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Träfflista för sökning "L773:0028 0836 OR L773:1476 4687 "

Sökning: L773:0028 0836 OR L773:1476 4687

  • Resultat 31-40 av 1369
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31.
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32.
  • Ahlberg, Per E. (författare)
  • Something fishy in the family tree
  • 1999
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 397, s. 564-565
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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33.
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34.
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35.
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36.
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37.
  • Ahlberg, Per (författare)
  • Science set in stone
  • 2007
  • Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 447, s. 37-38
  • Recension (populärvet., debatt m.m.)
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38.
  • Ahlberg, Per, et al. (författare)
  • The axial skeleton of the Devonian tetrapod Ichthyostega.
  • 2005
  • Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 437:7055, s. 137-140
  • Tidskriftsartikel (refereegranskat)abstract
    • Ichthyostega was the first Devonian tetrapod to be subject to a whole-body reconstruction and remains, together with Acanthostega one of only two Devonian tetrapods for which near-complete postcranial material is available. It is thus crucially important for our understanding of the earliest stages of tetrapod evolution and terrestrialization. Based on extensive re-examination of original material, augmented by recently collected specimens, we present a new reconstruction of Ichthyostega that differs substantially from those previously published and reveals hitherto unrecognised regionalization in the vertebral column. Ichthyostega is the earliest vertebrate to show obvious adaptations for non-swimming locomotion. Uniquely among early tetrapods, the presacral vertebral column shows pronounced regionalization of neural arch morphology, suggesting that it was adapted for dorsoventral rather than lateral flexion.
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39.
  • Ahlberg, Per, 1963-, et al. (författare)
  • Ventastega curonica and the origin of tetrapod morphology
  • 2008
  • Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 453:7199, s. 1199-1204
  • Tidskriftsartikel (refereegranskat)abstract
    • The gap in our understanding of the evolutionary transition from fish to tetrapod is beginning to close thanks to the discovery of new intermediate forms such as Tiktaalik roseae. Here we narrow it further by presenting the skull, exceptionally preserved braincase, shoulder girdle and partial pelvis of Ventastega curonica from the Late Devonian of Latvia, a transitional intermediate form between the 'elpistostegids' Panderichthys and Tiktaalik and the Devonian tetrapods (limbed vertebrates) Acanthostega and Ichthyostega. Ventastega is the most primitive Devonian tetrapod represented by extensive remains, and casts light on a part of the phylogeny otherwise only represented by fragmentary taxa: it illuminates the origin of principal tetrapod structures and the extent of morphological diversity among the transitional forms
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40.
  • Ahmadi, M., et al. (författare)
  • An improved limit on the charge of antihydrogen from stochastic acceleration
  • 2016
  • Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 529:7586, s. 373-
  • Tidskriftsartikel (refereegranskat)abstract
    • Antimatter continues to intrigue physicists because of its apparent absence in the observable Universe. Current theory requires that matter and antimatter appeared in equal quantities after the Big Bang, but the Standard Model of particle physics offers no quantitative explanation for the apparent disappearance of half the Universe. It has recently become possible to study trapped atoms(1-4) of antihydrogen to search for possible, as yet unobserved, differences in the physical behaviour of matter and antimatter. Here we consider the charge neutrality of the antihydrogen atom. By applying stochastic acceleration to trapped antihydrogen atoms, we determine an experimental bound on the antihydrogen charge, Qe, of vertical bar Q vertical bar < 0.71 parts per billion (one standard deviation), in which e is the elementary charge. This bound is a factor of 20 less than that determined from the best previous measurement(5) of the antihydrogen charge. The electrical charge of atoms and molecules of normal matter is known(6) to be no greater than about 10(-21)e for a diverse range of species including H-2, He and SF6. Charge-parity-time symmetry and quantum anomaly cancellation(7) demand that the charge of antihydrogen be similarly small. Thus, our measurement constitutes an improved limit and a test of fundamental aspects of the Standard Model. If we assume charge superposition and use the best measured value of the antiproton charge(8), then we can place a new limit on the positron charge anomaly (the relative difference between the positron and elementary charge) of about one part per billion (one standard deviation), a 25-fold reduction compared to the current best measurement(8),(9).
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