THE CRETACEOUS-TERTIARY BIOTIC TRANSITION

Mass extinctions are recognized through the study of fossil groups acr oss 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 Both approaches have inherent flaws: and data that once seemed reliable can be readily superseded by the discovery of new fossils and/or the application of new analytical techniques. He rein the current state of the Cretaceous-Tertiary (K-T) biostratigraph ical record is reviewed for most major fossil clades, including: calca reous nannoplankton, dinoflagellates, diatoms, radiolaria, foraminifer a, ostracodes, scleractinian corals, bryozoans, brachiopods, molluscs, echinoderms, fish, amphibians, reptiles and terrestrial plants (macro fossils and palynomorphs). These reviews take account of possible bias ing factors in the fossil record in order to extract the most comprehe nsive picture of the R-T biotic crisis available. Results suggest that many faunal and floral groups (ostracodes, bryozoa, ammonite cephalop ods, bivalves, archosaurs) were in decline throughout the latest Maast richtian while others (diatoms, radiolaria, benthic foraminifera, brac hiopods, gastropods, fish, amphibians, lepidosaurs, terrestrial plants ) passed through the K-T event horizon with only minor taxonomic richn ess and/or diversity changes. A few microfossil groups (calcareous nan noplankton, dinoflagellates, planktonic foraminifera) did experience a turnover of varying magnitudes in the latest Maastrichtian-earliest D anian. However, many of these turnovers, along with changes in ecologi cal dominance patterns among benthic foraminifera, began in the latest Maastrichtian. Improved taxonomic estimates of the overall pattern an d magnitude of the K-T extinction event must await the development of more reliable systematic and phylogenetic data for all Upper Cretaceou s clades.