Since the asteroid impact - mass extinction hypothesis first caught the attention of the scientific community and the public in 1980, an enormous amount of research effort has been expended on the question of whether an asteroid impact occurred near the close of the Cretaceous (65 million years ago), and what effect such an event might have had on the upper Cretaceous plant and animal faunas. However, the missing element in this research has been a careful and highly detailed analysis of the fossil record of plants and animals across this mass extinction interval. Such data are crucial for understanding the nature of the mass extinction itself and testing the various scenarios that put forward by earth scientists to account for the physical evidence.
Figure 1. The Cretaceous-Tertiary (K-T) boundary at Brazos River, Texas.
The Natural History Museum's research project into this chapter of Earth history - part of the Museum's Global Change and the Biosphere Programme -
represents a multi-investigator, multi-institutional effort that seeks to bring together the
expertise of palaeontologists, sedimentologists, palaeoceanographers, and geochemists
to reconstruct the record of upper Cretaceous - lower Tertiary biotic and environmental
events. The level of detail at which these investigations are proceeding is unprecedented
for this time interval.
Figure 2. The Cretaceous-Tertiary boundary in the Barranco del Gredero section, southern Spain, near the town of Caravaca.
By sampling sequences of strata that span the K-T boundary in places such as Texas (see the first illustration), Tunisia, Spain (see the illustration to the right), Israel, and Mexico, we have constructed a detailed model of K-T sedimentation patterns. These show which sections and deep sea cores contain a complete record of biotic events and which have time intervals missing at the K-T boundary.
Our results suggest that the Earth's biota underwent a long period (hundreds of thousands of years) of environmental change prior to the impact event (see the illustration below). The biotic effects of the impact itself appear to have been largely confined to the tropical and subtropical regions with high latitude biotas which experience few K-T extinctions. The existence of these high latitude areas, isolated from environmental change, together with the substantial numbers of extinctions that occur in the uppermost Cretaceous period, indicate that the K-T mass extinction was caused by a variety of factors. Those probably involved climate changes resulting from widespread volcanism and sea- level changes in addition to those caused by an asteroid impact.
Figure 3. shows the changes in the sizes of marine microfossil shells across the Cretaceous-
Tertiary (K-T) boundary at Brazos River, Texas. The patterns suggest that
environmental changes were affecting these populations long before the accumulation
of impact debris (at the K-T boundary). These pre-impact changes in the environment
probably reflect the influence of volcanism and sea-level changes on the Earth's climate
during the uppermost Cretaceous interval.
This project is part of the Ecological Patterns and Processes Research Theme.
For further information contact:
Dr Norman MacLeod
Department of Palaeontology
The Natural History Museum
Cromwell Road
London SW7 5BD UK
Tel: +44 (0) 171 938 9006 Fax: +44 (0) 171 938 9277
Email: nm@nhm.ac.uk
