Oxygen and Early Arthropod Micro-Benthos Evolution Figure 7. Recent ostracods with gills. Leuroleberis surugaensis Hiruta from Japan. A–C, scanning electron micrographs showing book gills in the branchial cavities, left lateral and posterior views. D, gill in transmitted light showing internal hemolymph sinuses. E, F, stained microtome serial sections, showing 8 pairs of gills on both sides of the soft body and detailed features through individual gills (e.g. hemolymph sinuses). Scale bars: 1 mm in A and B, 500 mm in D and E, 200 mm in C, and 50 mm in F. bg, book gill (integumental lamina attached to thoracic wall); ef, epipodial fan for ventilation; hs, hemolymph sinus in book gills. doi:10.1371/journal.pone.0028183.g007 demonstrates the beginnings of ventilatory adaptation which is unknown in the other ostracod groups (podocopes, myodocopes), a strategy that may have enabled it to withstand intervals of seabed anoxia. Preserved respiratory organs in fossil ostracods are rare, but examples from fossil Lagersta¨tten in the Silurian [39–41] and the Triassic [42] indicate that ancient myodocopids had book gills almost identical to those of their modern counterparts and fan-like exopodial features to ventilate their domiciliar cavity. The earliest ostracods of the Ordovician were small and it is highly probable that they lacked a specialized gas-exchange area (gills) and respired mainly by gaseous diffusion, as do most recent ostracods of similar body size. Indeed, the earliest ostracods possess no exoskeletal features that would indicate the presence of a hemolymph network. Phosphatocopids These animals possessed a dorsally articulated bivalve shield which covered the entire body, the largest of which (in Cyclotron) approaches 6 mm in length [43], but more typically is 1 to 3 mm in length. Unlike ostracods, phosphatocopids lacked adductor muscles and may have been unable to control valve opening and closing, suggested by the many specimens preserved with ‘butterfly’ orientation (e.g. [12,13,43]; Figure 8). The lobate valves of some phosphatocopids (e.g. Cyclotron, [44]) are reminiscent of PLoS ONE | www.plosone.org 7 December 2011 | Volume 6 | Issue 12 | e28183