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Yin Z, Sun W, Liu P, Chen J, Bottjer DJ, Li J, Zhu M. Diverse and complex developmental mechanisms of early Ediacaran embryo-like fossils from the Weng'an Biota, southwest China. Philos Trans R Soc Lond B Biol Sci 2022; 377:20210032. [PMID: 35125006 PMCID: PMC8819369 DOI: 10.1098/rstb.2021.0032] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
The origin and early evolution of animal development remain among the many deep, unresolved problems in evolutionary biology. As a compelling case for the existence of pre-Cambrian animals, the Ediacaran embryo-like fossils (EELFs) from the Weng'an Biota (approx. 609 Myr old, Doushantuo Formation, South China) have great potential to cast light on the origin and early evolution of animal development. However, their biological implications can be fully realized only when their phylogenetic positions are correctly established, and unfortunately, this is the key problem under debate. As a significant feature of developmental biology, the cell division pattern (CDP) characterized by the dynamic spatial arrangement of cells and associated developmental mechanisms is critical to reassess these hypotheses and evaluate the diversity of the EELFs; however, their phylogenetic implications have not been fully realized. Additionally, the scarcity of fossil specimens representing late developmental stages with cell differentiation accounts for much of this debate too. Here, we reconstructed a large number of EELFs using submicron resolution X-ray tomographic microscopy and focused on the CDPs and associated developmental mechanisms as well as features of cell differentiation. Four types of CDPs and specimens with cell differentiation were identified. Contrary to the prevailing view, our results together with recent studies suggest that the diversity and complexity of developmental mechanisms documented by the EELFs are much higher than is often claimed. The diverse CDPs and associated development features including palintomic cleavage, maternal nutrition, asymmetric cell divisions, symmetry breaking, establishment of polarity or axis, spatial cell migration and differentiation constrain some, if not all, EELFs as total-group metazoans. This article is part of the theme issue ‘The impact of Chinese palaeontology on evolutionary research’.
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Affiliation(s)
- Zongjun Yin
- State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China.,Centre for Excellence in Life and Palaeoenvironment, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China.,Nanjing College, University of Chinese Academy of Sciences, Nanjing 211135, People's Republic of China.,College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
| | - Weichen Sun
- State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China.,University of Science and Technology of China, Hefei 230026, People's Republic of China
| | - Pengju Liu
- Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, People's Republic of China
| | - Junyuan Chen
- State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China
| | - David J Bottjer
- Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA
| | - Jinhua Li
- Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Innovation Academy for Earth Science, Chinese Academy of Sciences, Beijing 100029, People's Republic of China
| | - Maoyan Zhu
- State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China.,Centre for Excellence in Life and Palaeoenvironment, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China.,Nanjing College, University of Chinese Academy of Sciences, Nanjing 211135, People's Republic of China.,College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
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Yin Z, Vargas K, Cunningham J, Bengtson S, Zhu M, Marone F, Donoghue P. The Early Ediacaran Caveasphaera Foreshadows the Evolutionary Origin of Animal-like Embryology. Curr Biol 2019; 29:4307-4314.e2. [DOI: 10.1016/j.cub.2019.10.057] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2019] [Revised: 08/21/2019] [Accepted: 10/29/2019] [Indexed: 01/01/2023]
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Sperling EA, Stockey RG. The Temporal and Environmental Context of Early Animal Evolution: Considering All the Ingredients of an "Explosion". Integr Comp Biol 2019; 58:605-622. [PMID: 30295813 DOI: 10.1093/icb/icy088] [Citation(s) in RCA: 57] [Impact Index Per Article: 11.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
Animals originated and evolved during a unique time in Earth history-the Neoproterozoic Era. This paper aims to discuss (1) when landmark events in early animal evolution occurred, and (2) the environmental context of these evolutionary milestones, and how such factors may have affected ecosystems and body plans. With respect to timing, molecular clock studies-utilizing a diversity of methodologies-agree that animal multicellularity had arisen by ∼800 million years ago (Ma) (Tonian period), the bilaterian body plan by ∼650 Ma (Cryogenian), and divergences between sister phyla occurred ∼560-540 Ma (late Ediacaran). Most purported Tonian and Cryogenian animal body fossils are unlikely to be correctly identified, but independent support for the presence of pre-Ediacaran animals is recorded by organic geochemical biomarkers produced by demosponges. This view of animal origins contrasts with data from the fossil record, and the taphonomic question of why animals were not preserved (if present) remains unresolved. Neoproterozoic environments demanding small, thin, body plans, and lower abundance/rarity in populations may have played a role. Considering environmental conditions, geochemical data suggest that animals evolved in a relatively low-oxygen ocean. Here, we present new analyses of sedimentary total organic carbon contents in shales suggesting that the Neoproterozoic ocean may also have had lower primary productivity-or at least lower quantities of organic carbon reaching the seafloor-compared with the Phanerozoic. Indeed, recent modeling efforts suggest that low primary productivity is an expected corollary of a low-O2 world. Combined with an inability to inhabit productive regions in a low-O2 ocean, earliest animal communities would likely have been more food limited than generally appreciated, impacting both ecosystem structure and organismal behavior. In light of this, we propose the "fire triangle" metaphor for environmental influences on early animal evolution. Moving toward consideration of all environmental aspects of the Cambrian radiation (fuel, heat, and oxidant) will ultimately lead to a more holistic view of the event.
