Mass extinctions are recognized through the study of fossil groups across event horizons, and from analyses of long-term trends in taxonomic richness and diversify. Both approaches have inherent ...flaws: and data that once seemed reliable can be readily superseded by the discovery of new fossils and/or the application of new analytical techniques. Herein the current state of the Cretaceous-Tertiary (K-T) biostratigraphical record is reviewed for most major fossil clades, including: calcareous nannoplankton, dinoflagellates, diatoms, radiolaria, foraminifera, ostracodes, scleractinian corals, bryozoans, brachiopods, molluscs, echinoderms, fish, amphibians, reptiles and terrestrial plants (macrofossils and palynomorphs). These reviews take account of possible biasing factors in the fossil record in order to extract the most comprehensive picture of the R-T biotic crisis available. Results suggest that many faunal and floral groups (ostracodes, bryozoa, ammonite cephalopods, bivalves, archosaurs) were in decline throughout the latest Maastrichtian while others (diatoms, radiolaria, benthic foraminifera, brachiopods, gastropods, fish, amphibians, lepidosaurs, terrestrial plants) passed through the K-T event horizon with only minor taxonomic richness and/or diversity changes. A few microfossil groups (calcareous nannoplankton, dinoflagellates, planktonic foraminifera) did experience a turnover of varying magnitudes in the latest Maastrichtian-earliest Danian. However, many of these turnovers, along with changes in ecological dominance patterns among benthic foraminifera, began in the latest Maastrichtian. Improved taxonomic estimates of the overall pattern and magnitude of the K-T extinction event must await the development of more reliable systematic and phylogenetic data for all Upper Cretaceous clades.
A study is presented of the electronic structure of the Ni--Zr system based on photoemission, susceptibility, and specific heat measurements. The results show that the band structure can be ...understood in a relatively simple mode, which has been partially presented before. For concentrations below 50 at.% Ni the contribution of Ni atoms to the electronic density of states at the Fermi level n(E sub F ) is small or negligible, while that of Zr atoms is similar to a pure metal. NiZr sub 2 is a superconductor with T sub c = 1.62 K; it also presents an atypical Debye temperature, which is related to soft modes, and a high electronic specific heat, which is attributable to the occurrence of a strong electron--phonon coupling. For concentrations between about 50 and 27.5 at.% Zr, due to the widening of the Ni d band, the Ni contribution to n(E sub F ) becomes appreciable and increases moderately with the Ni content. For concentrations higher than 72.5 at.%Ni, this contribution increases rapidly and a large enhancement factor to the susceptibility is noticed. These properties can be related to the atomic structure, in particular the number of Ni neighbours of the Ni atom. 24 ref.--AA.
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