What might geology be telling us about the fossil record?

One of the themes of our evolution module is that the geosphere and biosphere co-evolved throughout Earth history. Evolution of life profoundly affected the geochemistry of the planet, and changing geological conditions in turn repeatedly stirred the pot of evolution. Ars Technica has a couple of nice stories on the gaps in the geological and fossil records.

The first deals with the question of how faithfully the fossil record tells the history of life. Evolutionary biologists from Darwin onwards have fretted about gaps in the fossil record, and creationists have seized on these gaps to state that lack of transitional forms falsifies evolutionary theory. Clearly, the fossil record depends on the deposition of sedimentary rocks and how well different types of organisms, particularly those with only soft body parts, become fossilized.

Scott Johnson’s article “The rock record got a bad rap. Fossil diversity accurately reflects history” discusses a Science paper by Hannisdal and Peters (2011), showing that both the rate of sedimentary rock formation and fossil diversity depend on environmental changes. The same factors such as sea level changes affect both global sedimentation rates and biological diversity. Therefore, the fossil record may be a more accurate indicator than previously thought of the history of life’s diversity.

An example of common-cause relationships among an environmental parameter (sea level), the rock and fossil records, and biodiversity. Arrows denote the direction of causality; “+” and “−” signs indicate measured or inferred positive and negative relationships. From J Crampton 2011 Science 334:1073-1074

The second article by Scott Johnson, “Missing rocks may explain why life started playing shell games” addresses the Cambrian explosion and the paucity of the fossil record preceding this period of great diversification of animal life. The geological record has a globally widespread gap, called the “Great Unconformity.”

Stratigraphy of the Grand Canyon, Wikimedia Commons, as shown in Scott Johnson’s article, “Missing rocks may explain why life started playing shell games.”

Johnson discusses a paper in Nature by Peters and Gaines (2012), that both the Great Unconformity and the Cambrian explosion could be explained by sea level rise.

Fig. 4 from Peters & Gaines 2012 The shift from widespread continental denudation to widespread sedimentation on the continents defines the Great Unconformity.

The sea level rise and increased weathering led to a buildup of ions and salts in the ocean, including calcium, that led to the formation of shells. Such hard protective coatings could then have driven an evolutionary arms race and rapid increases in body size, that are preserved in the fossil record as the Cambrian Explosion.

These two papers, both with Shanan Peters at the U. of Wisconsin, and well-explained by Scott Johnson, suggest that the fossil record and the geological record both record changes in global environment that shaped the evolutionary history of life. By reading the two together, we can have greater confidence that we’re not missing large chunks of this historical record.

http://arstechnica.com/science/news/2012/04/missing-rocks-may-explain-why-life-started-playing-shell-games/

http://arstechnica.com/science/news/2012/02/fossil-hips-dont-lie-rock-record-got-a-bad-rap.ars

Crampton, J. 2011 What drives biodiversity changes? Science 334:1073-1074 DOI: 10.1126/science.1214829

Hannisdal, B and SE Peters 2011 Phanerozoic Earth system evolution and marine biodiversity Science 334: 1121-1124 DOI: 10.1126/science.1210695

Peters SE and RR Gaines 2012 Formation of the “Great Unconformity” as a trigger for the Cambrian explosion Nature 484: 363–366 doi:10.1038/nature10969

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About jchoigt

I'm an Associate Professor in the School of Biology at Georgia Tech, and Faculty Coordinator of the Professional MS Bioinformatics degree program.
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