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Affiliation(s)
- Erik A Sperling
- Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, CA 94305, USA
| | - Richard G Stockey
- Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, CA 94305, USA
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Crosby CH, Bailey JV. Experimental precipitation of apatite pseudofossils resembling fossil embryos. GEOBIOLOGY 2018; 16:80-87. [PMID: 29047205 DOI: 10.1111/gbi.12264] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/03/2017] [Accepted: 09/10/2017] [Indexed: 06/07/2023]
Abstract
Certain phosphatic grains preserved in the rock record are interpreted as microfossils representing a diversity of microorganisms from bacteria to fossil embryos. In addition to bona fide primary biological features, phosphatic microfossils and fossil embryos commonly exhibit features that result from abiotic precipitation or diagenetic alteration. Distinguishing between abiotic and primary biological features can be difficult, and some features thought to represent biological tissue could instead be artifacts that are unrelated to the original morphology of a preserved organism. Here, we present experimentally generated, abiotically produced mineral precipitates that morphologically resemble biologically produced features, some of which may be observed in the rock record or noted in extant organisms, including embryos. These findings extend the diversity of biomorphic features known to result from abiotic precipitation.
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Affiliation(s)
- C H Crosby
- Department of Earth Sciences, University of Minnesota-Twin Cities, Minneapolis, MN, USA
| | - J V Bailey
- Department of Earth Sciences, University of Minnesota-Twin Cities, Minneapolis, MN, USA
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Erwin DH. Eric Davidson and deep time. HISTORY AND PHILOSOPHY OF THE LIFE SCIENCES 2017; 39:29. [PMID: 29030723 DOI: 10.1007/s40656-017-0156-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Eric Davidson had a deep and abiding interest in the role developmental mechanisms played in generating evolutionary patterns documented in deep time, from the origin of the euechinoids to the processes responsible for the morphological architectures of major animal clades. Although not an evolutionary biologist, Davidson's interests long preceded the current excitement over comparative evolutionary developmental biology. Here I discuss three aspects at the intersection between his research and evolutionary patterns in deep time: First, understanding the mechanisms of body plan formation, particularly those associated with the early diversification of major metazoan clades. Second, a critique of early claims about ancestral metazoans based on the discoveries of highly conserved genes across bilaterian animals. Third, Davidson's own involvement in paleontology through a collaborative study of the fossil embryos from the Ediacaran Doushantuo Formation in south China.
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Affiliation(s)
- Douglas H Erwin
- Department of Paleobiology, MRC-121, National Museum of Natural History, Washington, DC, 20013-7012, USA.
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Abstract
This paper addresses the taphonomic processes responsible for fossil preservation in calcium phosphate, or phosphatization. Aside from silicification and rarer examples of carbonaceous compression, phosphatization is the only taphonomic mode claimed to preserve putative subcellular structures. Because this fossilization window can record such valuable information, a comprehensive understanding of its patterns of occurrence and the geochemical processes involved in the replication of soft tissues are critical endeavors. Fossil phosphatization was most abundant during the latest Neoproterozoic through the early Paleozoic, coinciding with the decline of non-pelletal phosphorite deposits. Its temporal abundance during this timeframe makes it a particularly valuable window for the study of early animal evolution. Several occurrences of phosphatization from the Ediacaran through the Permian Period, including Doushantuo-type preservation of embryo-like fossils and acritarchs, phosphatized gut tracts within Burgess Shale-type carbonaceous compressions, Orsten-type preservation of meiofaunas, and other cases from the later Paleozoic are reviewed. In addition, a comprehensive description of the geochemical controls of calcium phosphate precipitation from seawater is provided, with a focus on the rates of phosphate nucleation and growth, favorable nucleation substrates, and properties of substrate tissue and pore-fluid chemistry. It is hoped that the paleontological and geochemical summaries provided here offer a practical and valuable guide to the Neoproterozoic–Paleozoic phosphatization window.
