Figure 10. Early ostracods from the Cambrian and Ordovician. A, Kimsella luciae Salas, Vannier & Williams, lateral view of right valve. Tremadocian, lower part of the Parcha Formation, Abra de Sococha Section, Province of Salta, Argentina, CORD-MP 11186. B, Altajanella costulata Melnikova, lateral view of right valve. Late Cambrian, Tandoshka Formation, Gorny Altay, PIN N4346/1. C, Vojbokalina magnifica Melnikova, left lateral view of carapace. Middle Cambrian, Leningrad Region, Russia, PIN N4341/6. D, Nanopsis coquina Salas, Vannier & Williams, lateral view of left valve. Tremadocian, Upper Member of the Coquena Formation, Quebrada Chalala Section, Cordillera Oriental, Argentina, CORD-MP 11179. Scale bars are: 100 mm. For repositories of figured specimens see [20]. doi:10.1371/journal.pone.0028183.g010 Waldoria [43,44] in lithofacies which are devoid of benthic bradoriids. Similarly diverse phosphatocopid assemblages are known from the Alum Shales of Scandinavia, in the same stratigraphical interval (‘phophatocopine lithofacies’, see [63]), from a succession that also includes the earliest chemotrophic (olenid) trilobites that signal very low oxygen seabed conditions [65]. The Alum Shales environment appears to have varied between seabed anoxia and slightly more oxygenated conditions (dysoxic or hypoxic), with phosphatocopids occupying the latter environments. Phosphatocopid diversity declined during Stage 9 and they disappeared from the rock record a few million years after their peak diversity. Discussion In this section we examine the feedback mechanisms that may have driven changes in bradoriid and phosphatocopid diversity. We begin by assessing the level of Cambrian seawater oxygen. We then examine how changes in marine oxygen may have led to the demise of bradoriids and phosphatocopids, but favoured a third group of arthropod micro-benthos, the Ostracoda. Cambrian seawater oxygenation There is a consensus that oxygen levels were rising from the late Precambrian through into the Cambrian [2], from perhaps an initial value of between 10 and 15 kPa to values perhaps as high as 30 kPa by the late Cambrian [3]. Although surface ocean waters may have been more oxygenated, the deeper waters of Cambrian oceans probably remained anoxic [49]. Thus, overall increasing oxygen-levels in the oceans were punctuated by a series of ocean anoxic events, in part caused by intervals of marine transgression in which deep marine anoxic waters shoaled onto the shelf resulting in the accumulation of black shales [49,59]. Many of these events are equated with marked carbon isotope excursions Oxygen and Early Arthropod Micro-Benthos Evolution that are associated with marine extinction events [58]. Salzmann et al. [3] used a carbon and sulphur isotope mass balance model to show a dramatic increase in oxygen-levels through the interval from 502 to 496 Ma (through the Guzhangian – Pabian Stage boundary) that encapsulates the interval of the SPICE positive CIE (Figure 4). The SPICE event at the base of the Furongian Series likely represents an interval of increased organic carbon burial beneath euxinic ocean waters that extended onto the shelf. The positive carbon (d13Ccarb) and sulphur (d34SCAS – carbonate associated sulphur) isotope excursions associated with this event would have been associated with net O2 production [3]. Beginning with initial values of PO2 (taken from the Berner model), Salzmann et al. [3] calculated peak PO2 at about 30 kPa (Figure 4). Oxygen-levels may have remained high for several million years [3] and this build-up of oxygen would eventually have resulted in more oxygenated marine shelf waters. The build-up of atmospheric oxygen during the peak of the SPICE event would have produced a negative feedback mechanism on the spread of anoxic waters that had initiated the global mass extinction at the beginning of SPICE [3]. The Early Cambrian dominance of bradoriids Bradoriids originating during the ‘CARE’ mostly possessed large shields that exhibit clear evidence of hemolymph networks and a cardiovascular system [51] (Figure 9). These early Cambrian bradoriids appear to have adopted an active mode of life within (normoxic Cambrian-seawater) oxygenated marine shelf waters. The Chengjiang bradoriid Kunmingella douvillei [10,14] and the Burgess Shale Liangshanella (JV, unpublished) both possess biramous appendages. The endopods of these limbs had an obvious crawling function suggesting that the animals lived in contact with the seabed. The exopods are leaf-like and although no microstructures are preserved (e.g. hemolymph sinuses) the exopods may have functioned both for respiration and swimming. Bradoriids are likely to have been highly mobile arthropods the contrary of slow moving animals, moving rapidly at the water sediment-interface or just above it. There is abundant evidence of gills in early Cambrian arthropods (e.g. lamellae and filaments on every appendage; Figure 2) and of a network of branching sinuses adjacent to the inner side of the shield of many Cambrian arthropods (unpub- lished information of JV). Development of advanced cardiovascu- lar systems may have been the only way for relatively large animals to develop highly mobile lifestyles that required sufficient oxygen, particularly if early Cambrian shelf seas had relatively reduced oxygen-levels (10 kPa air-equilibrated water). A hemolymph circulation powered by a heart seems to have enabled more active behaviour in the water column among a variety of arthropods including the bradoriids. The ‘CARE’ radiation of bradoriids into a range of marine shelf environments during early Stage 3 (Atdabanian; Figures 3, 4) was curtailed during mid to late Stage 3 with the extinction of at least 25 genera (Figures 3, 4). These extinctions may correlate with widespread black shale deposition on several continents, com- mencing as early as the late Atdabanian in Australia and China (59), and associated with shelf anoxia and a positive carbon isotope excursion that has been christened the Sinsk event [59]. The early to mid Botomian interval (Figure 3) is characterised by a rapid decline in archaeocyathans, Tommotians and some trilobites. For organisms like bradoriids with an advanced cardiovascular system and an active mode of life the spread of deep-ocean anoxic waters onto the marine shelf would have been lethal. As the majority of earlier bradoriids are interpreted to have occupied epibenthic niches [66], and many such as Kunmingella, Auriculatella and Shangsiella possessed limited intercontinental dispersal capabilities PLoS ONE | www.plosone.org 10 December 2011 | Volume 6 | Issue 12 | e28183