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Unformatted text preview: Explaining the Cambrian Explosion of Animals Charles R. Marshall Department of Earth and Planetary Sciences, Department of Organismic and Evolutionary Biology, and Department of Invertebrate Paleontology, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138; email: firstname.lastname@example.org Annu. Rev. Earth Planet. Sci. 2006. 34:35584 First published online as a Review in Advance on January 16, 2006 The Annual Review of Earth and Planetary Science is online at earth.annualreviews.org doi: 10.1146/ annurev.earth.33.031504.103001 Copyright c 2006 by Annual Reviews. All rights reserved 0084-6597/06/0530- 0355$20.00 Key Words bilaterian phyla, evolution, paleontology, development, ecology Abstract The Cambrian explosion is a unique episode in Earth history, when essentially all the animal phyla first appear in the fossil record. A variety of environmental, de- velopmental (genetic), and ecological explanations for this complex and somewhat protracted event are reviewed, with a focus on how well each explains the observed increases in disparity and diversity, the time of onset of the radiation, its duration, and its uniqueness. The increase in disparity (the origin of the phyla) and diversity are best understood as being the result of the interplay of the combinatorial bilaterian develop- mental system and the increase in the number of needs the first bilaterians had to meet as complex ecological interactions developed. The time of onset is constrained by the evolution of the environment, whereas its duration appears to be controlled pri- marily by rates of developmental innovation. The uniqueness of the event is either due to ensuing developmental limitation, to ecological saturation, or simply to the exhaustion of ecologically viable morphologies that could be produced by the nascent bilaterian developmental system. 355 A n n u . R e v . E a r t h P l a n e t . S c i . 2 6 . 3 4 : 3 5 5- 3 8 4 . D o w n l o a d e d f r o m a r j o u r n a l s . a n n u a l r e v i e w s . o r g b y U n i v e r s i t y o f T e x a s- A u s t i n o n 2 / 2 / 8 . F o r p e r s o n a l u s e o n l y . INTRODUCTION The Cambrian explosion, or radiation, is perhaps the most significant evolutionary transition seen in the fossil record. Essentially all of the readily fossilizable animal body plans first appear in the fossil record during this interval (Valentine 2002). We move from the depths of the Precambrian world, where the sedimentary record is essentially devoid of animal fossils, to the Phanerozoic, where animal life leaves per- vasive evidence of its existence, both as body fossils and as disturbers of the sediment. Numerous explanations for the Cambrian explosion have been posited (note here that I am not considering in any detail explanations for the precursor to the Cambrian explosion, the Ediacaran radiation). Classification of this rich panoply of explanations is somewhat arbitrary but typically explanations center on one of the fol-...
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This note was uploaded on 03/06/2010 for the course BIOL 50755 taught by Professor Allgeier during the Spring '10 term at University of Texas-Tyler.
- Spring '10