Oxygen and Early Arthropod Micro-Benthos Evolution Figure 8. General morphology of phosphatocopid arthropods. A, Hesslandona angustata from the late Cambrian of western Hunan, China, ventral view of complete specimen with phosphatised appendages (see [77]). B, Klausmuelleria salopiensis Siveter, Waloszek & Williams from the Protolenus-Strenuella Limestone, early Cambrian (Series 2), Shropshire England, reconstruction of appendages in ventral view (courtesy of Dieter Waloszek, Ulm). Reconstruction is based on a specimen 0.34 mm long [11]. C, Vestrogothia spinata M u¨ller, from the late Cambrian ‘‘Orsten’’ of Sweden, three-dimensional model, lateral and frontal views (courtesy of Joachim Haug). Scale bar: 100 mm. doi:10.1371/journal.pone.0028183.g008 ostracods – the group to which they were first assigned [45], but their detailed soft anatomy, revealed for several different groups [13], indicates that phosphatocopids lacked modification of the first post-mandibular limb to form a maxillula and they are therefore not ostracods [12]. Most phosphatocopid crustaceans were small enough to enable oxygen diffusion through their cuticle (body and inner wall of carapace) and indeed none of the phosphatocopids have appendages bearing gills or integumental (circulatory system) networks. Although phosphatocopids appear in Series 2 of the Cambrian (e.g. Dabashanella, [46]), their lithofacies distribution suggests they favoured areas of the seabed characterised by low-oxygen level. Phosphatocopids such as Cyclotron, Vestrogothia, Trapezilites and Falites dominated late Guzhangian and Furongian dysoxic benthic lithofacies, especially the olenid trilobite-bearing black shales of the English Midlands [43,44,47] and the organic-rich mudstones and limestone environments of the Alum Shales in Sweden [13,48]. The Alum Shales are pyrite-rich, thinly laminated and show little evidence of bioturbation. They probably formed at a dysoxic or anoxic seabed ([48] and references therein) during an interval characterised by shoaling of anoxic waters onto the marine-shelf [49]. Bradoriids These small arthropods possessed a dorsally folded bivalve shield of between 2 mm (e.g. in Neokunmingella) to 17 mm length (e.g. in Petrianna) for adults and this probably covered much of the body (Fig. 9). Long equated with the Cambrian record of ostracods [50], rare evidence from soft-anatomy shows that bradoriids lacked specialised post-antennal head appendages and are therefore not Eucrustacea [10,14]. Like phosphatocopids, bradoriids lacked adductor muscles and many shields are preserved in butterfly’ orientation with the valves gaping open. Bradoriid shields present striking evidence for the development of a complex cardiovascular system in this group [51], while the leaf-like exopodial branches attached to the appendages of some species such as Kunmingella suggest that bradoriids had additional gill-like features. Bradoriids were probably deposit feeders or scavengers of the numerous carcasses that were rotting on the seabed in epibenthic Cambrian ecosystems. Their basal position in Cambrian ecosystems is suggested by their occurrence in coprolite-like aggregates (e.g. Chengjiang, [52]) which suggests that they were predated by larger invertebrates. The facies distribution and shield morphology of bradoriids suggests that most of these animals adopted an active epibenthic lifestyle in well-oxygenated marine settings [10,53]. Indeed, bradoriids are the most abundant animals in the epibenthic communities of the Burgess Shale [54,55]. According to the initial hypothesis set out in the Introduction one must consequently suggest that they were active animals ‘burning’ O2 as fast as it diffused along the gradient of O2 partial pressure from ambient to cellular compartment. As a result of their relative environmental ranges, bradoriids and phosphatocopids tend to be mutually exclusive in the Cambrian rock-record, except where a few pelagic bradoriids [56,57] are preserved in black mudstones at the seabed after death. Temporal patterns of micro-benthos diversity The temporal distribution of the Cambrian bivalve arthropod micro-benthos is summarised through Figures 3 and 4 and is based on the assessment of faunas from all continents including Antarctica [57]. The dataset represents fossil materials collected over 140 years from hundreds of localities. We plot genera as a proxy for species diversity: this is a viable way to represent bradoriid diversity as genera typically contain only between 1 and 3 species [57]: exceptions to this are documented in the explanation to Figure 4. Early Cambrian phosphatocopid genera (e.g. Comleyopsis, Klausmuelleria and Dabashanella [12,43,46]) also contain only few species. Later Cambrian phosphatocopid genera are more species-rich, and Bidimorpha, Cyclotron, Falites, Vestrogothia and Hesslandona all contain greater than 3 species. Also shown on Figure 4 are the major Carbon Isotope Excursions (CIEs) of the Cambrian [58]. Some of these were associated with intervals of increased marine shelf anoxia and black shale deposition [49,59]. The peak diversification of Bradoriida occurred during the Cambrian Arthropod Radiation Event (‘CARE’ sensu [58]). During this interval species radiated into a wide range of high and low latitude shelf sea environments [57] including deep shelf [60] and marginal marine settings [53,61]. Following the ‘CARE’ event bradoriids underwent major extinctions, during late Stage 3 (Atdabanian-Botomian boundary, and early to mid Botomian) with the loss of at least 25 genera and the demise of the entire Kunmingellidae and most of the Cambriidae Family (Figure 4). Taxa that survived these extinctions include several long-ranging genera such as Liangshanella, Beyrichona and Anabarochilina. Further originations of essentially short-ranging bradoriid taxa occurred during late Stage 4, Stage 5 and the Drumian (Figure 4). Bradoriid diversity declined significantly from late Stage 5 to the Drumian boundary (e.g. the loss of long-ranging genera such as Hipponicharion), PLoS ONE | www.plosone.org 8 December 2011 | Volume 6 | Issue 12 | e28183