2,534 research outputs found
Monophyly of brachiopods and phoronids: reconciliation of molecular evidence with Linnaean classification (the subphylum Phoroniformea nov.)
Molecular phylogenetic analyses of aligned 18S rDNA gene sequences from articulate and inarticulate brachiopods representing all major extant lineages, an enhanced set of phoronids and several unrelated protostome taxa, confirm previous indications that in such data, brachiopod and phoronids form a well-supported clade that (on previous evidence) is unambiguously affiliated with protostomes rather than deuterostomes. Within the brachiopod-phoronid clade, an association between phoronids and inarticulate brachiopods is moderately well supported, whilst a close relationship between phoronids and craniid inarticulates is weakly indicated. Brachiopod-phoronid monophyly is reconciled with the most recent Linnaean classification of brachiopods by abolition of the phylum Phoronida and rediagnosis of the phylum Brachiopoda to include tubiculous, shell-less forms. Recognition that brachiopods and phoronids are close genealogical allies of protostome phyla such as molluscs and annelids, but are much more distantly related to deuterostome phyla such as echinoderms and chordates, implies either (or both) that the morphology and ontogeny of blastopore, mesoderm and coelom formation have been widely misreported or misinterpreted, or that these characters have been subject to extensive homoplasy. This inference, if true, undermines virtually all morphology-based reconstructions of phylogeny made during the past century or more
Ordering of geometrically frustrated classical and quantum Ising magnets
A systematic study of both classical and quantum geometric frustrated Ising
models with a competing ordering mechanism is reported in this paper. The
ordering comes in the classical case from a coupling of 2D layers and in the
quantum model from the quantum dynamics induced by a transverse field. By
mapping the Ising models on a triangular lattice to elastic lattices of
non-crossing strings, we derive an exact relation between the spin variables
and the displacement field of the strings. Using this map both for the
classical (2+1)D stacked model and the quantum frustrated 2D system, we obtain
a microscopic derivation of an effective Hamiltonian which was proposed before
on phenomenological grounds within a Landau-Ginzburg-Wilson approach. In
contrast to the latter approach, our derivation provides the coupling constants
and hence the entire transverse field--versus--temperature phase diagram can be
deduced, including the universality classes of both the quantum and the
finite--temperature transitions. The structure of the ordered phase is obtained
from a detailed entropy argument. We compare our predictions to recent
simulations of the quantum system and find good agreement. We also analyze the
connections to a dimer model on the hexagonal lattice and its height profile
representation, providing a simple derivation of the continuum free energy and
a physical explanation for the universality of the stiffness of the height
profile for anisotropic couplings.Comment: 15 pages, 7 figure
Disorder Driven Roughening Transitions of Elastic Manifolds and Periodic Elastic Media
The simultaneous effect of both disorder and crystal-lattice pinning on the
equilibrium behavior of oriented elastic objects is studied using scaling
arguments and a functional renormalization group technique. Our analysis
applies to elastic manifolds, e.g., interfaces, as well as to periodic elastic
media, e.g., charge-density waves or flux-line lattices. The competition
between both pinning mechanisms leads to a continuous, disorder driven
roughening transition between a flat state where the mean relative displacement
saturates on large scales and a rough state with diverging relative
displacement. The transition can be approached by changing the impurity
concentration or, indirectly, by tuning the temperature since the pinning
strengths of the random and crystal potential have in general a different
temperature dependence. For D dimensional elastic manifolds interacting with
either random-field or random-bond disorder a transition exists for 2<D<4, and
the critical exponents are obtained to lowest order in \epsilon=4-D. At the
transition, the manifolds show a superuniversal logarithmic roughness. Dipolar
interactions render lattice effects relevant also in the physical case of D=2.
For periodic elastic media, a roughening transition exists only if the ratio p
of the periodicities of the medium and the crystal lattice exceeds the critical
value p_c=6/\pi\sqrt{\epsilon}. For p<p_c the medium is always flat. Critical
exponents are calculated in a double expansion in \mu=p^2/p_c^2-1 and
\epsilon=4-D and fulfill the scaling relations of random field models.Comment: 23 pages, 9 figure
The Mediterranean deep-sea fauna: historical evolution, bathymetric variations and geographical changes
The deep-water fauna of the Mediterranean is characterized by an absence of
distinctive characteristics and by a relative impoverishment. Both are a result
of events after the Messinian salinity crisis (Late Miocene). The three main
classes of phenomena involved in producing or recording these effects are
analysed and discussed: - Historical: Sequential faunal changes during the
Pliocene and thereafter in particular those during the Quaternary glaciations
and still in progress. - Bathymetric: Changes in the vertical aspects of the
Bathyal and Abyssal zones that took place under peculiar conditions, i.e.
homothermy, a relative oligotrophy, the barrier of the Gibraltar sill, and
water mass movement. The deeper the habitat of a species in the Mediterranean,
the more extensive is its distribution elsewhere. - Geographical: There are
strong affinities and relationships between Mediterranean and Atlantic faunas.
Endemic species remain a biogeographical problem. Species always become smaller
in size eastward where they occupy a progressively deeper habitat. Thus, the
existing deep Mediterranean Sea appears to be younger than any other deep-sea
constituent of the World Ocean
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