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.
K-T boundary extinction: Geologically instantaneous or gradual event? Evidence from deep-sea benthic foraminifera
A stepped pattern of benthic foraminiferal decimation, survival, and recovery identified across the Cretaceous-Tertiary (K-T) boundary at Caravaca (southeast Spain) suggests that an exceptionally large influx of nutrients reached the sea floor suddenly, at the time of the K-T extinction event. Rapid reestablishment of pre-K-T boundary environmental conditions followed. Our data demonstrate a close coupling between the collapse of the ecologic system in the upper water column at Caravaca and a correspondent decimation of bottom dwellers on the sea floor. The dramatic environmental collapse of deep-water benthic foraminiferal communities at the end of the Cretaceous at Caravaca suggests that the K-T boundary event was geologically instantaneous and catastrophic.
The Downtonian and Dittonian Facies of the Welsh Borderland
Abstract The succession of Lower Old Red Sandstone rocks found in the Welsh Borderland, is reinterpreted to reveal a shifting pattern of facies of far greater complexity than was hitherto realized. The sequence of vertebrate faunas is examined in conjunction with the sediments, and in this way a fuller account is given of the conditions of life of these early vertebrates, than was previously possible. The boundary between the Downtonian and Dittonian Stages is revised to coincide with the major palaeontological break in the region, and this is shown to approximate to a major lithological break, which is here accepted as the division between the local rock units known as the Downton and Ditton Series.