that includes morphological and Ubiquitin sequence data. A number of fossil types that can be interpreted as lobopods are known from the Early (possibly Tommotian) and Middle Cambrian (Ramsköld and Hou, 1991; Bengtson et al., 1986; Budd, 1993). Living lobopods have hemocoels but flexible body walls (and can therefore squeeze through narrow openings). These data leave open the question of the body plans associated with the branch points. Perhaps the most likely possibility is that the onychophoran and arthropodan ancestors branched from a hemocoelic, seriated form before jointed, sclerotized exoskeletons appeared.
Among the Vendian body fossils of uncertain afinity are a few that have sometimes been interpreted as segmented bilaterians, including sprigginids and vendomiids (Glaessner, 1984; Fedonkin, 1985a). These forms are serially constructed and, if bilaterians, have cephalic shields but lack appendages; they have also been reconstructed as frond-like forms (Seilacher, 1989; Bergström, 1989). In some of them at least, the putative jointing alternates across the midline, so that "segments" are laterally offset rather than continuous; however, offsetting of serial structures is known in many living bilaterians (e.g., muscle blocks in cephalochordates) (Jeffries, 1986). If these fossils are bilaterians, then they may belong to the grade of segmented, hemocoelic organisms postulated to have given rise to arthropods (Valentine, 1989, 1992).
Ancestral annelids may also have branched from proto-molluscan grade ancestors, but the formation of the annelid body plan involved the origin of the famous compartmented coelom, widely interpreted as a hydrostatic skeleton to aid in peristaltic burrowing (Clark, 1964) or horizontal plowing. Undoubted annelidan body fossils have not been found in rocks older than Middle Cambrian; the earliest penetrating burrows may have been made by so-called pseudocoelomates such as priapulids or paleoscolecids, both of which are known in the Lower Cambrian. Long tubes presumably constructed by vermiform organisms, the sabelliditids, occur in the Manykaian and are sometimes considered to be pogonophorans, but no body fossils of these forms are known. Like arthropods, annelids possess larval intramesodermal spaces. In various arthropod taxa the spaces are occluded, or become organ coeloms, or are incorporated into the blood-vascular system during development; in annelids they are lost during metamorphosis, although the adult coelom develops (by schizocoely) within tissues that are derived from larval mesoderm (Anderson, 1973). If the larval coeloms in these phyla are homologous they are presumably plesiomorphic. In the larvae of some phyla, an intramesodermal space serves as a nephridium (Ruppert and Balser, 1986), and possibly this was the original function in this case as well. The segmentation in annelids is not