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Sökning: L773:0028 0836 OR L773:1476 4687

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21.
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22.
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23.
  • Ahlberg, Per Erik, 1963-, et al. (författare)
  • Pelvic claspers confirm chondrichthyan-like internal fertilization in arthrodires
  • 2009
  • Ingår i: Nature. - : Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 460:7257, s. 888-889
  • Tidskriftsartikel (refereegranskat)abstract
    • Recent finds(1,2) demonstrate that internal fertilization and   viviparity (live birth) were more widespread in the Placodermi, an   extinct group of armoured fishes, than was previously realized.   Placoderms represent the sister group of the crown group jawed   vertebrates (Gnathostomata)(3,4), making their mode(s) of reproduction   potentially informative about primitive gnathostome conditions. An   ossified pelvic fin basipterygium discovered in the arthrodire   Incisoscutum ritchiei was hypothesized to be identical in males and   females, with males presumed to have an additional cartilaginous   element or series forming a clasper. Here we report the discovery of a   completely ossified pelvic clasper in Incisoscutum ritchiei (WAM   03.3.28) which shows that this interpretation was incorrect: the   basipterygium described previously(1) is in fact unique to females. The   male clasper is a slender rod attached to a square basal plate that   articulates directly with the pelvis. It carries a small cap of dermal   bone covered in denticles and small hooks that may be homologous with   the much larger dermal component of the ptyctodont clasper.
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24.
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25.
  • 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)
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26.
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27.
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28.
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29.
  • Ahlberg, Per, 1963-, et al. (författare)
  • Ventastega curonica and the origin of tetrapod morphology
  • 2008
  • Ingår i: Nature. - 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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30.
  • Ahmadi, M. (författare)
  • An improved limit on the charge of antihydrogen from stochastic acceleration
  • 2016
  • Ingår i: Nature. - 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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