Figure 1 Histograms of first appearances of metazoan phyla, classes, and orders in the marine fossil record. Filled bars, durably skeletonized taxa; open bars, poorly or non-skeletonized taxa. Bar widths equal 10 million years (my). [Reprinted with permission, from Valentine, 1969 (copyright Society for the Study of Evolution).]

Ages of First Appearances. Figure 2 depicts the ages associated with the sequence of Stages and Series of the late Precambrian and Early Cambrian, with most terminology based on the Russian sequence (Bowring et al., 1993). The past couple of decades have witnessed intense work on the early faunas, and during most of that time the base of the Tommotian has been taken as the base of the Cambrian. However, within the last few years new criteria have been developed and now the lowest Cambrian boundary is commonly based on the earliest appearance of the trace fossil Phycodes pedum (see Narbonne et al., 1987). Choosing this boundary has lowered the base of the Cambrian, enlarging that Period by about one half (Figure 2). Despite this expansion of the Cambrian, new absolute age estimates have caused the length of time believed to be available for the Cambrian explosion to be shortened (Bowring et al., 1993). The relationship of the dates given in Figure 2 to the boundaries of the Stages of the Lower Cambrian remains a difficult stratigraphic problem, but it is likely that the most critical Stages, the Tommotian and Atdabanian, are probably only 8–10 my in