Biogeography and affinities of the bradoriid arthropods: Cosmopolitan microbenthos of the Cambrian seas

Abstract

This paper reviews the global biogeography and affinities of the Bradoriida, a group of Cambrian–early Ordovician arthropods. Bradoriida appear in the fossil record during the early Cambrian, just prior to the earliest trilobite faunas. Seven families may form a natural (monophyletic) group of Bradoriida sensu stricto, comprising Cambriidae, Kunmingellidae, Comptalutidae, Bradoriidae, Hipponicharionidae, Beyrichonidae and Svealutidae. Amongst the Bradoriida sensu lato, some forms that lie outside these families have carapace designs that suggest an ostracod affinity, particularly Altajanella and Vojbokalina. Bradoriida formed a major element of the ‘Cambrian evolutionary fauna’ and are components of the Chengjiang, Burgess Shale, and Buen Formation Lagerstätten. Bradoriida achieved global distribution from the Atdabanian (early Cambrian). Their diversity peaked during the early and middle Cambrian, with highest diversity at the species and genus level amongst the palaeo-tropical faunas of the South China (17 genera) and east Gondwana palaeocontinental regions (23 genera). By contrast Laurentian faunas were of much lower diversity (10 genera for the whole Cambrian). Bradoriid diversity declined rapidly from the latest middle Cambrian, a trend that may be related to the major extinction of trilobites at the base of the late Cambrian. The youngest Bradoriida comprise a few, rare Ordovician forms. Bradoriids appear to have occupied well-oxygenated marine shelf facies. The biogeographical patterns of early and middle Cambrian Bradoriida suggest climatic control on their distribution (temperate and tropical faunas), together with palaeogeographical constraints that also reflect trilobite provinciality. Kunmingellids and comptalutids were restricted to palaeo-tropical/subtropical sites, but migrated between South China, Siberia and eastern Gondwana palaeocontinents. They are absent from the mid- and high latitude faunas of the western Gondwana, Avalonia and Baltica palaeocontinental areas that were dominated by hipponicharionids, beyrichonids and Bradoriidae. The Laurentia palaeocontinent was isolated from the main zones of bradoriid diversity, but its faunas include a number of early and middle Cambrian Bradoriidae ( Indota, Bradoria, Walcottella), and the region was colonised by cosmopolitan cambriids during the early Cambrian and by supposed pelagic svealutids ( Liangshanella and Anabarochilina) during the middle Cambrian. The genera Walcottella, Dielymella and Bullaluta were endemic to Laurentia. One species of Anabarochilina that possibly dwelt near the sea-surface achieved an equatorial to high southern latitude (70° S) distribution during the late middle Cambrian ( Lejopyge laevigata Biozone), perhaps reflecting a reduced latitudinal temperature gradient for near surface ocean waters.