Cambrian Explosion
"Stratigraphic sections spanning the Vendian-Cambrian boundary show a broadly similar pattern whereby the key events are bracketed by the 600-million-year (Myr)-old Neoproterozoic glacial deposits (tillites) and in the succeeding Cambrian diverse metazoan assemblages, typified by abundant skeletons, diverse trace fossils, and Burgess Shale-type faunas (Fig. 1). One key development is a series of accurate radiometric determinations (1). The Vendian-Cambrian boundary is now placed at 543 Myr, and the duration (45 Myr) of the Cambrian is substantially shorter than once thought. The preceding Ediacaran faunas have an approximate age range of 565-545 Myr. Accordingly, the overall time-scale for discussion is a relatively protracted 65 Myr, although the principal events of evolutionary interest are probably more tightly bracketed (550-530 Myr) between the diverse Ediacaran faunas of latest Neoproterozoic age (2) and the Chengjiang Burgess Shale-type faunas (3). Correlations are also assisted by emerging schemes of chemostratigraphy (2, 4), notably with reference to strontium (87Sr) and carbon (13C)." Simon Conway Morris The Cambrian "explosion": Slow-fuse or megatonnage? PNAS April 25, 2000 vol. 97 no. 9 4426-4429
Science & Technology at Scientific American.com: The Woodstock of Evolution -- The World Summit on Evolution, held in the Galapagos Islands, revealed a science rich in history and tradition, data and theory, as well as controversy and debate: "Stephen Jay Gould's Wonderful Life stimulated a lot of new ideas about the Cambrian explosion of life, he continued, and it soon became clear that there were a huge variety of organisms difficult to classify, such as those in the Burgess Shale. But there are a number of Cambrian fossil beds, such as in China, where important phyla such as Chordata evolved. 'But what does all this diversity mean?' Fortey asked. 'There are today 30 living phyla. In the Cambrian, some claim that there were as many as 100 phyla, but the evidence does not support this. We now believe that morphological diversity did not explode as much as Gould originally suggested, although the explosion in evolutionary experimentation was real. By the time we get to the Cambrian, like at the Burgess Shale, the systems are very complex, such as trilobite eyes. Evolution was experimenting with many wondrous varieties, such as all the armor on the heads of trilobites.' "
Science & Technology at Scientific American.com: The Woodstock of Evolution -- The World Summit on Evolution, held in the Galapagos Islands, revealed a science rich in history and tradition, data and theory, as well as controversy and debate: "Stephen Jay Gould's Wonderful Life stimulated a lot of new ideas about the Cambrian explosion of life, he continued, and it soon became clear that there were a huge variety of organisms difficult to classify, such as those in the Burgess Shale. But there are a number of Cambrian fossil beds, such as in China, where important phyla such as Chordata evolved. 'But what does all this diversity mean?' Fortey asked. 'There are today 30 living phyla. In the Cambrian, some claim that there were as many as 100 phyla, but the evidence does not support this. We now believe that morphological diversity did not explode as much as Gould originally suggested, although the explosion in evolutionary experimentation was real. By the time we get to the Cambrian, like at the Burgess Shale, the systems are very complex, such as trilobite eyes. Evolution was experimenting with many wondrous varieties, such as all the armor on the heads of trilobites.' "