ediacaran cleavage embryos, preserved as diagenetically phosphatized replacements and encrustations, are of uncertain affinity, and no co-occurring adult forms have been identified. The cleavage embryos generally seem to be restricted to early cleavage stages, with up to some hundred cells. In this study, I describe the diversity of internal morphological features found in early-stage cleavage embryos by use of srXTM. For example, a few exceptionally preserved 4-cell stage specimens exhibit subcellular structures indicative of eukaryotic affinity, such as complex nucleus-like structures, which are centrally located in each cell. Other specimens display spheroidal and ovoidal structures interpreted as lipid vesicles or yolk granules. Recently, Bailey and colleagues (2007, Nature 445: 198–201) published an alternative interpretation of these fossils, in which they propose that they represent giant sulphur bacteria. However, the discoveries of likely eukaryotic subcellular features, augmented by decay studies of modern animal embryos by Raff et al. (2006, Proc. Natl. Acad. Sci. USA 103: 5846–5851) and Gostling et al. (2008, Evol Dev. 10:3, 339–349), support the interpretation that these fossils from the Doushantuo Formation are of embryonic nature.
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| 7. |
- Huldtgren, Therese, et al.
(författare)
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Subcellular structures in Ediacaran fossil cleavage embryos
- 2008
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Ingår i: The Palaeontological Association.
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Konferensbidrag (refereegranskat)abstract
- The Ediacaran Doushantuo phosphoritic deposits in southern China hold exquisitely preserved microfossils, and a large number of papers on this biota have been published. The controversial cleavage embryos, preserved as diagenetically phosphatized replacements and encrustations, have been proposed to belong to animals, but no co-occurring adult forms have yet been identified. Recent studies, in which fossil embryos were analysed with synchrotron-radiation X-ray tomographic microscopy (srXTM), have shown that with this technique it is possible not only to distinguish between taphonomic and diagenetic features, but also to depict cellular and subcellular details of the internal structures. In this study, we describe some of the diversity of internal morphological features found in early-stage cleavage embryos by use of srXTM. A few exceptionally preserved 4-cell stage specimens exhibit subcellular structures indicative of eukaryotic affinity, such as complex nucleus-like structures, which are centrally located in each cell. One of these displays a spiral structure on the surface of each blastomere, suggesting affinity with Spiralicellula Xue et al. 1995. Other specimens display spheroidal and ovoidal structures interpreted as lipid vesicles or yolk granules. In 2007, Bailey et al. (Nature 445) published an alternative interpretation, in which they propose that the alleged cleavage fossils represent giant sulphur-oxidizing bacteria. However, recent decay studies of modern animal embryos in combination with the likely eukaryotic subcellular features described here supports the interpretation that these Ediacaran fossils from the Doushantuo Formation are in fact of embryonic nature.
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| 8. |
- Raff, Elizabeth C., et al.
(författare)
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Doushantuo fossils are not giant bacteria, but bacterial pseudomorphs of animal embryos
- 2008
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Ingår i: The Palaeontological Association.
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Konferensbidrag (refereegranskat)abstract
- Embryos from the Ediacaran Doushantuo Formation are among the most astonishing examples of exceptional fossilization. However, the mechanism of fossilization ispoorly understood, leading directly to debate over the interpretation of the fossils, someauthors even questioning their interpretation as embryos. It has been hypothesized thatmicrobial processes are responsible for preservation and mineralization of organic tissues.However, the actions of microbes in preservation of embryos have not been demonstratedexperimentally. We show that bacterial biofilms assemble rapidly in marine embryos,forming detailed pseudomorphs of cellular organization and structure. We define threeessential steps in embryo preservation: 1) blockage of autolysis by reducing or anaerobic conditions; 2) rapid formation of microbial biofilms that consume the embryo butform a replica that retains cell organization and morphology; 3) bacterially-catalyzedmineralization. We identified major bacterial taxa in embryo decay biofilms using16S rDNA sequencing. Decay processes were similar in different taphonomic conditions,but bacterial populations depended on specific conditions. Experimental taphonomyresembles preservation states of fossils. Our data show how fossilization of soft tissues insediments is mediated by bacterial replacement and mineralization, providing a foundationfor experimentally creating biofilms from defined microbial species to model fossilization asa biological process.
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| 9. |
- Raff, Elizabeth C., et al.
(författare)
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Embryo fossilization is a biological process mediated by microbial biofilms
- 2008
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Ingår i: Proceedings of the National Academy of Sciences. ; 105:49, s. 19360-19365
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Tidskriftsartikel (refereegranskat)abstract
- Fossilized embryos with extraordinary cellular preservation appear in the Late Neoproterozoic and Cambrian, coincident with the appearance of animal body fossils. It has been hypothesized that microbial processes are responsible for preservation and mineralization of organic tissues. However, the actions of microbes in preservation of embryos have not been demonstrated experimentally. Here, we show that bacterial biofilms assemble rapidly in dead marine embryos and form remarkable pseudomorphs in which the bacterial biofilm replaces and exquisitely models details of cellular organization and structure. The experimental model was the decay of cleavage stage embryos similar in size and morphology to fossil embryos. The data show that embryo preservation takes place in 3 distinct steps: (i) blockage of autolysis by reducing or anaerobic conditions, (ii) rapid formation of microbial biofilms that consume the embryo but form a replica that retains cell organization and morphology, and (iii) bacterially catalyzed mineralization. Major bacterial taxa in embryo decay biofilms were identified by using 16S rDNA sequencing. Decay processes were similar in different taphonomic conditions, but the composition of bacterial populations depended on specific conditions. Experimental taphonomy generates preservation states similar to those in fossil embryos. The data show how fossilization of soft tissues in sediments can be mediated by bacterial replacement and mineralization, providing a foundation for experimentally creating biofilms from defined microbial species to model fossilization as a biological process.
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