Oxygen and Early Arthropod Micro-Benthos Evolution Figure 9. General morphology of bradoriid arthropods. A, B, Kunmingella douvillei (Mansuy) from the early Cambrian Maotianshan Shale, Yunnan Province, China, dorsal views of two specimens showing bivalved open carapace in ‘‘butterfly’’ position and posterior appendages, Chen Jun- Yuan’s personal collections, Chengjiang and RCCBYU 10258, respectively (B, courtesy of Derek Siveter). C, Anabarochilina sp., middle Cambrian from Kazakhstan, PIN N4343/55, partly exfoliated right valve. D, Anabarochilina primordialis (Linnarsson), middle Cambrian from Va¨sterg o¨tland, Sweden, SGU 8662, partly exfoliated right valve. E, Tsunyiella gridinae Melnikova from the early Cambrian (Atdabanian) of North Central Kazakhstan, PIN N4343/12, left valve, internal mould. F, Cambria melnikovi Ivanova from the early Cambrian (Atdabanian) of the eastern part of Siberia, Russia, PIN N2175/1, largely exfoliated left valve. Small white arrows indicate supposed integumental hemolymph networks (see [51]). All scale bars: 1 mm. For repositories of figured specimens see [14,75]. doi:10.1371/journal.pone.0028183.g009 and at the Drumian-Guzhangian boundary (Figure 4). As a result, bradoriids become extremely rare in late Cambrian strata worldwide, with the exception of one group, the svealutids [57] (Figure 9). The longevity of svealutids such as Anabarochilina (Figure 4) was perhaps facilitated by their ability to disperse widely [57]. One bradoriid taxon, Beyrichona (represented by the very long-ranging species B. triceps [43]), ranged into the earliest Ordovician but disappeared from the rock record in the Angelina sedgwickii Biozone at the end of the Tremadocian [43]. The latest originations of bivalved arthropods traditionally assigned to the Bradoriida occurred during the Guzhangian and Pabian, and include Septadella, Altajanella, Bullaluta and Uskutchiella (Figure 4). Of these, Bullaluta appears assignable to the Svealutidae and Uskutchiella to the Hipponicharionidae [57]. The two other taxa are characterised by small size (1–2 mm) and lack evidence from their shield morphology for the development of a cardiovascular system. The lobation of their shields is indistinguishable from those of early Ordovician ostracods (Figure 10) and they do not occur in dysoxic black shale lithofacies with phosphatocopids. They have been mooted as the earliest ostracods [20,57]. Phosphatocopids also appear during the ‘CARE’ (Figure 4), but have a sporadic distribution in the rock record. In southern Britain, for example, where the Cambrian record has been intensively studied for 150 years they are known only from the Comley Limestones of Shropshire [62] and the black shales of South Wales and the English Midlands [43,44]. Phosphatocopids are most common at horizons associated with dysoxic environ- ments. Peak abundances and diversity of Phosphatocopida are associated with sedimentary deposits of the Guzhangian and Pabian (Figure 4), the latter originations (e.g. Trapezilites, Cyclotron) being stratigraphically coincident with the beginning and peak of the SPICE event [63,64]. In England, Wales and Newfoundland, this interval is associated with black shale deposition that yields species of Cyclotron, Vestrogothia, Falites, Veldotron, Trapezilites and PLoS ONE | www.plosone.org 9 December 2011 | Volume 6 | Issue 12 | e28183