Oxygen and Early Arthropod Micro-Benthos Evolution Figure 4. Temporal distribution of Cambrian arthropods conventionally assigned to the Bradoriida and Phosphatocopida. Bradoriid ranges are compiled from [57,71,72,78] and references therein, representing a global dataset. Thick lines indicate definite ranges, whilst thin lines represent a questionable or imprecisely defined range. Only data from [46] and [71] are used to reconstruct the ranges of Bradoriida from China. The phosphatocopid data are from China [46,77], Britain [43], Scandinavia and north Germany [13,56,63,73,79,80,81,82,83]: phosphatocopids are also known from the Antarctic [74], North America [66], Australia [84] and Kazakhstan [75]. The Borrega˚rd Member of Bornholm (equivalent to the Exsulans Limestone) was originally identified as the oldest horizon in Scandinavia with the phosphatocopids Hesslandona, Vestrogothia, Bidimorpha and Falites [79]. The Borrega˚rd Member is equivalent to the P. gibbus Biozone of uppermost Stage 5 [47,83]. However, later the same author [82, p. 887] referred the material of [79] to the ‘Andrarum Limestone Breccia’, a horizon equivalent to the P. forchhammeri Biozone. Accordingly, we take the lower ranges of these phosphatocopids as Guzhangian. Our range for Waldoria includes material referred to Falidoria in [82]. We plot genera as a proxy for species diversity. This is reasonable given that most bradoriid genera contain only between 1 and 3 species. Exceptions to this include Cambria, Liangshanella, Anabarochilina, Indiana and Hipponicharion. For phosphatocopids, a multitude of species are referred to Dabashanella, but most of these are synonyms of D. hemicyclica [46]. Later Cambrian phosphatocopids including Falites, Cyclotron, Hesslandona, Vestrogothia and Bidimorpha all contain more than 3 species [78], emphasising the diversity of Guzhangian and Pabian phosphatocopid assemblages. Also shown are the major Carbon Isotope Excursions (CIEs) after [58], and oxygen levels reconstructed from [3]. For the latter, the red line represents the oxygen reconstruction (with error shown in pink envelope) of [3] and the blue line [with error envelope] is the Berner reconstruction quoted therein. In the text and figures we use the terms ‘early’, ‘middle’ and ‘late’ Cambrian informally to denote Cambrian Series 1 and 2 combined, Series 3, and Series 4 respectively. Note that some authors [13] would tentatively include Epactridion, Dielymella, Liangshanella, Flemingopsis, Alutella, Oepikaluta and Gladioscutum in the Phosphatocopida. For morphological reasons outlined in [57] we include these taxa within the Bradoriida. Abbreviations for CIEs are: BAsal Cambrian Carbon isotope Excursion (BACE); ZHUjiaqing Carbon isotope Excursion (ZHUCE); SHIyantou Carbon isotope Excursion (SHICE); Cambrian Arthropod Radiation isotope Excursion (CARE); MIngxinsi Carbon Isotope Excursion (MICE); Archaeocyathid Extinction Carbon isotope Excursion (AECE); Redlichiid-Olenellid Extinction Carbon isotope Excursion (ROECE); Drumian Carbon isotope Excursion (DICE); StePtoean Carbon Isotope Excursion (SPICE); Top of Cambrian Excursion (TOCE). doi:10.1371/journal.pone.0028183.g004 behavioural response is more complex than for podocopids. During daytime they shelter within self-made nests (aggregates of sediment and mucous-substances) that create a hypoxic environ- ment. Even more interestingly, this is the result of a behavioural and social strategy as groups of Cylindroberis mariae build the nests in which they are buried during daytime, re-breathing together in a confined environment. They become active during the night and swim into the water-column in normoxic water. Although podocopids and myodocopids show major anatomi- cal, physiological and behavioural differences, they both seem to respond in a similar way: both groups will avoid long-term exposure to normoxic conditions and migrate to low-oxygen conditions. Podocopids, which are sluggish and move slowly, do this continuously through the day. Myodocopids which are only active at night avoid normoxic water during daytime when they are at rest, buried in the top millimetres of sediment where oxygen PLoS ONE | www.plosone.org 5 December 2011 | Volume 6 | Issue 12 | e28183