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Cunningham JA, Vargas K, Pengju L, Belivanova V, Marone F, Martínez-Pérez C, Guizar-Sicairos M, Holler M, Bengtson S, Donoghue PCJ. Critical appraisal of tubular putative eumetazoans from the Ediacaran Weng'an Doushantuo biota. Proc Biol Sci 2016; 282:20151169. [PMID: 26180072 PMCID: PMC4528530 DOI: 10.1098/rspb.2015.1169] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Molecular clock analyses estimate that crown-group animals began diversifying hundreds of millions of years before the start of the Cambrian period. However, the fossil record has not yielded unequivocal evidence for animals during this interval. Some of the most promising candidates for Precambrian animals occur in the Weng'an biota of South China, including a suite of tubular fossils assigned to Sinocyclocyclicus, Ramitubus, Crassitubus and Quadratitubus, that have been interpreted as soft-bodied eumetazoans comparable to tabulate corals. Here, we present new insights into the anatomy, original composition and phylogenetic affinities of these taxa based on data from synchrotron radiation X-ray tomographic microscopy, ptychographic nanotomography, scanning electron microscopy and electron probe microanalysis. The patterns of deformation observed suggest that the cross walls of Sinocyclocyclicus and Quadratitubus were more rigid than those of Ramitubus and Crassitubus. Ramitubus and Crassitubus specimens preserve enigmatic cellular clusters at terminal positions in the tubes. Specimens of Sinocyclocyclicus and Ramitubus have biological features that might be cellular tissue or subcellular structures filling the spaces between the cross walls. These observations are incompatible with a cnidarian interpretation, in which the spaces between cross walls are abandoned parts of the former living positions of the polyp. The affinity of the Weng'an tubular fossils may lie within the algae.
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Affiliation(s)
- John A Cunningham
- School of Earth Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol BS8 1TQ, UK Department of Palaeobiology and Nordic Center for Earth Evolution, Swedish Museum of Natural History, Stockholm 10405, Sweden
| | - Kelly Vargas
- School of Earth Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol BS8 1TQ, UK
| | - Liu Pengju
- Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, People's Republic of China
| | - Veneta Belivanova
- Department of Palaeobiology and Nordic Center for Earth Evolution, Swedish Museum of Natural History, Stockholm 10405, Sweden
| | - Federica Marone
- Swiss Light Source, Paul Scherrer Institute, Villigen 5232, Switzerland
| | - Carlos Martínez-Pérez
- School of Earth Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol BS8 1TQ, UK Department of Geology, University of Valencia, C/Dr. Moliner 50, Burjassot, Valencia 46100, Spain
| | | | - Mirko Holler
- Swiss Light Source, Paul Scherrer Institute, Villigen 5232, Switzerland
| | - Stefan Bengtson
- Department of Palaeobiology and Nordic Center for Earth Evolution, Swedish Museum of Natural History, Stockholm 10405, Sweden
| | - Philip C J Donoghue
- School of Earth Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol BS8 1TQ, UK
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Brasier M. Deep questions about the nature of early-life signals: a commentary on Lister (1673) 'A description of certain stones figured like plants'. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2015; 373:rsta.2014.0254. [PMID: 25750150 PMCID: PMC4360089 DOI: 10.1098/rsta.2014.0254] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
In 1673, Martin Lister explored the preservation of 'St Cuthbert's beads' plus other fossil crinoid remains from approximately 350 Ma Carboniferous limestone in northern England. He used taphonomic evidence (transport, disarticulation, burial and cementation) to infer an origin as petrified plant remains, in contrast with his views expressed elsewhere that fossil mollusc shells could have formed abiogenically, by 'plastic forces' within rock. Lister also observed pentagonal symmetry, now seen as characteristic of living echinoderm skeletons. A postscript from John Ray supports Lister's 'taphonomic' observations and accepts the biogenicity of these fossil 'vegetables'. Ray then concluded with a prophecy, predicting the discovery of comparable living fossils in remote ocean waters. These early discussions compare with current debates about the character of candidate microfossils from the early Earth and Mars. Interesting biomorphs are now tested against the abiogenic null hypotheses, making use of features such as those pioneered by Lister, including evidence for geological context, rules for growth and taphonomy. Advanced techniques now allow us to extend this list of criteria to include the nanoscale mapping of biology-like behaviour patterns plus metabolic pathways. Whereas the science of palaeobiology once began with tests for biogenicity, the same is now true for geobiology and astrobiology. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.
