Detritus in the ocean bed helps reveal Earth's story

Biostratigraphy
March 23, 2007

Biostratigraphy is the application of the fossil record of biological evolution to the dating and correlation of the sedimentary rocks in which they occur. With its origins in the late 18th century, it is a relatively young science, but it has given us the geological timescale, enabled us to determine the age of sediments worldwide and so to reconstruct changing patterns of environments through geological time. It also provides the best documented examples of evolutionary change available from the geological record.

Thousands of man-years have been spent to build our knowledge of the succession of species of ammonites, graptolites, planktonic foraminifera, coccoliths and other biostratigraphically useful fossils so that now we can reliably date fossil-bearing strata from almost any geological age or environment. But strangely, there has been remarkably little theoretical debate about such work. In this volume, Bruce McGowran, who has had a long and distinguished career in the field, attempts to fill this philosophical vacuum by examining how biostratigraphy evolved as a science, how it interrelates with other aspects of earth science, and where it is going.

As he hints in his subtitle, McGowran does this primarily by examining how microfossils, especially planktonic foraminifera, have over the past 50 years transformed our knowledge of the past 65 million years of earth history, the Cenozoic. The Cenozoic began with the meteorite impact and related environmental changes that caused, or completed, the extinction of dino-saurs and many other organisms. For biostratigraphers, the most important group to go extinct were the ammonites. Throughout the Mesozoic, these pelagic molluscs evolved rapidly, dispersed widely and left their shells in marine sediments across the world. They provided geologists with an accessible tool for correlating Mesozoic sediments, but there is no equivalent for the Cenozoic. So, whereas Mesozoic stratigraphy was mostly worked out in the 19th century, Cenozoic stratigraphy was a mire of uncertainty until relatively recently.

The key to escaping the morass was provided by planktonic foraminifera, single-celled protozoa that, like ammonites, produce calcareous shells but are much smaller and less obviously variable. They were largely overlooked until the 1950s, when the demand from oil exploration for accurate correlation of strata between wells led to intensive study of them and realisation that, like ammonites, they evolved quickly and were dispersed widely. This led rapidly to the development of standard zonal sequences that provided the key to reliably subdividing Cenozoic time and correlating sediments around the world.

Since then, other microfossil groups have been documented, their various zonations have been cross-correlated and all this has integrated with timescales from geomagnetic reversals, sea-level change and orbitally driven climate change. The result is a comprehensive framework for dating sediments at ever higher precision and, increasingly, a history of global environmental change. It is the integration of the framework of time and the history of global change that most interests McGowran and where he is most interesting in terms of developing ideas. In particular, he emphasises the need to integrate more lines of data into our emerging picture of long-term global change.

Interwoven with this theme of the integrative role of biostratigraphy in earth science is discussion of biostratigraphy as an evolutionary science.

In this area, McGowran produces interesting reviews of the way in which evolutionary pattern has affected biostratigraphic methodology in a range of different groups. He also discusses the degree to which these detailed studies and new molecular genetic evidence for cryptic speciation support the alternative models of phyletic gradualism and punctuated stasis. If he brings fewer new insights to this area of the subject, his book will nonetheless be a useful source for evolutionary biologists with the humility to complement theorising with consideration of the best available records of how evolution actually works.

Cambridge University Press perhaps optimistically describes this as a textbook. It is not, nor is it a popular science account of the subject or a reference guide to the field. Rather, it is a practitioner's exploration of the historical and philosophical development of the subject, its role in the earth sciences and the emerging agenda for future research in the field. It is not an easy book, but it may prove to be an influential one.

It develops a series of ideas and does so clearly and based on authoritative knowledge of the field. There is an immense amount of content here to fuel research, illuminate courses on historical geology or provide the basis for several more approachable books. It is not a book for the interested layman, but anyone teaching or researching in the field should buy a copy.

Jeremy R. Young is head of micropalaeontology, the Natural History Museum, London.

Biostratigraphy: Microfossils and Geological Time

Author - Bruce McGowran
Publisher - Cambridge University Press
Pages - 424
Price - £50.00
ISBN - 0 521 83750 2

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