Skip to page content

Fossil record cycles are not mass extinctions

09 August 2005

Regular changes in the diversity of marine organisms over geological time may not be due to cycles of 'boom and bust' in diversity.

Diverse community of fossil marine invertebrates, Silurian limestone, Wenlock, UK. © Andrew Smith

Diverse community of fossil marine invertebrates, Silurian limestone, Wenlock, England. © Andrew Smith

New research by Museum scientists suggests these cycles are largely explained by the nature of the rock record. The more rock preserved from a particular age, the more fossils found.

Boom and bust
In 2005, in the journal Nature, Rohde and Muller identified a 62-million-year 'boom and bust' cycle in genus-level diversity of marine organisms in the fossil record.  Numbers of recorded fossils were shown to swing from low to high and back again repeatedly over the past 542 million years - the period of time known as the Phanerozoic eon.

Cycles of diversity
Explanations for these patterns and the apparent cycles of diversification and extinction have been hard to find. Scientists have looked at many different processes that might explain these cycles, including volcanic activity, comet impacts and sea level and climate change.  None, however, show a matching cyclicity. Although sea level changes were considered, Rohde and Muller did not measure them in a way that incorporated geologists’ understanding of how changes in sea level affect the preservation of fossils in the geological record.

Fossil bearing rocks of Abu Dhabi © The Natural History Museum, London 2005

Fossil bearing rocks of Abu Dhabi © The Natural History Museum, London 2005

New research findings
Research by palaeontologists Andrew Smith and Alistair McGowan from the Natural History Museum, strongly suggests that much of the variation in marine diversity is not connected to biological factors, but is caused by differences in the amount of rock available for sampling the fossils.

‘The observed diversity of marine organisms is systematically higher when there is more rock preserved for geologists and palaeontologists to study,’ says McGowan. ‘Differences in the erosion and deposition of marine sediments, and the range of habitats they sample, could be as important for generating the apparent “boom and bust” patterns in marine biodiversity’.

McGowan adds, ‘we have to try to ensure we understand whether our samples are based on samples from the whole range of marine habitats, or from a more limited range of environments. Based on statistical analyses, around about half of the change in diversity can be attributed to changes in the rock record. The other half could be genuine biological effects.’ 

More details explaining the cyclical fossil record research .

The research paper is from Biology Letters and a summary is published in the 23 July issue of the New Scientist magazine.

Further Information

External Links