This monograph is part of a series of publications by the two authors and their collaborators (Williams et al., 1994; Hou et al., 1996; Melnikova et al. 1997; Williams and Siveter in press) which aim to make a comprehensive inventory of an important component of the Palaeozoic arthropod fauna: the bradoriids and the phosphatocopids. These two groups of small bivalved arthropods are known to occur worldwide from the early Cambrian through to the early Ordovician. From the time of their discovery in the late nineteenth century these fossils were referred to the ostracod crustaceans until their soft parts were discovered in the Upper Cambrian of Sweden (phosphatocopids of the Orsten faunal assemblage; Müller 1979, 1982), the Middle Cambrian of Australia (Walossek et al. 1993), the Lower Cambrian of Great Britain (a phosphatocopide baby, Hinz 1983), and the early Cambrian of China (Chengjiang fauna; Hou et al. 1996 for the bradoriid Kunmingella). These recent discoveries revealed that bradoriids and phosphatocopids had body plans fundamentally different from those of Recent and fossil ostracodes, most of them (e.g. the phosphatocopids) being thought to represent advanced stem group Crustacea (Walossek & Müller, 1992).
The debate concerning the affinity of phosphatocopids and bradoriids, however, is not the focus of Siveter and William’s paper. They 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 restudied an impressive amount of material housed in American and European institutions and have also made extensive new collections of the faunas originally described by Matthew (e.g., 1886) in New Brunswick and Nova Scotia more than a hundred years ago. Palaeogeographically, these faunas come from two broad areas: Avalonia (New Brunswick, Nova Scotia, East Newfoundland) and Laurentia (West Newfoundland, Vermont, New York State, Pennsylvania, Tenessee, Oklahoma, Texas, Mexico, Wyoming, Utah, Arizona, British Columbia, Alberta and North Greenland). Stratigraphically, they cover most of the Cambrian Series. The systematic study is accurate, and re-assesses the former diagnostic features (outline, ornament) of the two groups. It also provides clear definitions of key-taxa, such as Bradoria, a typical «bag genus» which formerly contained more than 50 different species and sub-species. The resulting total fauna comprises 16 genera and 26 species and is much less diverse at the specific level than indicated in the previous records. A new bradoriid species Liangshanella burgessensis sp. nov. is described from the Burgess Shale adding to the list of the bivalved arthropods with no preserved soft tissues present in this famous Middle Cambrian lagerstätte (e.g. Isoxys, Tuzoia, Carnarvonia; see Briggs et al., 1994).
Although the descriptive part of the monograph is very informative and beautifully illustrated by nine photographic plates (with numerous stereo-pairs of key specimens), we would have liked to find more data concerning the architecture (dorsal fold or hingement, body attachment), the functional morphology, and the ultrastructure of the head shield. The authors seem to be convinced that head-shield composition is no longer a reliable criterion for distinguishing bradoriids from phosphatocopids, but do not tackle the problem of the origin of phosphate (primary or secondary phosphatization). No attempt is made to reconstruct the animals (see for example, Hinz, 1993, or Vannier et al. 1997 for bradoriids) or the exoskeletons in three dimensions. In a chapter entitled «Palaeogeographical setting and Palaeoecology», the authors come to the interesting conclusion that Cambrian bradoriid and phosphatocopid faunas probably occupied both shallow and deeper shelf marine settings like typical post-Cambrian ostracodes. The idea of a possible ecological replacement of bradoriids/phosphatocopids by ostracodes would have deserved more comments. Inferred lifestyles (benthic, nekto-benthic, pelagic) of bradoriids and phosphatocopids lack additional lines of evidence (e.g. from Recent crustaceans; Middle Cambrian phosphatocopines from Australia lack exopods on their legs, clear evidence for a benthic crawling life style; Walossek et al. 1993).
Because of the short vertical range and local geographical occurrences of the species, the biostratigraphical and correlative potential of the fauna appears to be limited. Notable exceptions are a few taxa (e.g. Cyclotron lapworthi) found at similar horizons in the Canadian Atlantic provinces, southern Britain and Scandinavia. A short paragraph deals with zoogeography but is not accompanied by a map. The faunal distribution appear to be consistent with the current models of the Cambrian palaeogeography in which Avalonia and Laurentia were separated by the Iapetus Ocean. Numerous provincial taxa are recognized, for example Beyrichona, Cyclotron, Falites, Hipponicharion, Indiana, Wimanicharion in the temperate latitude Avalonia and Baltica and Dielymella and Walcottella from the low latitude tropical Laurentia. Cosmopolitan genera occur, such as Indota and Liangshanella found also, for example, in Australia and China, respectively. Anabarochilina occurs worldwide in the upper Middle and lower Upper Cambrian. Interestingly, the authors note that all these taxa occupy relatively deeper marine shelf settings. Interpretations and comparisons with other invertebrate groups which display similar patterns in the lower Palaeozoic (see Cocks and Fortey, 1988) would perhaps, have enhanced the interest of the information to the non-specialist.
All in all, this monograph contains a wealth of useful and reliable data concerning the morphology and distribution of North American bradoriids and phosphatocopids and the palaeontology of the two groups in general. This work together with other monographs co-authored by D. Siveter and M. Williams (Williams & Siveter, in press; European fauna) and in preparation (Chinese fauna with X. Hou) should make a major contribution to the knowledge of Cambrian biodiversity.
Jean Vannier, Université Claude Bernard Lyon 1, UMR 5565 du CNRS, 43, bd du 11 novembre 1918, 69622 Villeurbanne, France, jean.vannier@univ-lyon1.fr
Dieter Walossek, Section for Biosystematic Documentation, University of Ulm, Liststrasse 3, D-89079 Ulm, Germany, dieter.walossek@biologie.uni-ulm.de
References
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Hinz, I. 1993: Evolutionary trends in archaeocopid ostracods. In McKenzie, K.G. and Jones, P.J. (eds.): Ostracoda in the Earth and Life Sciences, 3-12. Balkema, Rotterdam.
Hou, Xianguang, Siveter, D. J., Williams, M., Walossek, D. & Bergström, J. 1996: Appendages of the arthropod Kunmingella from the early Cambrian of China: its bearing on the systematic position of the Bradoriida and the fossil record of the Ostracoda. Philosophical Transactions of the Royal Society, Series B, 351, 1131-1145.
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Melnikova, L. M., Siveter, D. J. & Williams, M. 1997: Cambrian Bradoriida and Phosphatocopida (Arthropoda) of the former Soviet Union. Journal of Micropalaeontology 16, 179-191.
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Williams, M., Rushton, A. W. A. & Berg-Madsen, V. 1994: The Upper Cambrian bradoriid ostracod Cyclotron lapworthi is a hesslandonid. Transactions of the Royal Society of Edinburgh: Earth Sciences, 85, 123-130.
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