Oxygen, ecology, and the Cambrian radiation of animals
- Departments of aEarth and Planetary Sciences and
- dOrganismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138;
- bCenter for Marine Biodiversity and Conservation and Integrative Oceanography Division, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093; and
- cMarine Biological Laboratory, Department of Zoology, Andhra University, Waltair, Visakhapatnam 530003, India
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Contributed by Andrew H. Knoll, July 5, 2013 (sent for review May 8, 2013)

Abstract
The Proterozoic-Cambrian transition records the appearance of essentially all animal body plans (phyla), yet to date no single hypothesis adequately explains both the timing of the event and the evident increase in diversity and disparity. Ecological triggers focused on escalatory predator–prey “arms races” can explain the evolutionary pattern but not its timing, whereas environmental triggers, particularly ocean/atmosphere oxygenation, do the reverse. Using modern oxygen minimum zones as an analog for Proterozoic oceans, we explore the effect of low oxygen levels on the feeding ecology of polychaetes, the dominant macrofaunal animals in deep-sea sediments. Here we show that low oxygen is clearly linked to low proportions of carnivores in a community and low diversity of carnivorous taxa, whereas higher oxygen levels support more complex food webs. The recognition of a physiological control on carnivory therefore links environmental triggers and ecological drivers, providing an integrated explanation for both the pattern and timing of Cambrian animal radiation.
Footnotes
- ↵1To whom correspondence may be addressed. E-mail: sperling{at}fas.harvard.edu or aknoll{at}oeb.harvard.edu.
Author contributions: E.A.S., L.A.L., and A.H.K. designed research; E.A.S. and C.A.F. performed research; C.A.F. contributed new reagents/analytic tools; E.A.S., C.A.F., and P.R.G. analyzed data; and E.A.S., C.A.F., A.V.R., P.R.G., L.A.L., and A.H.K. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1312778110/-/DCSupplemental.
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