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Affiliation(s)
- Martin Brasier
- Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK
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Sponge grade body fossil with cellular resolution dating 60 Myr before the Cambrian. Proc Natl Acad Sci U S A 2015; 112:E1453-60. [PMID: 25775601 DOI: 10.1073/pnas.1414577112] [Citation(s) in RCA: 148] [Impact Index Per Article: 16.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
An extraordinarily well preserved, 600-million-year (Myr)-old, three-dimensionally phosphatized fossil displaying multiple independent characters of modern adult sponges has been analyzed by SEM and synchrotron X-ray tomography. The fossilized animal (Eocyathispongia qiania gen. et sp. nov.) is slightly more than 1.2 mm wide and 1.1 mm tall, is composed of hundreds of thousands of cells, and has a gross structure consisting of three adjacent hollow tubes sharing a common base. The main tube is crowned with a large open funnel, and the others end in osculum-like openings to the exterior. The external surface is densely covered with flat tile-like cells closely resembling sponge pinacocytes, and this layer is punctuated with smaller pores. A dense patch of external structures that display the form of a lawn of sponge papillae has also survived. Within the main funnel, an area where features of the inner surface are preserved displays a regular pattern of uniform pits. Many of them are surrounded individually by distinct collars, mounted in a supporting reticulum. The possibility cannot be excluded that these pits are the remains of a field of choanocytes. The character set evinced by this specimen, ranging from general anatomy to cell type, uniquely indicates that this specimen is a fossil of probable poriferan affinity. So far, we have only this single specimen, and although its organized and complex cellular structure precludes any reasonable interpretation that its origin is abiogenic, confirmation that it is indeed a fossilized sponge will clearly require discovery of additional specimens.
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Xiao S, Muscente AD, Chen L, Zhou C, Schiffbauer JD, Wood AD, Polys NF, Yuan X. The Weng'an biota and the Ediacaran radiation of multicellular eukaryotes. Natl Sci Rev 2014. [DOI: 10.1093/nsr/nwu061] [Citation(s) in RCA: 97] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Abstract
The rise of multicellularity represents a major evolutionary transition and it occurred independently in multiple eukaryote clades. Although simple multicellular organisms may have evolved in the Mesoproterozoic Era or even earlier, complex multicellular eukaryotes began to diversify only in the Ediacaran Period, just before the Cambrian explosion. Thus, the Ediacaran fossil record can provide key paleontological evidence about the early radiation of multicellular eukaryotes that ultimately culminated in the Cambrian explosion. The Ediacaran Weng'an biota in South China hosts exceptionally preserved eukaryote fossils, including various acanthomorphic acritarchs, pseudoparenchymatous thalli, tubular microfossils, and spheroidal fossils such as Megasphaera, Helicoforamina, Spiralicellula, and Caveasphaera. Many of these fossils have been interpreted as multicellular eukaryotes, although alternative interpretations have also been proposed. In this review, we critically examine these various interpretations, focusing particularly on Megasphaera, which has been variously interpreted as a sulfur-oxidizing bacterium, a unicellular protist, a mesomycetozoean-like holozoan, a volvocine green alga, a stem-group animal, or a crown-group animal. We conclude that Megasphaera is a multicellular eukaryote with evidence for cell-to-cell adhesion, a flexible membrane unconstrained by a rigid cell wall, spatial cellular differentiation, germ–soma separation, and programmed cell death. These features are inconsistent with the bacterium, unicellular protist, and mesomycetozoean-like holozoan interpretations. Thus, the surviving hypotheses, particularly the stem-group animal and algal interpretations, should be further tested with additional evidence. The Weng'an biota also hosts cellularly differentiated pseudoparenchymatous thalli with specialized reproductive structures indicative of an affinity with florideophyte red algae. The other Weng'an fossils reviewed here may also be multicellular eukaryotes, although direct cellular evidence is lacking in some and phylogenetic affinities are poorly constrained in others. The Weng'an biota offers many research opportunities to resolve the life histories and phylogenetic diversity of early multicellular eukaryotes and to illuminate the evolutionary prelude to the Cambrian explosion.
