Mass Extinctions in the Geological Record: Causes and Consequences

    Speaker:  Dr. Norman MacLeod, Keeper of Paleontology at the British Museum of Natural History

     With its biblical overtones, the phenomenon of ‘mass extinction’ has intrigued and puzzled scientists in many different disciplines ever since the middle 1800s. Over the intervening 150 years an impressive number of hypotheses and mechanisms have been proposed to account for these global biotic turnover events (e.g., Benton 1990). Unfortunately, few of these hypotheses have been tested against empirical observations.
This situation changed in 1980 with the proposal of the bolide extinction hypothesis for the Cretaceous-Paleogene (K-Pg) extinction event (Alvarez et al. 1980) which predicted the existence of a globally distributed layer of sediments enriched in rare earth elements coincident with this extinction horizon. When the predicted enriched zone was discovered in many K-Pg boundary localities worldwide the prima facie case for an extraterrestrial cause of this mass extinction appeared to have been made. However, most palaeontologists remained skeptical that a bolide impact was a necessary or sufficient explanation for the K-Pg event in particular (Archibald et al. 2010) or for other ancient extinction events preserved in the fossil record.
     Now, after almost 35 years of painstaking research, an unexpectedly eventful history of these biotic events is emerging, our understanding of fossil record across these event has changed, and schools of thought that have been locked in opposition over the question of mass extinction causality for decades are beginning to come together. Survivorship of species across mass extinction horizons is far more widespread than had been suspected previously a fact that is now supported by data ranging from the most mundane biostratigraphical observations to the most sophisticated molecular phylogenetic calibrations. In addition, a physical record of major environmental disruptions having occurred prior to, at, and after the traditional mass extinction horizons is now beginning to match up with this complex fossil record.
     These data all point to a contingent, multiple-cause model involving large igneous province volcanism, sea-level regression, and (in the case of the K-Pg event) the Chicxulub (and perhaps other) impact(s), and as the necessary and sufficient explanations for these extreme events not all of which are driven by extinction per se. This model applies to all the major extinction events of the last 250 million years and may reach as far back as the Late Paleozoic. Even more importantly realistic assessments of the modern extinction event are now being made using the data of the fossil record as a critical source of comparative information. These investigations not only provide a much needed context for understand both ancient and modern extinction events, they underscore the need to continue research into the phenomenon of extinction as a critical twenty-first century research priority.

Further Talks by Dr. MacLeod will be held 4:00, Monday, October 8 in Galvin 283, and 12:30, Tuesday, October 9 in Galvin 190.