rocks of probable Atdabanian age from the Baltic Shield (Dzik and Lendzion, 1988 and refs. therein), China (Hou et al., 1991), and Greenland (Conway Morris et al., 1987), together with a variety of animals that cannot be assigned with any confidence to living phyla (Dinomischus, paleoscolicids, halkieriids, etc.).
Trace fossils in the Early Cambrian are greater in abundance and diversity than in the late Precambrian (Crimes, 1992). Penetrating vertical burrows, exceedingly rare and small earlier, become larger, longer, and more common, and bioturbation increases in depth and intensity in increasingly younger sediments (Crimes and Droser, 1992; Droser and Bottjer, 1993). The increased biological activity indicated by traces is consistent with that indicated by the explosion of body fossil types.
Middle Cambrian Faunas. The rate of appearance of novel body plans slows greatly during the Middle Cambrian with the exception of the fauna of the Burgess Shale and its correlatives, which create a diversity "spike" (Whittington, 1985; Conway Morris, 1992). The Burgess Shale fauna is exceptionally diverse and brings to light many of the less easily preserved members of the Cambrian fauna. The Chinese Atdabanian fauna is also exceptionally preserved and contains numbers of taxa in common with the Burgess Shale, suggesting that many of the Burgess Shale forms, or at least the higher taxa to which they belong, would be found to have originated in the Early Cambrian if that fauna were better known.
Summary. It is consistent with the fossil record that all of the body plans now ranked at the phylum level originated by the close of the Early Cambrian, although some that are not easily fossilized do not appear until later, and indeed some (platyhelminths, gnathostomulids, gastrotrichs, acanthocephalans, loriciferans, and kinorhynchs) are unknown as body fossils. As the very first bearers of any given body plan are unlikely to be found, any corrections for a smearing out of first appearances towards the Recent may add to the abruptness of the Cambrian explosion. The first appearances of fossil groups whose relationships are problematic but that have very distinctive skeletal or body plans and that may be phyla or subphyla are also concentrated during the Early Cambrian. The Cambrian explosion was geologically abrupt and taxonomically broad.
Metazoans evidently originated from unicellular (perhaps colonial) choanoflagellates or their allies (Wainright et al., 1993). By the time of the Cambrian explosion, some metazoan bodies were as complex as primi-