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Affiliation(s)
- Shuhai Xiao
- Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
| | - A. D. Muscente
- Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
| | - Lei Chen
- State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China
- College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, China
| | - Chuanming Zhou
- Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China
| | - James D. Schiffbauer
- Department of Geological Sciences, University of Missouri, Columbia, MO 65211, USA
| | - Andrew D. Wood
- Advanced Research Computing, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
| | - Nicholas F. Polys
- Advanced Research Computing, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
| | - Xunlai Yuan
- State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China
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Chen L, Xiao S, Pang K, Zhou C, Yuan X. Cell differentiation and germ–soma separation in Ediacaran animal embryo-like fossils. Nature 2014; 516:238-41. [DOI: 10.1038/nature13766] [Citation(s) in RCA: 67] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2014] [Accepted: 08/07/2014] [Indexed: 11/09/2022]
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Cunningham JA, Rahman IA, Lautenschlager S, Rayfield EJ, Donoghue PCJ. A virtual world of paleontology. Trends Ecol Evol 2014; 29:347-57. [PMID: 24821516 DOI: 10.1016/j.tree.2014.04.004] [Citation(s) in RCA: 151] [Impact Index Per Article: 15.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2013] [Revised: 03/26/2014] [Accepted: 04/08/2014] [Indexed: 11/28/2022]
Abstract
Computer-aided visualization and analysis of fossils has revolutionized the study of extinct organisms. Novel techniques allow fossils to be characterized in three dimensions and in unprecedented detail. This has enabled paleontologists to gain important insights into their anatomy, development, and preservation. New protocols allow more objective reconstructions of fossil organisms, including soft tissues, from incomplete remains. The resulting digital reconstructions can be used in functional analyses, rigorously testing long-standing hypotheses regarding the paleobiology of extinct organisms. These approaches are transforming our understanding of long-studied fossil groups, and of the narratives of organismal and ecological evolution that have been built upon them.
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Affiliation(s)
- John A Cunningham
- School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK
| | - Imran A Rahman
- School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK
| | - Stephan Lautenschlager
- School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK
| | - Emily J Rayfield
- School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK.
| | - Philip C J Donoghue
- School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK.
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Nielsen C. Life cycle evolution: was the eumetazoan ancestor a holopelagic, planktotrophic gastraea? BMC Evol Biol 2013; 13:171. [PMID: 23957497 PMCID: PMC3751718 DOI: 10.1186/1471-2148-13-171] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/21/2013] [Accepted: 08/06/2013] [Indexed: 11/10/2022] Open
Abstract
BACKGROUND Two theories for the origin of animal life cycles with planktotrophic larvae are now discussed seriously: The terminal addition theory proposes a holopelagic, planktotrophic gastraea as the ancestor of the eumetazoans with addition of benthic adult stages and retention of the planktotrophic stages as larvae, i.e. the ancestral life cycles were indirect. The intercalation theory now proposes a benthic, deposit-feeding gastraea as the bilaterian ancestor with a direct development, and with planktotrophic larvae evolving independently in numerous lineages through specializations of juveniles. RESULTS Information from the fossil record, from mapping of developmental types onto known phylogenies, from occurrence of apical organs, and from genetics gives no direct information about the ancestral eumetazoan life cycle; however, there are plenty of examples of evolution from an indirect development to direct development, and no unequivocal example of evolution in the opposite direction. Analyses of scenarios for the two types of evolution are highly informative. The evolution of the indirect spiralian life cycle with a trochophora larva from a planktotrophic gastraea is explained by the trochophora theory as a continuous series of ancestors, where each evolutionary step had an adaptational advantage. The loss of ciliated larvae in the ecdysozoans is associated with the loss of outer ciliated epithelia. A scenario for the intercalation theory shows the origin of the planktotrophic larvae of the spiralians through a series of specializations of the general ciliation of the juvenile. The early steps associated with the enhancement of swimming seem probable, but the following steps which should lead to the complicated downstream-collecting ciliary system are without any advantage, or even seem disadvantageous, until the whole structure is functional. None of the theories account for the origin of the ancestral deuterostome (ambulacrarian) life cycle. CONCLUSIONS All the available information is strongly in favor of multiple evolution of non-planktotrophic development, and only the terminal addition theory is in accordance with the Darwinian theory by explaining the evolution through continuous series of adaptational changes. This implies that the ancestor of the eumetazoans was a holopelagic, planktotrophic gastraea, and that the adult stages of cnidarians (sessile) and bilaterians (creeping) were later additions to the life cycle. It further implies that the various larval types are of considerable phylogenetic value.
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Affiliation(s)
- Claus Nielsen
- Zoological Museum, The Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, DK-2100, Copenhagen, Denmark.
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