– at generic level they are strongly provincial [57], they may have been unable to escape anoxic seabed waters. In such a scenario the low PO2 gradient between the external seawater medium and the bradoriid tissues would have rendered respiratory organs ineffi- cient, especially also because they lack ventilatory plates to efficiently ventilate their gills. Nevertheless some bradoriid taxa that originated during Stage 3, such as Beyrichona and Liangshanella survived these extinctions (Figure 4) and became long-ranging taxa. In the case of the svealutid Liangshanella its survival may have been facilitated by species of this genus adopting an active swimming lifestyle that enabled them to escape anoxic dead zones. Species of the epibenthic Beyrichona were amongst the most widely dispersed of Cambrian bradoriids occurring in both the Redlichiid and Olenellid trilobite realms and from low to high latitudes [57], a range that would have facilitated survival (in oxygenated refugia) during periods of widespread ocean anoxia. Extinctions and originations of bradoriids continued through Cambrian stages 4 and 5, but bradoriid diversity appears to have been significantly curtailed for a second time at the Stage 5- Drumian boundary (Figure 4), an interval associated with marine transgression [63] and black shale deposition [47,63]. Thus, in the black shales of the Menevian Group of south Wales [47] benthic bradoriids are absent, excluded by dysoxic seabed conditions, though pelagic svealutids are present [43]. In the Scandinavian succession bradoriid-bearing strata referable to the B. oelandicus trilobite biozone of Stage 5 are succeeded by black shales of the P.gibbus biozone just below the top of Stage 5. Further bradoriid extinctions occurred at the Drumian-Guzhangian boundary, including the loss of several long-ranging taxa such as Comptaluta and Phasoia (Figure 4). Thus, by the onset of the Pabian Stage SPICE event, which was also associated with widespread shelf sea anoxia [3,49,64], virtually all of the large, earlier Cambrian bradoriids were extinct. The single group of large bradoriids which remain widespread in Pabian age rocks, the pelagic svealutids (Figure 9C–E), possessed well-developed cardio-vascular systems that supported an active free-swimming lifestyle and lived in well- oxygenated waters above areas affected by anoxia. Benthic bradoriids remained rare in the late Cambrian, as did all small bivalved arthropod micro-benthos [57]. This may reflect: 1) the initial widespread seabed anoxia during the SPICE event [49]; 2) more oxygenated surface marine waters following this interval; and [3] the physiological inability of benthic bradoriids to avoid seawater O2 levels well above those in which the early Cambrian bradoriid fauna had initially evolved. Bradoriids were too large to occupy the interstices between sediment grains (as ostracods do to escape normoxic conditions) and they also lacked the ability to tightly close their shields, thus negating their ability to control oxygen levels in the waters immediately adjacent to their respiratory organs. Phosphatocopids and dysoxic seabeds Even in those successions most intensively studied for bivalved arthropod micro-benthos, the occurrence of phosphatocopids is sporadic [43,46,66]. However at certain horizons, particularly in late Guzhangian and early Furongian black shales [44], and organic-rich limestones [48,63] phosphatocopids became an abundant component of the benthos. These ‘phosphatocopine facies’ lie adjacent environments characterised by pyrite-rich shales, chemotrophic olenids or pelagic agnostids [65], in sedimentary deposits formed in low-oxygen or anoxic seabed conditions [48,63,65] that excluded benthic bradoriids with cardiovascular systems adapted for oxygenated waters (see above). As phosphatocopid arthropods lacked a cardiovascular system [13], they must have possessed a metabolism that favoured dysoxic Oxygen and Early Arthropod Micro-Benthos Evolution benthic conditions and their diversity and abundance peaked during the widespread shelf anoxia of the SPICE event [49]. Their lower abundance in post Olenus Biozone strata [43], and disappearance from the fossil record in the late Cambrian (peltura Biozone, ca 490 Ma) may be linked to the elevated oxygen-levels of late Cambrian shelf seas that kept deep water anoxia at bay [3]. The ostracod micro-benthos Post-DICE event (Figure 4) shelf-marine benthic environments that were not oxygen-poor contained an impoverished small arthropod micro-benthos. This micro-benthos became even poorer during and after SPICE (Figure 4). In contrast, the peak diversity of phosphatocopids in benthic environments occurred during SPICE (Figure 4), with the spread of anoxic waters onto the shelf. In the interval following DICE, originations in the arthropod micro- benthos are characterised by tiny (millimetre-scale) sized animals (Figure 10), much smaller than the bradoriids that originated during ‘CARE’. These small animals include Altajanella and Vojbokalina that possess shields which appear to conform to ostracod ‘body plan 2’ of Vannier and Abe [28], with a small elliptical shape and apparently no sophisticated respiratory and circulatory systems: at least the latter are not reflected in the morphology of the shield (Figure 10). These small bivalved arthropods of the post-DICE Cambrian world were confronted with four major environmental changes: 1) the spread of marine anoxia onto the shelf during SPICE; 2) more oxygenated shallow marine waters post-SPICE [3]; 3) open ecospace to colonise (the absence of Bradoriida in epibenthic niches); and 4) substrate change as microbial mats retreated [67]. While phosphatocopid arthropods disappear from the Cambrian record after the peltura trilobite Biozone (Stage 10; see Figure 4) and may have been unable to adapt to rising seawater PO2, even by retreating into sediment, organisms with a range of more ‘oxygen- tolerant’ life strategies and anatomies (e.g. greater resistance to O2 diffusion; more active O2 consumption in tissues; retreat to zones of lower water oxygenation; ability to regulate the O2 content of waters adjacent respiratory tissues) may have been selectively favoured. Ostracods tolerate normoxic sea conditions and even during intervals of elevated oxygen can retreat to the interstices of sediment where hypoxic conditions prevail [30]. The earliest ostracods may have originated in very shallow marine waters that remained oxygenated even during the maximum spread of seabed anoxia at the beginning of SPICE. Certainly the earliest Tremadocian ostracods from rocks 15 million years later than SPICE are found dominantly in shallow shelf marine environments that appear to have been well-oxygenated [20,24]. Vojbokalina, Septadella and Altajanella are therefore plausible early ostracod micro-benthos. In tandem with changes in marine oxygenation, there were also significant changes in marine substrate evolution that may have benefited a burrowing (ostracod) micro-benthos. During the Cambrian, seabed sediments became increasingly bioturbated, for example by worms [68], which redistributed organic matter through the sediment and thus provided new food sources and new habitats (micro-environments). These conditions facilitated the colonization of sediment by a range of meiofauna [69], though these did not, initially, include ostracods. In tandem with the increasing availability of oxygen, and greater seabed bioturbation, the distribution of microbial mats was curtailed in the late Cambrian [67]. The ‘‘substrate revolution’’ [70] may have enabled more easy penetration of seabed environments by small organisms inhabiting the flocculent layer, and facilitated feedbacks that favoured the evolution of small size. Certainly ostracods with their small size and well developed musculature for closing the protective shield were amongst the first successful colonizers of these niches. In addition, if their physiology was similar to that of PLoS ONE | www.plosone.org 11 December 2011 | Volume 6 | Issue 12 | e28183