recent podocopid ostracods, a substrate with oxygen levels lower than those in the water column would have been the most suitable habitat for them to colonise. Bradoriids were excluded from such environments by their large size and life attitude with the two valves widely open - poorly consistent with digging habits, and by their absence of musculature to control shield closure. Phospha- tocopids were excluded by elevated levels of oxygen that were hostile to a physiology adapted for very low-oxygen seabed conditions. The arthropod micro-benthos remained impoverished through- out the post-SPICE interval (Figure 4) even when widespread marine shelf anoxia retreated. This poor diversity may have been compounded by the elevated PO2 levels of the late Cambrian that may have restricted the environmental distribution of ostracods, despite their ability to retreat from normoxic waters. 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