The Unique Significance and Origin of the Cretaceous-Tertiary Boundary: Historical Context and Burdens of Proof
The abruptness and intensity of the Cretaceous-Tertiary boundary have been deemphasized by some authors over recent years, mainly by those skeptical of an impact origin for the boundary. However, it was recognized at the birth of stratigraphy as both abrupt and of major importance. It was used to define the change from the Mesozoic to the Cenozoic; the boundary has become continually more precisely defined and its global sequences more correlatable. It is now unique in being an event boundary marked by an iridium-bearing layer of global extent, rather than an arbitrary boundary in a sequence of little change. The Permian-Triassic boundary, in contrast, is arbitrary and the transition is not yet proven to be abrupt, the extinctions that define it perhaps having taken place in pulses over several millions of years. Some of those who have denied the importance (and in some cases even the existence) of an impact in the Cretaceous-Tertiary extinctions have placed burdens of proof on the impact hypothesis that they do not place on strictly terrestrial mechanisms. Terrestrial mechanisms have always been unsatisfactory (or at least unconvincing for global, massive, multienvironment faunal change) and are now even more so. Some authors have required of the impact hypothesis attributes that are not inherent in it, including particular patterns of extinction selectivity and timing.
Major extinctions of land-dwelling vertebrates at the Cretaceous-Tertiary boundary, eastern Montana
A large database recording species of terrestrial vertebrates present in formations above and below the Cretaceous-Tertiary (K-T) boundary in eastern Montana was assembled by J. D. Archibald and L. J. Bryant. Division of the species in this database into freshwater and land-dwelling vertebrate assemblages reveals that the K-T vertebrate extinction was concentrated in land- dwelling forms. In data corrected for the effects of rare taxa, 90% of the species in the freshwater assemblage survived into the Tertiary, but only 12% of the land-dwelling forms survived. The pattern of differential extinction of terrestrial vertebrates in eastern Montana may be in large part the result of the dependence of land-based communities on primary productivity. This is in contrast to the riverine communities, which may derive much of their organic carbon from detritus. The pattern of extinction and survival is compatible with the hypothesis of an asteroid impact after which there was a temporary cessation of primary, photosynthetic productivity.
Palaeocope Ostracodes from the Well Leba I in Pomerania
Abstract The Siluro-Devonian sequence at Leba in Pomerania abounds in ostracodes, occurring in calcareous shales or in limestone lenticles in mudstones. Three consecutive faunas may be discerned, viz. (1) an older fauna with primitiopsines, showing aspects of the youngest Ludlovian faunas of Gotland but mainly composed of new species of genera known from Gotland;, (2) a fauna with elements and aspects of the youngest ostracodiferous parts of the Oved-Ramsasa Beds in Scania, and (3) a fauna with elements from the Beyrichienkalk, occurring only in pebbles in the Zechstein conglomerate. The families Craspedobolbinidae, Beyrichiidae, Primitiopsidae, Hollinidae, and Aechminidae are represented. The new genera Undulirete (Primitiopsidae) and Borussulus (incertae familias) are described; there are nine-new species (see Contents), including one Hemsiella species with open nomenclature.