A compound scenario for the end-Cretaceous mass extinctions

Abstract

Two major controversies have arisen in research on the end-Cretaceous mass extinctions, concerning the extent to which they were sudden or gradual, and terrestrially or extraterrestrially induced. A review of recent work supports more or less gradual extinction for a number of terrestrial and marine groups such as dinosaurs and-ammonites, but the spectacular crash of the calcareous plankton and correlative ecological disaster in land plants in part of the northern hemisphere suggest a short-terrn catastrophic event. With regard to extinction selectivity in the marine realm, tropical groups and suspension feeders dependent on phytoplankton were relatively vulnerable, while on the continents large terrestrial reptiles were more prone to extinction than their freshwater relatives, while plants in western North America and eastern Asia suffered more severely than elsewhere. Chemical and physical signatures of the Cretaceous-Tertiary boundary, including iridium anomalies, shocked quartz, microspherules and carbon, oxygen and strontium isotopes, are discussed and the evidence for and against a bolide impact-induced or volcanic catastrophe is reviewed, with the conclusion that it is not yet decisive either way. Evidence for longer-term changes is also cited, with strontium-isotope data supporting that from stratigraphy in suggesting a signiflcant fall in sea leve! shortly before the end of the Cretaceous. A large body of evidence also supports a latest Cretaceous fall in seawater and air temperature, but this has recently been disputed for western North America on the basis of leaf studies. It is concluded that both longer term causes, intrinsic to this planet, and a final catastrophe either involving bolide impact or volcanism on a spectacular scale, or perhaps a combination of the two, are required to account for the pattern of end Cretaceous extinctions.