Vol. 334 no. 6059 pp. 1091-1097
RESEARCH ARTICLE
The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals
Douglas H. Erwin1,2,*, Marc Laflamme1, Sarah M. Tweedt1,3, Erik A. Sperling4, Davide Pisani5, Kevin J. Peterson6,*
ABSTRACT
Diverse bilaterian clades emerged apparently within a few million years during the early Cambrian, and various environmental, developmental, and ecological causes have been proposed to explain this abrupt appearance. A compilation of the patterns of fossil and molecular diversification, comparative developmental data, and information on ecological feeding strategies indicate that the major animal clades diverged many tens of millions of years before their first appearance in the fossil record, demonstrating a macroevolutionary lag between the establishment of their developmental toolkits during the Cryogenian [(850 to 635 million years ago (Ma)], and the later ecological success of metazoans during the Ediacaran (635 to 541 Ma) and Cambrian (541 to 488 Ma) periods. We argue that this diversification involved new forms of developmental regulation, as well as innovations in networks of ecological interaction within the context of permissive environmental circumstances.
Here's a chart of the origin and diversification of animals.
The origin and diversification of animals as inferred from the geologic and genetic fossil records. The dramatic rise in the number of animal fossils (see scale on left) in the Cambrian relative to the Ediacaran conveys the impact of the Cambrian explosion of animal life. Little high-level morphological innovation occurred during the subsequent 500 million years in that much of animal disparity, as measured by the Linnean taxonomic ranking, was achieved early in the radiation. Overlying the geologic record is the pattern of animal origination as inferred from the molecular clock. Seven different housekeeping genes from 118 taxa were used to generate this chronogram (see SOM 2 for methodological details and database S1). Twenty-four calibrations (open circles) were used and treated as soft bounds. Divergence times for key nodes and their 95% highest posterior intervals are reported in database S2. All estimates appear to be robust to numerous experimental manipulations performed to assess whether the results were dependent on the parameters used in the analyses (Materials and Methods, SOM Text 2, and figs. S5 to S10). There is general concordance of bilaterian phylum-level crown groups (colored circles; the color of each circle is the same as the corresponding taxonomic bar and label on the far right), with the first appearance of most animal groups at the Ediacaran-Cambrian boundary. In contrast, the origins of the demosponge (dark blue) and cnidarian (yellow) as well as the bilaterian (black) and metazoan (gray) crown groups are deep in the Cryogenian. Geological period abbreviations: Є, Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; Tr, Triassic; J, Jurassic; K, Cretaceous; Pe, Paleogene; N, Neogene. A high-resolution image is available in the SOM.