Foreshadowing the Cretaceous–Tertiary extinction: none if by land, two if by sea

Abstract

There is broad consensus that the end of the Cretaceous period [144–65 million years ago (Mya)] was punctuated by a major bolide impact, which unleashed global environmental catastrophe. However, paleontologists are still scuffling over the biological consequences of that impact. Was everything rosy until that fateful Cretaceous day? Or, were earth-based processes, such as climate change and volcanism, destabilizing biological communities and setting the stage for mass extinction? The timing and rate of extinctions are crucial to these mechanistic hypotheses. Two recent papers bring perspectives from among the biggest and smallest organisms to suffer Cretaceous–Tertiary (K–T) extinctions.Peter Sheehan et al.1xDinosaur abundance was not declining in a ‘3 m gap’ at the top of the Hell Creek Formation, Montana and North Dakota. Sheehan, P.M. et al. Geology. 2000; 28: 523–526CrossrefSee all References1 argue that the abundance of dinosaur fossils in southwestern North Dakota and adjacent areas of Montana was not declining in the upper meters of Cretaceous sediments. The team used 200 volunteers to search more than 11 000 000 m2xLate Cretaceous sea-level changes in Tunisia: a multi-disciplinary approach. Li, L. et al. J. Geol. Soc. 2000; 157: 446–458Crossref | Scopus (71)See all References2 of Cretaceous outcrop at 1.5-m and 3.0-m intervals, and to map the distribution of dinosaur fossils. Although dinosaur fossils are not evenly distributed within the latest Cretaceous sediments, the upper meters of the Cretaceous were not among the most dinosaur-poor zones. This result contrasts with previous reports of a ‘3-m gap’ below the K–T boundary, in which vertebrate fossils are uncommon or absent. The ‘gap’ has led some to speculate that dinosaurs were rare or extinct millennia before the terminal-Cretaceous impact and to propose that other forces led to their demise. Sheehan et al. conclude that dinosaurs were regionally abundant until the K–T impact, thus supporting the causal role of the bolide in their extinction.By contrast, Gerta Keller and colleagues have long argued that patterns of extinction in marine foraminifera do not support a single, catastrophic extinction mechanism. New results, which combine paleontological data with geological and geochemical perspectives, show that climate-related changes in the marine system foreshadow biotic turnover. In previous work using stable oxygen isotope data from a series of globally distributed marine cores, Li and Keller showed that both intermediate water and sea surface temperatures increased by 3–4°C between 450–200 Ka (thousand years) before the end of the Cretaceous. Warming was followed by a cooling of 2–3°C during the past 200 Ka of the Cretaceous. In their most recent paper, Li et al.2xLate Cretaceous sea-level changes in Tunisia: a multi-disciplinary approach. Li, L. et al. J. Geol. Soc. 2000; 157: 446–458Crossref | Scopus (71)See all References2 detailed the analysis of a series of sections in Tunisia showing that the warming and cooling events were paralleled by rising and falling sea level2xLate Cretaceous sea-level changes in Tunisia: a multi-disciplinary approach. Li, L. et al. J. Geol. Soc. 2000; 157: 446–458Crossref | Scopus (71)See all References2. They conclude that these events played an important role in marine extinctions across the K–T boundary.It does not seem surprising that lineages as disparate as foraminifera and dinosaurs would respond differently to the same series of geologic events. Rather, these papers highlight that the end of the Cretaceous was a complex time, for which multifaceted hypotheses will be required. Sorting out the complexity of the biological signal is the first step to such comprehensive hypotheses of extinction and survival across the K–T boundary.