[8] that possessed potentially large oxygen exchange surfaces (e.g. rangeomorphs). But this respiratory mode has strong limitations by imposing a relatively small distance between sea-water and tissues and a low or sluggish metabolism. The transportation of oxygen to the tissues via a circulating fluid (hemolymph) and a cardiovascular system (heart and vessels; Figure 1) and the increase of exchange surfaces via gills are two key-innovations that opened enormous opportunities to animal life: that of combining anatomical complexity, larger size and higher metabolic rates. The development of a sophisticated respiratory and circulatory system is particularly well exemplified by arthropods. This major phylum in present-day ecosystems was already well represented in the early Cambrian marine fauna, both in biodiversity and numerical abundance [9], and provides one of the best examples to study how respiratory strategies evolved in the Early Palaeozoic and how marine animals responded to the variations of oxygen level. Indeed, exceptionally preserved biota show that early arthropods already possessed sophisticated respiratory and circulatory systems as exemplified by the development of gills in many groups (see Waptia fieldensis, Figure 2) and also integument hemolymph networks. Here we focus on three groups of Early Palaeozoic small bivalved arthropods with a well-documented fossil record: the phosphatoco- pids, the bradoriids and the ostracods and examine how they may Oxygen and Early Arthropod Micro-Benthos Evolution have responded to changes in oxygen level at the beginning of the Palaeozoic Era. The three groups belong to the evolutionary history of crustaceans sensu lato (see [10–14]. More precisely in this paper we investigate environmental feedbacks caused by the spread of anoxia onto the marine shelf, and the rise of oxygen levels in seawater coupled to changes in seabed substrate that may have selected against bradoriid and phosphatocopid arthropods, but ultimately favoured the development of an ostracod micro-benthos. Results The temporal distribution of the bivalve arthropod micro- benthos in the Cambrian is shown through Figures 3 and 4. Here we use the morphology of the three main groups coupled with lithofacies distribution to suggest the mode of life and respiratory function of the animals. Though their record from the Cambrian is contentious, the most detailed discussion concentrates on the ostracods, for which physiological evidence from modern fauna is available and also because this is the only group that survived 500 million years of marine evolution to become a major Phanerozoic component of the micro-benthos. Ostracods These are small (millimetre-length) aquatic crustaceans with a dorsally articulated bivalve carapace. The most numerically Figure 1. Respiration in water-breathers from simple gaseous diffusion (1) to ventilatory and circulatory convection (5). PwO2, PeO2, PcellO2, Pw9O2, PaO2, PvO2 correspond to the partial pressure of oxygen in water, in the extracellular medium, at cell level, within external chamber (e.g. branchial chamber), in arterial hemolymph, and in venous hemolymph. Today, the PO2 in the blood and tissues of water-breathing animals (see [4] for a review), is remarkably low, ranging between 1 and 3 kPa and is largely independent of ambient PO2. PcellO2 results from the equilibrium between O2 supply and O2 consumption (90% by mitochondria). PwO2.Pw9O2.PaO2.PeO2.PcellO2. doi:10.1371/journal.pone.0028183.g001 PLoS ONE | www.plosone.org 2 December 2011 | Volume 6 | Issue 12 | e28183