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forms show tentacles, which suggest prey capture, and there are other indications that feeding was by ingestion (Fedonkin, 1994). Other frond-like forms resemble sea pens; Conway Morris (1993) has studied similar forms from the Burgess Shale, wherein preservation is better, and has concluded that both Vendian and Burgess Shale forms are cnidarians. It seems likely on present evidence that most of the late Precambrian forms do have cnidarian affinities. Even so, the late Precambrian body fossils do not represent direct ancestors of any of the higher metazoans and do not help to resolve the puzzle of the origin of the remaining phyla. A few other body fossils may represent bilaterians (see below), but such an assignment is disputed also.
The other mode of preservation of late Precambrian forms is as trace fossils—markings made by animal activities (Glaessner, 1969; Fedonkin, 1985b; Crimes, 1989). Some traces are of winding, rather featureless, trails, but others display transverse rugae and contain pellets that can be interpreted as of fecal origin. The bilaterian nature of these traces is not in dispute. Furthermore, such traces must have been made by worms, some of which had lengths measured in centimeters, with through guts, which were capable of displacing sediment during some form of peristaltic locomotion, implying a system of body wall muscles antagonized by a hydrostatic skeleton. Such worms are more complex than flatworms, which cannot create such trails and do not leave fecal strings (Fedonkin and Runnegar, 1992; Fedonkin, 1994). It is among the trace makers that the ancestors of the Cambrian clades are most likely to be represented.
Early Cambrian Faunas. During the Manykaian Stage mineralized skeletons begin to appear, and during the Tommotian and Atdabanian they increase spectacularly in numbers and diversity (Bengtson and Conway Morris, 1992). Many of these fossils are dissociated sclerites that give few clues as to the nature of their scleritomes or of the animals that bore them. In a few cases, however, sclerites have been found in life associations or preserved with soft-bodied remains to provide indications of a body plan (Conway Morris and Peel, 1990; Hinz et al., 1990; Ramsköld and Hou, 1991). Of living phyla, skeletons of brachiopods, mollusks, arthropods (trilobites), and echinoderms appear in the Tommotian and Atdabanian Stages, and nearly all durably skeletonized phyla are known by the end of the Early Cambrian. The exceptions are chordates (Middle Cambrian) and bryozoans (Early Ordovician); however, the body plans of neither of those phyla require a mineralized skeleton and both may have been present well before they appear as fossils. Body fossils of some phyla or subphyla lacking mineralized skeletons (priapulans, onychophoran relatives, etc.) are known from