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.
A Geographic Database Approach to the KT Boundary
A comprehensive database on the Cretaceous-Tertiary (KT) boundary is presented. The database (KTbase) is designed to evaluate spatial patterns in data that are important to constrain the causes and mechanisms of the K/T event. KTbase contains paleontological, sedimentological, mineralogical, and geochemical data on currently 350 KT sites. Although still incomplete, KTbase can already be used to summarize global aspects of the ecological catastrophe coincident with the KT boundary, to suggest some regional patterns related to the impact of an extraterrestrial body in the Gulf of Mexico and to document the distribution of ejecta debris from the Chicxulub crater.
Some Aspects of the Siluro-Devonian Boundary Problem
The writer made a journey recently on the Continent to study the various facies of the Lower Devonian in connection with work on a fauna from New Zealand. Although not directly concerned with the lower limit of the Devonian System he gathered certain information regarding faunas near that boundary. This was especially a consequence of an invitation to the international excursion in May, 1937, arranged by the Prussian Geological Survey to discuss the Siluro-Devonian boundary and view a series of trenches at Hüinghausen, Westphalia, which, it was hoped, would expose the passage beds between these systems. The Prussian Geological Survey generously allowed the writer to collect somewhat later from these trenches. Professor Pruvost also invited the writer to examine the collections from Liévin at Lille, and later loaned a series of specimens for more detailed examination. For all these kindnesses the writer expresses his thanks. Mr. S. H. Straw has kindly read through the manuscript and made many valuable suggestions, and the writer had the benefit of discussion with Professor Richter of Frankfurt who also helped with material for comparison. To the authorities of the British Museum, the Museum of the Geological Survey, and the Sedgwick Museum the writer is indebted for the loan of, or access to, figured specimens. He also acknowledges a grant from the Royal Society which enabled him to make a journey on the Continent.