Figure 2 Geochronological time scale for the late Precambrian and Cambrian, with stratigraphic terms derived from the Russian geological column and with recorded metazoan diversity indicated by orders and classes (the earliest records are poorly constrained in time). Dates are from U-Pb zircon ages assigned to the stratigraphic levels indicated by filled circles or squares. [Reprinted with permission, from Bowring et al., 1993 (copyright American Association for the Advancement of Science); diversity data from Sepkoski, 1992.]

duration; over 50 metazoan orders first appear in the record during that interval (Valentine et al., 1991).

Vendian Faunas. The earliest fossils that may be metazoans are preserved in two modes. One is as body fossils, chiefly as impressions in this case. Most of these fossils somewhat resemble cnidarians but many cannot easily be assigned to living classes, and their affinities even as to kingdom are in dispute. Seilacher (1989) has suggested that most of them are not metazoans at all but represent a separate, extinct multicellular clade; they are suggested to have fed via symbionts as chemautotrophs (Seilacher, 1989), phototrophs, or osmotrophs (McMenamin, 1993). However, especially well-preserved specimens of "medusoid"

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