Bivalved arthropods from the Lower Cambrian Mernmerna Formation, Arrowie Basin, South Australia and their implications for identification of Cambrian 'small shelly fossils'

A WIDE RANGE OF ARTHROPODS with bivalved head-shields are known from the Cambrian. Many of these arthropods did not have mineralised exoskeletons, and consequently are known mainly from celebrated Cambrian Lagerstatten such as the Burgess Shale of British Columbia and the Maotianshan Shale of Chengjiang, China (Briggs 1983; Hou & Bergstrom 1997). The vast majority of Cambrian arthropods with bivalved head-shields belong to the orders Bradoriida Raymond, 1935 and Phosphatocopida Muller, 1964. The shields of both these groups were often mineralised by calcium phosphate, facilitating their preservation as fossils in different environments (Siveter & Williams 1997). The shields of bradoriids and phosphatocopids are superficially similar to ostracods, to which they have often been referred, but modern studies show only a distant relatationship to extant and fossil Ostracoda (Hou et al. 1996; Shu et al. 1999; Siveter et al. 2003; Maas et al. 2003). The first bradoriids appear slightly below the oldest trilobites in China (Hou et al. 2002) and were thus among the first arthropods to leave identifiable remains in the fossil record. Both bradoriids and phosphatocopids continued to form important components in many Cambrian faunas, and persisted into the early and possibly middle Ordovician (Copeland 1974; Shu 1990; Siveter & Williams 1997; Williams & Siveter 1998). They may be very common locally, such as at the Chengjiang Lagerstatte, where bradoriids are the most abundant of all fossils (Hou et al. 1996). Ecologically, bradoriids may have been among the most common microscavengers/recyclers of nutrients in Cambrian benthic communities (Shu et al. 1999) and appear to have been an important food source for larger animals (Vannier & Chen 2005). In Australia, bradoriids and phosphatocopids from the uppermost Lower and Middle Cambrian (Ordian and Templetonian) of Queensland have been well studied (e.g. Opik 1961, 1968; Fleming 1973; Jones & McKenzie 1980; Hinz-Schallreuter 1993a, 1999; and references therein), but faunas from pre-Ordian rocks, or from other parts of Australia are less well known. Chapman (1918) was the first to describe bradoriids from the Lower Cambrian of South Australia, and limited faunas have since been described by Bengtson (in Bengtson et al. 1990) and Melnikova (in Gravestock et al. 2001). SKOVSTED, C.B., BROCK, G.A. & PATERSON, J.R. 2006:07:29. Bivalved arthropods from the Lower Cambrian Mernmerna Formation, Arrowie Basin, South Australia and their implications for identification of Cambrian ‘small shelly fossils’. Memoirs of the Association of Australasian Palaeontologists 32, 7-41. ISSN 0810-8889.