Oxygen and Early Arthropod Micro-Benthos Evolution Figure 2. External respiratory features of Cambrian arthropods. A, B, Waptia fieldensis Walcott from the middle Cambrian Burgess Shale, British Columbia, Canada, USNM 138231 with trunk appendages bearing gill-like feature, dorsal view and close-up. C, Naraoia longicaudata Zhang and Hou from the early Cambrian Maotianshan Shale, Yunnan Province, China, NIGPAS 115315, biramous appendage with numerous gill-like filaments attached to exopodial branch. Scale bars: 1 cm in A and 1 mm in B and C. doi:10.1371/journal.pone.0028183.g002 abundant arthropods in the fossil record, through their half billion-year history they have adapted to environments ranging from the abyssal depths of the oceans, to freshwater lakes and damp leaf litter. Earlier studies of the fossil record have documented the radiation of ostracods from the benthos into the marine zooplankton [15,16] and from marine to freshwater environments [17,18]. Whilst these adaptive radiations involved fundamental changes in ostracod morphology (e.g. the development of swimming appendages), and metabolism, the primary origin of an ostracod micro-benthos is obscure [19,20]. Ostracods fulfil an important role in aquatic food webs as basal species feeding on primary producers and detritus from plants and animals, and are themselves predated by larger invertebrates and vertebrates. Some myodocopid ostracods live as micro-predators of other invertebrates in the water column, as scavengers [21], and even live as parasites on the gills of sharks [22]. The basal position of ostracods in ancient food webs is suggested by their occurrence in coprolites [23]. Using uniformitarian principles applied to carapace morphol- ogy the oldest benthic ostracods have been identified from early Ordovician strata (ca 485 Ma; [19,24]). These early ostracods formed low-diversity (1 to 2 species) associations, are small (about 1 mm length), possess a bivalved dorsally articulated carapace and have lobal structures that are homologous to stratigraphically later fossil ostracods [25]. They are found associated with a range of marine benthos, which in their earliest occurrences [19,20] includes trilobites and brachiopods. An ostracod micro-benthos was therefore established as a component of the earliest ‘Paleozoic fauna’ (sensu [26]) and these Tremadocian ostracods form the root of the major ostracod biodiversification event of the Middle Ordovician [25,27]. Still earlier, putative ostracods are found in the Cambrian (see below). Vannier and Abe [28] characterised ostracods as belonging to two morphological groups. ‘Body plan 1’ possesses bilateral symmetry, an ellipsoidal shape, an active mode of life (e.g. swimming) and a well-developed circulatory system and is typified by present and fossil myodocopid ostracods (Figures 5, 6). Some myodocopids have book gills as additional exchange surfaces attached in the latero-posterior part of their body (Figure 7). ‘Body plan 2’ corresponds to podocope ostracods sensu lato (podocopids and platycopids; see [29] for classification) with a ventral or lateral polarity ( = valve asymmetry), a heavily calcified shell, and is characterised by a crawling mode of life (mostly in the flocculent layer or sub-sediment surface) and no cardiovascular features (i.e. absence of a heart). Ostracods with ‘body plan 2’ often form a component of the meiofauna living interstitially between sediment grains in an environment which is both protected from predators (though some meiofaunal organisms feed on them), but rich in detrital, bacterial or algal food supply. These ostracods are small, typically in the range of 0.5 to 3 mm length [28]. Oxygen exchange occurs only via diffusion directly from the water through the integument of the carapace inner lamella and body. Although the water column where most Recent ostracods live is under or close to normoxic conditions (PO2 close to 21 kPa air-equilibrated water in open marine conditions), experimental studies [30] indicate that benthic ostracods migrate through the O2 gradient of the sediment in order to reach sediment layers where the PO2 of the water is much lower (ca 3–5 kPa). This low-oxygenation (hypoxic) level that occurs within a few millimetres below the water-sediment interface seems to be the preferential environment of numerous podocopid ostracods. All ostracods possess a pair of ‘ventilatory appendages’ that beat rhythmically, creating currents between the inner side of the valves and the animal body over the main gas exchange tissue surfaces. Ventilation is produced by fan-like exopodial features with primary radiating bristles and closely packed secondary setae that offer resistance to water, typically on the 5th limb of myodocopid ostracods. Importantly, the ventilatory movements are similar to what is observed in living green crabs Carcinus maenas or crayfish Astacus leptodactylus [30–32]. Laboratory experiments [30] per- formed with five podocopid genera (Leptocythere, Cyprideis, Lox- oconcha, Cytheropteron and Argilloecia) indicate that podocopids lack regulatory mechanisms for adapting their ventilation to changes in water oxygenation in contrast to decapod crustaceans which can control their ventilation. Corbari et al. [30] propose that the migration of podocopid ostracods to poorly oxygenated (hypoxic) layers is a behavioural response to adjusting their tissue oxygenation to low-oxygen levels (of 1–3 kPa). Similar experi- ments were conducted with myodocopid ostracods [31]. Some myodocopids are pelagic (e.g. Conchoecia, [33]), others live at the water-sediment interface but show abilities to burrow into the sediment [34] and swim into the water column (e.g. nocturnal PLoS ONE | www.plosone.org 3 December 2011 | Volume 6 | Issue 12 | e28183