CAMBRIAN BIVALVED ARTHROPODS

This monograph is part of a series of publications by the two authors and their collaborators (Williams et al. 1994; Hou et al. 1996; Melnik ova et ai. 1997; Williams & Siveter, in press) aiming toward a compreh ensive inventory of an important component of the Palaeozoic arthropod fauna: the bradoriids and the phosphatocopids. These two groups of sm all bivalved arthropods occur worldwide from the early Cambrian to the early Ordovician. From the time of their discovery in the late :19th century, these fossils were referred to the ostracod crustaceans until their soft parts were discovered in the Upper Cambrian of Sweden (pho sphatocopids of the orsten faunal assemblage; Muller 1979, 1982), the Middle Cambrian of Australia (Walossek et al. 1993), the Lower Cambria n of Great Britain (a phosphatocopid baby, Hint 1983), and the early C ambrian of China (Chengjiang fauna; Hou et al. 1996 for the,radoriid K unmingella). These recent discoveries revealed that bradoriids and pho sphatocopids had body plans fundamentally different from those of Rece nt and fossil ostracodes, most of them (e.g., the phosphatocopids) bei ng thought to represent advanced stem-group crustaceans (Walossek Sc M uller 1992). The debate concerning the affinity of phosphatocopids and bradoriids, however, is not the focus of Siveter & William's paper. T hey have produced a much needed classical descriptive treatment of the two groups based on the morphology of the head shields (traditionally termed 'carapaces') in North American faunas. The authors have restud ied an impressive amount of material housed in American and European i nstitutions and have also made extensive new collections of the faunas originally described by Matthew (e.g., 1885) in New Brunswick and Nov a Scotia more than a hundred years ago. Palaeogeographically, these fa unas come from two broad areas: Avalonia (New Brunswick, Nova Scotia,E ast Newfoundland) and Laurentia (West Newfoundland, Vermont, New York Stare, Pennsylvania, Tennessee, Oklahoma, Texas, Mexico, Wyoming, Utah , Arizona, British Columbia, Alberta and North Greenland). Stratigraph ically, they cover most of the Cambrian Series.